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  • 1.
    Abrehdary, Majid
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Bagherbandi, Mohammad
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    The spherical terrain correction and its effect on the gravimetric-isostatic Moho determination2016In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 204, no 1, p. 262-273Article in journal (Refereed)
    Abstract [en]

    In this study, the Moho depth is estimated based on the refined spherical Bouguer gravity disturbance and DTM2006 topographic data using the Vening Meinesz-Moritz gravimetric-isostatic hypothesis. In this context, we compute the refined spherical Bouguer gravity disturbances in a set of 1 degrees x 1 degrees blocks. The spherical terrain correction, a residual correction to each Bouguer shell, is computed using rock heights and ice sheet thicknesses from the DTM2006 and Earth2014 models. The study illustrates that the defined simple Bouguer gravity disturbance corrected for the density variations of the oceans, ice sheets and sediment basins and also the non-isostatic effects needs a significant terrain correction to become the refined Bouguer gravity disturbance, and that the isostatic gravity disturbance is significantly better defined by the latter disturbance plus a compensation attraction. Our study shows that despite the fact that the lateral variation of the crustal depth is rather smooth, the terrain affects the result most significantly in many areas. The global numerical results show that the estimated Moho depths by the simple and refined spherical Bouguer gravity disturbances and the seismic CRUST1.0 model agree to 5.6 and 2.7 km in RMS, respectively. Also, the mean value differences are 1.7 and 0.2 km, respectively. Two regional numerical studies show that the RMS differences between the Moho depths estimated based on the simple and refined spherical Bouguer gravity disturbance and that using CRUST1.0 model yield fits of 4.9 and 3.2 km in South America and yield 3.2 and 3.4 km in Fennoscandia, respectively.

  • 2.
    Abreu, Rafael
    et al.
    Westfalische Wilhelms Univ Munster, Inst Geophys, Corrensstr 24, D-48149 Munster, Germany.;Univ Granada, Inst Andaluz Geofis, Campus Cartuja S-N, E-18071 Granada, Spain..
    Kamm, Jochen
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Westfalische Wilhelms Univ Munster, Inst Geophys, Corrensstr 24, D-48149 Munster, Germany.
    Reiss, Anne-Sophie
    Westfalische Wilhelms Univ Munster, Inst Geophys, Corrensstr 24, D-48149 Munster, Germany..
    Micropolar modelling of rotational waves in seismology2017In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 210, no 2, p. 1021-1046Article in journal (Refereed)
    Abstract [en]

    In this contribution we study elastic wave propagation via the introduction of the micropolar theory. As a generalization of a classical linear elastic medium, a micropolar medium allows each particle to have intrinsic rotational degrees of freedom (spin). We perform numerical experiments using the Pseudospectral method. We find analytical harmonic micropolar solutions for different problem configurations, which result in waveform differences between the classical linear elastic and micropolar media. In contrast to linear elastic media, wave propagation in micropolar media is dispersive. We study how the spin waveform depends on the micropolar elastic parameters and frequency content of the simulation. The micropolar effect on numerical seismograms has a direct implication on the phase, amplitude and arrival time. For frequencies lower than the cut-off frequency, the spin waveform has the same amplitude as the macrorotation field. For frequencies higher than the cut-off frequency, the amplitude of the spin waveform decreases with increasing frequency, so that then it is no longer comparable to the amplitude of macroscopic rotations. When both frequencies are equal there is no wave propagation. This work attempts to clarify the theory of micropolar media for its applications in seismology. We argue that micropolar theory should be further investigated for its potential uses in seismology to, for example, describe energy dissipation, seismograms recorded with rotational seismometers and rupture processes.

  • 3.
    Abril, Claudia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Relocating earthquakes with empirical traveltimes2018In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 214, no 3, p. 2098-2114Article in journal (Refereed)
    Abstract [en]

    A strategy is proposed to incorporate effects of 3-D velocity variations on earthquake locationsusing empirical traveltimes (ETTs). Traveltime residuals are interpolated from those predictedusing a 1-D velocity reference, mapped on to the hypocentres of corresponding earthquakesfor each station in a network. First, station corrections are computed by averaging the residualsover a fixed scale. Then, summary-ray variograms are used to estimate uncertainty and that,in turn, is used to tune a local fit to neighbouring residuals to refine the corrections. Resulting3-D traveltime estimates are then used as a description of the forward problem in a nonlineargrid-search relocation. Data are weighted according to the estimated uncertainty. Data fromthe Icelandic Southern Iceland Lowlands (SIL) national seismic network are used to test thestrategy. ETTs are estimated forP- andS-waves at 65 stations in the SIL network, basedon four million arrival time readings of 300.000 events registered between 1990 and 2012.ETTs are strongly correlated for the two wave types. The spatial variations of the predictedcorrections are consistently comparable or somewhat less forS-waves thanP-waves. Thisfeature suggests variations of theVP/VSratio in the Icelandic crust. Error estimates are alsostrongly correlated for the two wave types and between nearby stations. Relocations aretested by comparison with explosions and small populations of well-located events withindenser subnetworks. Relocations result in modestly enhanced clustering of explosions andearthquakes and significantly improved depth estimates. Estimates of the random relocationerror are statistically better behaved than those of the SIL system. They are in general reduced,as is expected since 3-D heterogeneity is now partly taken into account.

  • 4. Adamczyk, A.
    et al.
    Malinowski, M.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    High-resolution near-surface velocity model building using full-waveform inversion-a case study from southwest Sweden2014In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 197, no 3, p. 1693-1704Article in journal (Refereed)
    Abstract [en]

    Full-waveform inversion (FWI) is an iterative optimization technique that provides high-resolution models of subsurface properties. Frequency-domain, acoustic FWI was applied to seismic data acquired over a known quick-clay landslide scar in southwest Sweden. We inverted data from three 2-D seismic profiles, 261-572 m long, two of them shot with small charges of dynamite and one with a sledgehammer. To our best knowledge this is the first published application of FWI to sledgehammer data. Both sources provided data suitable for waveform inversion, the sledgehammer data containing even wider frequency spectrum. Inversion was performed for frequency groups between 27.5 and 43.1 Hz for the explosive data and 27.5-51.0 Hz for the sledgehammer. The lowest inverted frequency was limited by the resonance frequency of the standard 28-Hz geophones used in the survey. High-velocity granitic bedrock in the area is undulated and very shallow (15-100 m below the surface), and exhibits a large P-wave velocity contrast to the overlying normally consolidated sediments. In order to mitigate the non-linearity of the inverse problem we designed a multiscale layer-stripping inversion strategy. Obtained P-wave velocity models allowed to delineate the top of the bedrock and revealed distinct layers within the overlying sediments of clays and coarse-grained materials. Models were verified in an extensive set of validating procedures and used for pre-stack depth migration, which confirmed their robustness.

  • 5. Artyushkov, E V
    et al.
    Mörner, Nils-Axel
    Tarling, D H
    The cause of loss of lithospheric rigidity in areas far from plate tectonic activity2000In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 143, no 3, p. 752-776Article, review/survey (Refereed)
    Abstract [en]

    Significant losses of lithospheric strength are generally considered to be almost entirely associated with abnormal heating or steep lithospheric bending and/or stretching near to active plate boundaries. Several areas-the western Greater Caucasus, the North Crimean basin, the Carpathian foredeep, the Peri-Caspian basin and the Trans-Caspian areas-are shown to have steep basement slopes, usually comprising a difference in height of several kilometres over lateral distances of only 20-30 km, corresponding to very low, similar to 3-5 km, effective elastic thicknesses of the lithosphere. Each of these areas is shown to have undergone rapid steepening of the basement slope, usually within 1-2 Myr but in up to 10 Myr in some areas. At such times, these localities were far from active plate boundaries and in positions where bending forces could not have been transmitted to them from far-distant plate activities. Surface and/or subsurface loading can similarly be excluded as mechanisms for such steepening, and there is no apparent outflow of crustal materials into adjacent regions. It is suggested that such rapid subsidence far from plate tectonic activity is caused by rapid increases in the local density of the lithosphere. This could occur as a result of, for example, a gabbro-eclogite transformation in the lower crust, catalysed by the infiltration of volatiles from the asthenosphere. The resultant contraction of the mafic rocks would be non-uniform in space and produce high deviatory stresses, reducing the viscosity in the lower crust to similar to 10(23) Pa s. This would result in the rapid subsidence of the top of this layer, accompanied by steep ductile bending of the overlying upper crust. Such steer downwarping of the basement would be accompanied by a similar steepening of the underlying weakened mantle. The formation of such steep slopes thus indicates a weakening of the entire lithospheric layer, most probably due to the infiltration of volatiles from the asthenosphere, and unrelated to coeval plate tectonic activity.

  • 6.
    Bazargan, Mohsen
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Hieronymus, Christoph F.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Vachon, Remi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Evolution of the statistical distribution of crystal orientations in time- and space-varying viscous flows2019In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 218, no 2, p. 773-786Article in journal (Refereed)
    Abstract [en]

    Magmas and other viscously deforming fluids in the Earth frequently contain embedded crystals or other solid inclusions. These inclusions generally rotate about their own axis and, under certain conditions, align themselves in a direction dictated by the details of the flow. This rotational behaviour has been studied extensively for homogeneous flows. Here, we couple the crystal rotation dynamics with the fluid mechanical Navier-Stokes equations for the large-scale flow, thus allowing the analysis of crystal rotations in settings that are variable in both space and time. The solution is valid provided that the intercrystal spacing is sufficiently large to preclude interaction between crystals. Additionally, we derive an evolution equation for the probability density function (PDF) of crystal orientations based on the fundamental concept of conservation of generic properties in continuum mechanics. The resulting system of equations is extensively tested against previous analytical and numerical solutions. Given the focus on method validation, we limit the fluid mechanics to simple systems with analytical solutions for the velocity field. Even for the simple examples computed, all of which are characterized by fluid flow that is constant in time, the crystal orientation patterns are spatially complex and change in time. Pressure-driven flow in a channel results in coherent bands of crystal orientations with band thickness decreasing towards the channel walls. In corner flow constrained by two mutually perpendicular walls, the pattern of crystal orientations does not exhibit any significant similarity with the flow field. Given that there is no local one-to-one correspondence between the flow and the PDF pattern, a combined and larger-scale solution of the two systems is generally required. The simple flow examples shown demonstrate the viability of this new approach. Application to more complex flow geometries which may commonly occur in nature is deferred to future studies.

  • 7.
    Biete, Cristina
    et al.
    CSIC, ICTJA, Inst Earth Sci, Lluis Sole i Sabaris S-N, E-08028 Barcelona, Spain;Univ Barcelona, Dept Dinam Terra & Ocea, Barcelona 02028, Spain.
    Brown, Dennis
    CSIC, ICTJA, Inst Earth Sci, Lluis Sole i Sabaris S-N, E-08028 Barcelona, Spain;Natl Cent Univ, Dept Earth Sci, Taoyuan 32001, Taiwan.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Alvarez-Marron, Joaquina
    CSIC, ICTJA, Inst Earth Sci, Lluis Sole i Sabaris S-N, E-08028 Barcelona, Spain.
    Wu, Yih-Min
    Natl Taiwan Univ, Dept Geosci, Taipei 10617, Taiwan;Acad Sinica, Inst Earth Sci, Taipei 11529, Taiwan;Natl Taiwan Univ, NTU Res Ctr Future Earth, Taipei 10617, Taiwan.
    Kuo-Chen, Hao
    Natl Cent Univ, Dept Earth Sci, Taoyuan 32001, Taiwan.
    Ho, Chun-Wei
    Natl Cent Univ, Dept Earth Sci, Taoyuan 32001, Taiwan;Cent Weather Bur, Taipei 10048, Taiwan.
    The influence of inherited continental margin structures on the stress and strain fields of the south-central Taiwan fold-and-thrust belt2019In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 219, no 1, p. 430-448Article in journal (Refereed)
    Abstract [en]

    In this paper we test whether or not structural and morphological features inherited from the Eurasian continental margin are affecting the contemporary stress and strain fields in south-central Taiwan. Principal stress directions (sigma(1), sigma(2) and sigma(3)) are estimated from the inversion of clustered earthquake focal mechanisms and the direction of the maximum compressive horizontal stress (S-H) is calculated throughout the study area. From these data the most likely fault plane orientations and their kinematics are inferred. The results of the stress inversion are then discussed together with the directions of displacement, compressional strain rate and maximum shear strain rate derived from GPS data. These data show that there is a marked contrast in the direction of SH from north to south across the study area, with the direction of SH remaining roughly subparallel to the relative plate motion vector in the north, whereas in the south it rotates nearly 45 degrees counter-clockwise. The direction of the horizontal maximum compression strain rate (epsilon(H)) and associated maximum shear planes, together with the displacement field display an overall similar pattern between them, although undergoing a less marked rotation. We interpret the southward change in the SH, eH and the dextral maximum shear plane directions, together with that of the horizontal displacement field to be related to the reactivation of east-northeast striking faults inherited from the rifted Eurasian margin and to the shelf/slope break. Inherited faults in the basement are typically reactivated as strike-slip faults, whereas newly formed faults in the fold-and-thrust belt are commonly thrusts or oblique thrusts. Eastwards, the stress inversions and strain data show that the western flank of the Central Range is undergoing extension in the upper crust. SH in the Central Range is roughly parallel to the relative plate convergence vector, but in southwestern Taiwan it undergoes a marked counter-clockwise rotation westwards across the Chaochou fault. Farther north, however, there is no significant change across the Lishan fault. This north to south difference is likely due to different margin structures, although local topographic effects may also play a role.

  • 8.
    Blanc, Emilie
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Division of Scientific Computing. Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Numerical Analysis.
    Komatitsch, Dimitri
    Chaljub, Emmanuel
    Lombard, Bruno
    Xie, Zhinan
    Highly accurate stability-preserving optimization of the Zener viscoelastic model, with application to wave propagation in the presence of strong attenuation2016In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 205, p. 427-439Article in journal (Refereed)
  • 9. Bruijn, Rolf
    et al.
    Almqvist, Bjarne
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Hirt, Ann
    Benson, Philip
    Decoupling of paramagnetic and ferrimagnetic AMS development during the experimental chemical compaction of illite shale powder2013In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246XArticle in journal (Refereed)
  • 10.
    Burchardt, Steffi
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Koyi, Hemin
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Schmeling, Harro
    Fuchs, Lukas
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Sinking of anhydrite blocks within a Newtonian salt diapir: modelling the influence of block aspect ratio and salt stratification2012In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 188, no 3, p. 763-778Article in journal (Refereed)
    Abstract [en]

    2-D Finite Differences models are used to analyse the strain produced by gravity-driven sinking of dense rectangular inclusions through homogeneous and vertically stratified Newtonian salt. We systematically modelled the descent of dense blocks of different sizes and initial orientations (aspect ratios) representing the Main Anhydrite fragments documented within, for example, the Gorleben salt diapir. Model results demonstrate that size of the blocks is a governing parameter which dictates the amount of strain produced within the block and in the surrounding host salt. Initial block orientation (aspect ratio), on the other hand, causes fundamental differences in block deformation, while the resulting structures produced in the salt are principally the same in all models with homogeneous salt, covering shear zones and folding of passive markers. In models with vertically stratified salt with different viscosities, block descent takes place along complex paths. This results from greater strain accommodation by the salt formation with the lowest viscosity and an asymmetrical distribution of initial vertical shear stresses around the block. Consequently, in these models, block strain is lower compared with the models with homogeneous salt (for the same viscosity as the high-viscosity salt), and sinking is accompanied by block rotation. The latter causes diapir-scale disturbance of the pre-sinking salt stratigraphy and complex sinking paths of the blocks. In particular, vertically oriented blocks sink into high-viscosity salt and drag with them some low-viscosity salt, while horizontal blocks sink in the low-viscosity salt. The resultant sinking velocities vary strongly depending on the sinking path of the block. Based on model results and observed structural configuration within the Gorleben salt diapir, we conclude that the internal complexity of a salt diapir governs its post-ascent deformation. Salt structure and its interaction with dense blocks should hence be considered in the assessment of the long-term stability of storage sites for hazardous waste.

  • 11.
    Chemia, Z.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Koyi, H.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Schmeling, H.
    Numerical modelling of rise and fall of a dense layer in salt diapirs2008In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 172, no 2, p. 798-816Article in journal (Refereed)
    Abstract [en]

    Numerical models are used to study the entrainment of a dense anhydrite layer by a diapir. The anhydrite layer is initially horizontally embedded within a viscous salt layer. The diapir is down-built by aggradation of non-Newtonian sediments (n = 4, constant temperature) placed on the top of the salt layer. Several parameters (sedimentation rate, salt viscosity, perturbation width and stratigraphic position of the anhydrite layer) are studied systematically to understand their role in governing the entrainment of the anhydrite layer. High sedimentation rates during the early stages of the diapir evolution bury the initial perturbation and, thus, no diapir forms. The anhydrite layer sinks within the buried salt layer. For the same sedimentation rate, increasing viscosity of the salt layer decreases the rise rate of the diapir and reduces the amount (volume) of the anhydrite layer transported into the diapir. Model results show that viscous salt is capable of carrying separate blocks of the anhydrite layer to relatively higher stratigraphic levels. Varying the width of the initial perturbation (in our calculations 400-800 m), from which a diapir triggers, shows that wider diapirs can more easily entrain an embedded anhydrite layer than the narrower diapirs. The anhydrite layer is entrained as long as rise rate of the diapir exceeds the descent rate of the denser anhydrite layer. We conclude that the four parameters mentioned above govern the ability of a salt diapir to entrain an embedded dense layer. However, the model results show that the entrained blocks inevitably sink back if the rise rate of the diapir is less than the rate of descent of the anhydrite layer or the diapir is permanently covered by a stiff overburden in case of high sedimentation rates.

  • 12.
    Chen, Chaojian
    et al.
    Cent S Univ, Sch Geosci & Infophys, Changsha 410083, Hunan, Peoples R China.
    Ren, Zhengyong
    Cent S Univ, Sch Geosci & Infophys, Changsha 410083, Hunan, Peoples R China.
    Pan, Kejia
    Cent S Univ, Sch Math & Stat, Changsha 410083, Hunan, Peoples R China.
    Tang, Jingtian
    Cent S Univ, Sch Geosci & Infophys, Changsha 410083, Hunan, Peoples R China.
    Kalscheuer, Thomas
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Maurer, Hansruedi
    Swiss Fed Inst Technol, Inst Geophys, Dept Earth Sci, CH-8091 Zurich, Switzerland.
    Sun, Ya
    Cent S Univ, Sch Geosci & Infophys, Changsha 410083, Hunan, Peoples R China.
    Li, Yang
    Chinese Acad Sci, Inst Geol & Geophys, Beijing 100029, Peoples R China.
    Exact solutions of the vertical gravitational anomaly for a polyhedral prism with vertical polynomial density contrast of arbitrary orders2018In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 214, no 3, p. 2115-2132Article in journal (Refereed)
    Abstract [en]

    We present general closed-form solutions for the vertical gravitational anomaly caused by a polyhedral prism with mass density contrast varying with depth. Our equations are the first ones to implement a polynomial vertical mass density contrast of arbitrary order. Singularities in the gravity field which arise when the observation site is close to or in the anomalous polyhedral prism are removed in our analytic expressions. Therefore, the observation site can be located outside, on the faces of or inside the anomalous mass bodies. A simple prismatic body of anomalous density is adopted to test the accuracy of our newly developed closed-form solution. Cases of constant, linear, quadratic, cubic and quartic polynomial orders of mass density contrast are tested. For cases of constant, linear, quadratic and cubic polynomial orders, the relative errors between our results and other published exact solutions are less than 10(-11)%. For the case of quartic polynomial order, relative errors less than 10(-10)% are obtained between our solutions and those computed by a high-order Gaussian quadrature rule (512 x 512 x 512 = 134 217 728 quadrature points), where our new analytic solution needs significantly less computational time (0.0009 versus 31.106 s). These numerical experiments not only verified the accuracy of our new formula but also demonstrated their potential in computing exact gravity anomalies for complicated mass density distributions in the Earth.

  • 13.
    Cherevatova, M.
    et al.
    Institute for Geophysics, University of Münster, 48149 Münster, Germany.
    Egbert, G. D.
    College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA.
    Smirnov, M. Yu
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    A multi-resolution approach to electromagnetic modelling2018In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 214, no 1, p. 656-671Article in journal (Refereed)
    Abstract [en]

    We present a multi-resolution approach for 3-D magnetotelluric forward modelling. Our approach is motivated by the fact that fine-grid resolution is typically required at shallow levels to adequately represent near surface inhomogeneities, topography and bathymetry, while a much coarser grid may be adequate at depth where the diffusively propagating electromagnetic fields are much smoother. With a conventional structured finite difference grid, the fine discretization required to adequately represent rapid variations near the surface is continued to all depths, resulting in higher computational costs. Increasing the computational efficiency of the forward modelling is especially important for solving regularized inversion problems. We implement a multi-resolution finite difference scheme that allows us to decrease the horizontal grid resolution with depth, as is done with vertical discretization. In our implementation, the multi-resolution grid is represented as a vertical stack of subgrids, with each subgrid being a standard Cartesian tensor product staggered grid. Thus, our approach is similar to the octree discretization previously used for electromagnetic modelling, but simpler in that we allow refinement only with depth. The major difficulty arose in deriving the forward modelling operators on interfaces between adjacent subgrids. We considered three ways of handling the interface layers and suggest a preferable one, which results in similar accuracy as the staggered grid solution, while retaining the symmetry of coefficient matrix. A comparison between multi-resolution and staggered solvers for various models shows that multi-resolution approach improves on computational efficiency without compromising the accuracy of the solution.

  • 14.
    Eken, Tuna
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Plomerova, Jaroslova
    GFU, Prague.
    Vecsey, Ludek
    GFU, Prague.
    Babuska, Vladislav
    GFU, Prague.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Shomali, Hossein
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bödvarsson, Reynir
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Effects of seismic anisotropy on P-velocity tomography of the Baltic Shield2012In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 188, no 2, p. 600-612Article in journal (Refereed)
    Abstract [en]

    We investigate possible effects of neglecting seismic anisotropy on standard isotropic P-velocity tomographic images of the upper mantle beneath the Baltic shield. Isotropic inversions of teleseismic P- and S-wave traveltimes exhibit alternating high- and low-velocity heterogeneities down to depths of over 400 km. Differences in tomographic inversions of SV- and SH-wave traveltimes are distinct down to depths of about 200 km and are associated with anisotropy of the lithospheric mantle. Anisotropic structures of the upper mantle affect both the P and S traveltimes, shear-wave splitting as well as the P polarization directions. Joint inversion for isotropic and anisotropic velocity perturbations is not feasible due to the limited 3-D ray coverage of available data. Therefore, we correct the input traveltimes for anisotropic contributions derived from independent analyses and then perform standard isotropic inversions. These corrections are derived either directly from directional deviations of P-wave propagation or are calculated in anisotropic models retrieved by joint inversions of body-wave anisotropic parameters (P-residual spheres and shear-wave splitting). These anisotropic models are also used to fit backazimuth variations of P-wave polarization directions. General features of tomographic images calculated from the original and the anisotropy-corrected data are similar. Amplitudes of the velocity perturbations decrease below similar to 200 km depth, that is in the sub-lithospheric mantle. In general, large-scale anisotropy related to the fabrics of the continental mantle lithosphere can contaminate tomographic images in some parts of models and should not be ignored.

  • 15.
    Eken, Tuna
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Shomali, Z. Hossein
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Bödvarsson, Reynir
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Upper-mantle structure of the Baltic Shield below the Swedish National Seismological Network (SNSN) resolved by teleseismic tomography2007In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 169, no 2, p. 617-630Article in journal (Refereed)
    Abstract [en]

    Upper-mantle structure under the Baltic Shield is studied using non-linear high resolution teleseismic P-phase tomography. Observed relative arrival-time residuals from 52 teleseismic earthquakes recorded by the Swedish National Seismological Network (SNSN) are inverted to delineate the structure of the upper mantle. The network consists of 47 (currently working) three-component broad-band stations located in an area about 450 km wide and 1450 km long. In order to reduce complications due to possible significant three-dimensionality of Earth structure, events chosen for this study lay close to in-line with the long-axis of the array  (±30°) . Results indicate P-wave velocity perturbations of ±3 per cent down to at least 470 km below the network. The size of the array allows inversion for structures even at greater depths, and lateral variations of velocity at depths of up to 680 km appear to be resolved. Below the central part of the array (60°–64° N), where ray coverage is best, the data reveals a large region of relatively low velocity at depths of over about 300 km. At depths less than about 250–300 km, the models include a number of features, including an apparent slab-like structure dipping gently towards the north.

  • 16.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Integrated seismic studies of the Baltic shield using data in the Gulf of Bothnia region1993In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 112, no 3, p. 305-324Article in journal (Refereed)
    Abstract [en]

    In the autumn of 1989 a co-operative experiment involving 12 research institutions in northwestern Europe collected 2268 km of deep seismic reflection profiles in the Gulf of Bothnia and the Baltic Sea. the 121 litre airgun array used for this profiling was also recorded by 62 muiticomponent land stations to provide coincident refraction surveys, fan-spreads, and 3-D seismic coverage of much of the Gulf of Bothnia. We thus have potentially both high-resolution impedance contrast images as well as more regional 3-D velocity models in both P- and S-waves. In the Bothnian Bay a south-dipping, non-reflective zone coincides with the conductive Archaean-Proterozoic boundary onshore in Finland. Between the Bothnian Bay and Bothnian Sea observed reflectivity geometries and velocity models at Moho depths suggest structures inherited from a 1.9Ga subduction zone; the upper crust here appears to have anomalously low velocity. Within the Bothnian Sea, reflectivity varies considerably beneath the metasedimentary/granitoid rocks of the Central Svecofennian Province (CSP) and the surrounding metavolcanic-arc rocks. Numerous dipping reflectors appear throughout the metavolcanic crust, whereas the CSP has little reflectivity. Wide-angle reflections indicate that the metasedimentary crust of the Bothnian Basin is 10 km thicker than the neighbouring Svecofennian subprovinces. Near the Åland archipelago Rapakivi granite plutons exhibit bright reflections, a contrast to the usual non-reflective plutons elsewhere in western Europe. Additional dipping reflections deep in the crust of this area may support models of rifting and crustal thinning during emplacement of the 1.70-1.54 Ga Rapakivi granites. Coeval gabbroic/anorthositic magmatism may explain the high reflectivity and high velocity of these plutons. the c. 1.25 Ga mafic sills and feeder dykes of the Central Scandinavian Dolerite Group also produce clear reflections on both near- and far-offset seismic sections. Continued modelling will produce better velocity models of the crust and better constrained contour maps of crustal thickness in this part of the Baltic shield.

  • 17.
    Elming, Sten-åke
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Bylund, Göran
    Palaeomagnetism and the Siljan impact structure, central Sweden1991In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 105, no 3, p. 757-770Article in journal (Refereed)
    Abstract [en]

    A palaeomagnetic study of rocks, from inside and outside the impact structure has been carried out, with the aim of identifying natural remanent magnetizations (NRMs) related to the impact event. Three different directions of magnetizations, not recognized elsewhere, were identified within the central part of the structure. These three magnetizations are defined in terms of coercivity and blocking temperature. The implications of the results are discussed in relation to ages obtained from Ar-Ar dating of impact melt

  • 18.
    Elming, Sten-åke
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    D'Agrella-Filho, M.S.
    Universidade de São Paulo.
    Page, L.M.
    University of Lund.
    Tohver, E.
    School of Earth and Geographical Sciences, University of Western Australia, Crawley.
    Trindade, R. I. F.
    Universidade de São Paulo.
    Pacca, I.I.G.
    Universidade de São Paulo.
    Geraldes, M.C.
    Faculdade de Geologia, Universidade do Estado do Rio de Janeiro.
    Teixeira, W.
    Instituto de Geociências, Universidade de São Paulo.
    A palaeomagnetic and Ar-40/Ar-39 study of late precambrian sills in the SW part of the Amazonian craton: Amazonia in the Rodinia reconstruction2009In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 178, no 1, p. 106-122Article in journal (Refereed)
    Abstract [en]

    A new key palaeomagnetic pole (Plat. = 64.3°S, Plon. = 271.0°E, N = 14, A95= 9.2°; Q = 5) is calculated from a primary magnetization isolated in early Neoproterozoic Aguapei basic sills and dykes hosted by 1.3-1.0 Ga sedimentary rocks from the southwestern part of the Amazon craton. The characteristic remanence carried by stable, pseudo-single domain titanomagnetite shows two antipodal polarities that pass a reversals test. Magnetic anisotropy for most sites shows fabric orientations that are typical of sills, with horizontal magnetic foliations concordant to the flat-lying bedding of the host sedimentary rocks. 40Ar/39Ar analyses for one of the sills reveal a well-defined plateau age at 981 ± 2 Myr. A tectonic reconstruction for Amazonia relative Laurentia based on this new pole 'is consistent with' a position of the present northwestern part of Amazonia attached with eastern Laurentia close to Greenland at ca. 981 Ma. On basis of palaeomagnetic and geological data, we propose a scenario where Amazonia moved northeastwards along the present southeast coast of Laurentia from ca. 1200 to 980 Ma. By 980 Ma, Amazonia is placed alongside Laurentia and Baltica, in a position similar to other reconstructions of Rodinia but with a significantly different orientation.

  • 19.
    Elming, Sten-åke
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Layer, P.
    Geophysical Institute, University of Alaska.
    Ubieta, K.
    Centro de Investigaciones Geoscientificas (CIGEO), the Autonomous University of Nicaragua (UNAN), Managua.
    A palaeomagnetic study and age determinations of Tertiary rocks in Nicaragua, Central America2001In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 147, no 2, p. 294-309Article in journal (Refereed)
    Abstract [en]

    Palaeomagnetic, K-Ar and Ar40/Ar39 measurements have been made to elucidate Nicaragua's plate tectonic history. These show that the Nicaraguan Highland rotated 90° counterclockwise between 30 and 18 Myr ago, but the Pacific Coastal Plain has not rotated.A lack of data has prevented a direct comparison with the tectonic history of adjacent areas, in particular, the Chortis Block, which is the part of the Caribbean Plate that is geologically most closely related to the Nicaraguan Highland. However, our data suggest that the Caribbean Plate in this part of Central America was not consolidated until c. 15 Ma.

  • 20.
    Elming, Sten-åke
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Maokhar, M.O.
    Razi University, Department of Physics, Faculty of Basic Sciences.
    Layer, P.
    University of Alaska, Institute for Geophysics, Fairbanks.
    Donadini, F.
    Scripps Institution of Oceanography, La Jolla, United States of America.
    Uplift deduced from remanent magnetization of a proterozoic basic dyke and the baked country rock in the Hoting area, central Sweden: a palaeomagnetic and Ar-40/Ar-39 study2009In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 179, no 1, p. 59-78Article in journal (Refereed)
    Abstract [en]

    Remanent magnetization in dyke contact zones in the Hoting area of the Fennoscandian Shield in the central part of Sweden has been studied in order to establish the ambient temperature of the host rock and the depth of burial of the present erosion surface at the time of intrusion. A positive baked contact test for two Subjotnian dykes demonstrates the primary nature of the dyke magnetization. From the magnetic properties and the palaeomagnetic data, it can be concluded that the overprinting in the hybrid zone of one of the basic dykes is a partial thermoremanent magnetization. Reliable results were also obtained from a palaeointensity study of samples from the hybrid zone in the baked host rock. The study was performed in the laboratories at Luleå and at Scripps and a mean intensity of the Earth's magnetic field of 5.8 ± 1.9 μT was determined. The maximum temperature due to the dyke intrusion in the hybrid zone has been defined and from that an ambient temperature of 375 °C in the host rock at the time of intrusion has been calculated. This calculated temperature is not contradicted by the 40Ar/39Ar data. A palaeothermal gradient in the crust at ca. 1.6 Ga is calculated at ca. 34 °C km-1 and yields a depth of burial of the present erosion surface at ca. 10.4 km. This implies an uplift rate of 0.65 km (100 Ma)-1. A slow cooling of the gabbroic host rock (ca. 3.5 °C Ma-1) has been calculated from the difference in the U-Pb age of zircon (1.786 ± 0.010 Ga) and the 40Ar/39Ar biotite ages (1.648 ± 0.012 Ga; 1614 ± 0.024 Ga) of this study. This slow cooling resulted in a palaeomagnetic age of ca. 1.7 Ga for the gabbro, which is also the age of the determined palaeointensity. The calculated ca. 1.614 Ga palaeomagnetic pole from the basic dykes fulfils most of the criteria for a reliable pole and may be regarded as a new key-pole for Fennoscandia.

  • 21.
    Elming, Sten-åke
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Rasmussen, Thorkild Maack
    Danmarks og Grønlands Geologiske Undersøgelse, GEUS.
    Results of magnetotelluric and gravimetric measurements in western Nicaragua, central America1997In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 128, no 3, p. 647-58Article in journal (Refereed)
    Abstract [en]

    Studies a ca. 170 km long traverse running from the Pacific coast of Nicaragua in the west to the Nicaraguan Highland in the east. This part of Nicaragua is characterized by sedimentary rocks of the Pacific Coastal Plain, separated from the Tertiary volcanic rocks of the Highland by the NW-SE-trending Nicaraguan Depression. The purpose of this study is to provide electric conductivity and density constraints on geological crustal structures along the transect. This may then serve as a base for the understanding of the tectonic evolution of this part of Central America. Questions to be answered are: (1) is the Pacific coastal province an accreted terrane? (2) What is the character of the depression? Is this a hidden contact between an accreted terrane and the continental Chortis block? (3) If so, is there a different physical signature for the crust of the Pacific province compared to that of the Highlands?

  • 22. Engen, Oyvind
    et al.
    Gjengedal, Jakob Andreas
    Faleide, Jan Inge
    Kristoffersen, Yngve
    Eldholm, Olav
    Seismic stratigraphy and sediment thickness of the Nansen Basin, Arctic Ocean2009In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 176, no 3, p. 805-821Article in journal (Refereed)
    Abstract [en]

    A Norwegian expedition to the western Nansen Basin, Yermak Plateau and the Hinlopen margin in 2001 acquired about 1100 km of 2-D multichannel seismic profiles and 50 wide-angle sonobuoy record sections. Analysis of these data establishes a regional seismic stratigraphic framework for the western Nansen Basin integrating previously published stratigraphic schemes. P-wave velocities and sediment thickness were derived within 7-8 per cent uncertainty from 2-D seismic ray tracing models of each sonobuoy section. Sediment thickness reaches 2 km in the studied area and increases towards the depocentre of the giant Franz-Victoria fan on the Barents-Kara continental margin. High-relief oceanic crystalline crust with 3.7 km s(-1) average near-top velocity is infilled by four seismic sediment units with typical velocities 2.4, 2.2, 2.0 and 1.8 km s(-1). A prominent regional seismic horizon between units 2 and 3 is tentatively correlated by basement onlap and sedimentation rates to a Miocene (similar to 10 Ma) palaeoceanographic event, possibly the opening of the Fram Strait. The youngest unit is correlated to prograding sequences on the margin and to the onset of major slope failure caused by intensified glacio-fluvial drainage and ice sheet erosion during Northern Hemisphere glaciations (2.6-0.01 Ma).

  • 23.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    A theoretical discussion on Vening Meinesz-Moritz inverse problem of isostasy2016In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 207, no 3, p. 1420-1431Article in journal (Refereed)
    Abstract [en]

    The Moho surface can be determined according to isostatic theories and the recent Vening Meinesz-Moritz (VMM) theory of isostasy has been successful for this purpose. In this paper, we will study this method from a theoretical prospective and try to find its connection to the Airy-Heiskanen (AH) and Vening Meinesz original theories. We develop Jeffrey’s inverse solution to isostasy according to the recent developments of the VMM method and compare both methods in similar situations. We will show that they are generalisations of the AH model in a global and continuous domain. In the VMM spherical harmonic solution for Moho depth, the mean Moho depth contributes only to the zero-degree term of the series, whilst in Jeffrey’s solution it contributes to all frequencies. We improve the VMM spherical harmonic series further so that the mean Moho can contribute to all frequencies of the solution. This modification makes the VMM global solution superior to the Jeffrey one, but in a global scale, the difference between both solutions is less than 3 km. Both solutions are asymptotically-convergent and we present two methods to obtain smooth solutions for Moho from them. 

  • 24.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Mathematics, Computer and Surveying Engineering.
    Elastic thickness determination based on Vening Meinesz-Moritz and flexural theories of isostasy2018In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 213, no 3, p. 1682-1692Article in journal (Refereed)
    Abstract [en]

    Elastic thickness (Te) is one of mechanical properties of the Earth's lithosphere. The lithosphere is assumed to be a thin elastic shell, which is bended under the topographic, bathymetric and sediment loads on. The flexure of this elastic shell depends on its thickness or Te. Those shells having larger Te flex less. In this paper, a forward computational method is presented based on the Vening Meinesz–Moritz (VMM) and flexural theories of isostasy. Two Moho flexure models are determined using these theories, considering effects of surface and subsurface loads. Different values are selected for Te in the flexural method to see by which one, the closest Moho flexure to that of the VMM is achieved. The effects of topographic/bathymetric, sediments and crustal crystalline masses, and laterally variable upper mantle density, Young's modulus and Poisson's ratio are considered in whole computational process. Our mathematical derivations are based on spherical harmonics, which can be used to estimate Te at any single point, meaning that there is no edge effect in the method. However, the Te map needs to be filtered to remove noise at some points. A median filter with a window size of 5° × 5° and overlap of 4° works well for this purpose. The method is applied to estimate Te over South America using the data of CRUST1.0 and a global gravity model.

  • 25.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    Local recovery of lithospheric stress tensor from GOCE gravitational tensor2017In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 209, no 1, p. 317-333Article in journal (Refereed)
    Abstract [en]

    The sub-lithospheric stress due to mantle convection can be computed from gravity data and propagated through the lithosphere by solving the boundary-value problem of elasticity for the Earth's lithosphere. In this case, a full tensor of stress can be computed at any point inside this elastic layer. Here, we present mathematical foundations for recovering such a tensor from gravitational tensor measured at satellite altitudes. The mathematical relations will be much simpler in this way than the case of using gravity data as no derivative of spherical harmonics or Legendre polynomials is involved in the expressions. Here, new relations between the spherical harmonic coefficients of the stress and gravitational tensor elements are presented. Thereafter integral equations are established from them to recover the elements of stress tensor from those of the gravitational tensor. The integrals have no closed-form kernels, but they are easy to invert and their spatial truncation errors are reducible. The integral equations are used to invert the real data of the gravity field and steady-state ocean circulation explorer (GOCE) mission, in November 2009, over the South American plate and its surroundings to recover the stress tensor at a depth of 35 km. The recovered stress fields are in good agreement with the tectonic and geological features of the area.

  • 26.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Mathematics, Computer and Surveying Engineering.
    On the approximations in formulation of the Vening Meinesz-Moritz theory of isostasy2017In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 210, no 1, p. 500-508Article in journal (Refereed)
    Abstract [en]

    Different approximations are used in Moho modelling based on isostatic theories. The well-known approximation is considering a plate shell model for isostatic equilibrium, which is an oversimplified assumption for the Earth’s crust. Considering a spherical shellmodel, as used in the Vening Meinesz-Moritz (VMM) theory, is a more realistic assumption, but it suffers from different types of mathematical approximations. In this paper, the idea is to investigate such approximations and present their magnitudes and locations all over the globe. Furthermore, we show that the mathematical model of Moho depth according to the VMM principle can be simplified to that of the plate shell model after four approximations. Linearisation of the binomial term involving the topographic/bathymetric heights is sufficient as long as their spherical harmonic expansion is limited to degree and order 180. The impact of the higher order terms is less than 2 km. The Taylor expansion of the binomial term involving the Moho depth (T) up to second order with the assumption of T-2 = TT0, T-0 is the mean compensation depth, improves this approximation further by up to 4 km over continents. This approximation has a significant role in Moho modelling over continents; otherwise, loss of frequency occurs in the Moho solution. On the other hand, the linear approximation performs better over oceans and considering higher order terms creates unrealistic frequencies reaching to a magnitude of 5 km in the Moho solution. Involving gravity data according to the VMM principle influences the Moho depth significantly up to 15 km in some areas.

  • 27.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Mathematics, Computer and Surveying Engineering.
    Steinberger, Bernhard
    Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, Potsdam, Germany & Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Postboks 1028 Blindern, Oslo, Norway.
    Tenzer, Robert
    Hong Kong Polytechnic University, Department of Land Surveying and Geo-Informatics, 11 Yuk Chai Rd, Hung Hom, Hong Kong.
    Tassara, Andrés
    Universidad de Concepción, Departamento de Ciencias de la Tierra, Facultad de Ciencias Químicas, Victor Lamas 1290, Concepción, Chile.
    Comparison of gravimetric and mantle flow solutions for sub-lithopsheric stress modeling and their combination2018In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 213, no 2, p. 1013-1028Article in journal (Refereed)
    Abstract [en]

    Based on Hager and O’Connell’s solution to mantle flow equations, the stresses induced by mantle convection are determined using the density and viscosity structure in addition to topographic data and a plate velocity model. The solution to mantle flow equations requires the knowledge of mantle properties that are typically retrieved from seismic information. Large parts of the world are, however, not yet covered sufficiently by seismic surveys. An alternative method of modeling the stress field was introduced by Runcorn. He formulated a direct relation between the stress field and gravity data, while adopting several assumptions, particularly disregarding the toroidal mantle flow component and mantle viscosity variations. A possible way to overcome theoretical deficiencies of Runcorn’s theory as well as some practical limitations of applying Hager and O’Connell’s theory (in the absence of seismic data) is to combine these two methods. In this study, we apply a least-squares analysis to combine these two methods based on the gravity data inversion constraint on mantle flow equations. In particular, we use vertical gravity gradients from the Gravity field and steady state Ocean Circulation Explorer that are corrected for the gravitational contribution of crustal density heterogeneities prior to applying a localized gravity-gradient inversion. This gravitational contribution is estimated based on combining the Vening Meinesz-Moritz and flexural isostatic theories. Moreover, we treat the non-isostatic effect implicitly by applying a band-limited kernel of the integral equation during the inversion. In numerical studies of modeling, the stress field within the South American continental lithosphere we compare the results obtained after applying Runcorn and Hager and O’Connell’s methods as well as their combination. The results show that, according to Hager and O’Connell’s (mantle flow) solution, the maximum stress intensity is inferred under the northern Andes. Additional large stress anomalies are detected along the central and southern Andes, while stresses under most of old, stable cratonic formations aremuch less pronounced or absent. A prevailing stress-vector orientation realistically resembles a convergent mantle flow and downward currents under continental basins that separate Andean Orogeny from the Amazonian Shield and adjacent cratons. Runcorn’s (gravimetric) solution, on the other hand, reflects a tectonic response of the lithosphere to mantle flow, with the maximum stress intensity detected along the subduction zone between the Nazca and Altiplano plates and along the convergent tectonic margin between the Altiplano and South American plates. The results also reveal a very close agreement between the results obtained from the combined and Hager and O’Connell’s solutions. © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society.

  • 28.
    Fuchs, Lukas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Schmeling, H.
    Koyi, Hemin
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Numerical models of salt diapir formation by down-building: the role of sedimentation rate, viscosity contrast, initial amplitude and wavelength2011In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 186, no 2, p. 390-400Article in journal (Refereed)
    Abstract [en]

    Formation of salt diapirs has been described to be due to upbuilding (i. e. Rayleigh-Taylor like instability of salt diapirs piercing through a denser sedimentary overburden) or syndepositional down-building process (i. e. the top of the salt diapir remains at the surface all the time). Here we systematically analyse this second end-member mechanism by numerical modelling. Four parameters are varied: sedimentation rate nu(sed), salt viscosity eta(salt), amplitude delta of the initial perturbation of the sedimentation layer and thewavenumber k of this perturbation. The shape of the resulting salt diapirs strongly depends on these parameters. Small diapirs with subvertical side walls are found for small values of nu(sed) and eta(salt) or large values of delta, whereas taller diapirs with pronounced narrow stems build for larges values of nu(sed) and eta(salt) or small values of delta. Two domains are identified in the four-parameter space, which separates successful down-building models from non-successful models. By applying a simple channel flow law, the domain boundary can be described by the non-dimensional law nu(sedcrit)' = C(1)1/2 delta(0)'rho(sed)'k'(2/)k'(2) + C2, where rho(sed)' is the sediment density scaled by the density contrast Delta rho between sediment and salt, the wavelength is scaled by the salt layer thickness h(salt), and velocity is scaled by eta(salt)/(h(salt)(2)Delta rho g), where eta(salt) is the salt viscosity and g is the gravitational acceleration. From the numerical models, the constants C(1) and C(2) are determined as 0.0283 and 0.1171, respectively.

  • 29.
    Fuchs, Lukas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Schmeling, Harro
    Goethe-University, Institute of Geoscience, Frankfurt am Main, Germany.
    A new numerical method to calculate inhomogeneous and time dependent large deformations of two-dimensional geodynamic flows with application to diapirism2013In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 194, no 2, p. 623-639Article in journal (Refereed)
    Abstract [en]

    A key to understand many geodynamic processes is studying the associated large deformation fields. Finite deformation can be measured in the field by using geological strain markers giving the logarithmic strain f = log 10(R), where R is the ellipticity of the strain ellipse. It has been challenging to accurately quantify finite deformation of geodynamic models for inhomogeneous and time-dependent large deformation cases. We present a new formulation invoking a 2-D marker-in-cell approach. Mathematically, one can describe finite deformation by a coordinate transformation to a Lagrangian reference frame. For a known velocity field the deformation gradient tensor, F, can be calculated by integrating the differential equation DtFij = LikFkj, where L is the velocity gradient tensor and Dt the Lagrangian derivative. The tensor F contains all information about the minor and major semi-half axes and orientation of the strain ellipse and the rotation. To integrate the equation centrally in time and space along a particle's path, we use the numerical 2-D finite difference code FDCON in combination with a marker-in-cell approach. For a sufficiently high marker density we can accurately calculate F for any 2-D inhomogeneous and time-dependent creeping flow at any point for a deformation f up to 4. Comparison between the analytical and numerical solution for the finite deformation within a Poiseuille–Couette flow shows an error of less than 2 per cent for a deformation up to f = 1.7. Moreover, we determine the finite deformation and strain partitioning within Rayleigh–Taylor instabilities (RTIs) of different viscosity and layer thickness ratios. These models provide a finite strain complement to the RTI benchmark of van Keken et al. Large finite deformation of up to f = 4 accumulates in RTIs within the stem and near the compositional boundaries. Distinction between different stages of diapirism shows a strong correlation between a maximum occurring deformation of f = 1, 3 and 4, and the early, intermediate and late stages of diapirism, respectively. Furthermore, we find that the overall strain of a RTI is concentrated in the less viscous regions. Thus, spatial distributions and magnitudes of finite deformation may be used to identify stages and viscosity ratios of natural cases.

  • 30.
    Fuchs, Lukas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Schmeling, Harro
    Goethe-University, Institute of Geoscience, Frankfurt am Main, Germany.
    Koyi, Hemin
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Thermo-mechanical modelling of progressive deformation and seismic anisotropy at the lithosphere-asthenosphere boundaryIn: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246XArticle in journal (Other academic)
    Abstract [en]

       Deformation at the lithosphere-asthenosphere boundary is strongly governed by its effective viscosity, which depends on temperature, strain rate, and grain size. Moreover, deformation can cause lattice preferred orientation resulting in seismic anisotropy and shear wave splitting. We used a 1D model approach to calculate shear strain and characteristic depths for an oceanic plate as a function of age. We assume a composite rheology (dislocation and diffusion creep) in combination with a half-space cooling model temperature field for constant and variable thermal parameters, and different potential mantle temperatures. Systematically, sensitivity of characteristic depths, deformation pattern, and seismic delay times δt on temperature, plate velocity, steady state grain size, and rheology have been analyzed. Model results show that the characteristic depths are only affected by local variations in the temperature field or a shift in the dominant deformation mechanism. The other parameters, however, do strongly affect the maximum total shear strain. Due to a continuous simple shear of the upper mantle governed by the motion of the plate, anisotropy, thickness of the anisotropic layer, and δt reach relatively large values in comparison to observed data. However, a small amount of dislocation creep (25-40 %), due to a modified rheology or small grain sizes, leads to a significantly thinner anisotropic layer. As a result, δt is reduced by 50 % or more. The change of the characteristics of the anisotropic layer and degree of its anisotropy may reflect and be of significance for the viscous (de)coupling between the lithosphere and asthenosphere.

  • 31.
    Hagos, Lijam
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Shomali, Hossein
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Re-evaluation of focal depths and source mechanisms of selected earthquakes in the Afar depression2006In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 167, no 1, p. 297-308Article in journal (Refereed)
    Abstract [en]

    We present a stepwise inversion procedure to assess the focal depth and model earthquake source complexity of seven moderate-sized earthquakes (6.2 > M-w > 5.1) that occurred in the Afar depression and the surrounding region. The Afar depression is a region of highly extended and intruded lithosphere, and zones of incipient seafloor spreading. A time-domain inversion of full moment tensor was performed to model direct P and SH waves of teleseismic data. Waveform inversion of the selected events estimated focal depths in the range of 17-22 km, deeper than previously published results. This suggests that the brittle-ductile transition zone beneath parts of the Afar depression extends more than 22 km. The effect of near-source velocity structure on the moment tensor elements was also investigated and was found to respond little to the models considered. Synthetic tests indicate that the size of the estimated, non-physical, non-isotropic source component is rather sensitive to incorrect depth estimation. The dominant double couple part of the moment tensor solutions for most of the events indicates that their occurrence is mainly due to shearing. Parameters associated with source directivity (rupture velocity and azimuth) were also investigated. Re-evaluation of the analysed events shows predominantly normal faulting consistent with the relative plate motions in the region.

  • 32. Heidari, Reza
    et al.
    Shomali, Zaher Hossein
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Ghayamghamian, Mohammad Reza
    Magnitude-scaling relations using period parameters tau(c) and tau(max)(p), for Tehran region, Iran2013In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 192, no 1, p. 275-284Article in journal (Refereed)
    Abstract [en]

    In this study, the first step towards establishing an onsite earthquake early warning system (EWS) in the Tehran region is presented. The system uses the period parameters tau(max)(p) and tau(c) from the first 3 s of the vertical and horizontal components of a P wave, separately and combined. Various regression relations between the magnitude and period parameters were determined for different seismic networks operating in the study area. The data set used in this study contains small ground motions including 194 events with magnitudes between M-L 2.5 and 4.6 located within approximately 80 km from the epicentre in the Tehran region. The SDs of the magnitude-scaling relations for all the component categories (vertical, horizontal and total components) based on the tau(max)(p) and tau(c) approaches were estimated to be on the order of +/- 1.0 and +/- 1.1 unit of magnitude, respectively. These relations were determined from the small magnitude range of the velocity records (M-L 2.5-4.6) as input seismograms. Additional tests were conducted to verify the reliability and robustness of the determined magnitude-scaling relations using acceleration records from the 2002 June 22, M-L 6.5 Changureh-Avaj earthquake; 2004 May 28, M-L 6.1 Firoozabad-Koojour earthquake; 2009 October 17, M-L 3.9 Shahre-Rey earthquake and 2011 February 20, M-L 4.1 Sharif-Abad earthquake; the first two events (Changureh-Avaj and Firoozabad-Koojour) occurred outside the study area. Among the various regression scaling relations obtained, the estimated magnitude based on the tau(max)(p) approach using the vertical components yielded the most stable and reliable results of 6.4 (+/- 0.4), 5.9 (+/- 0.4), 3.3 (+/- 0.5) and 3.8 (+/- 0.3) for the Changureh-Avaj, Firoozabad-Koojour, Shahre-Rey and Sharif-Abad earthquakes, respectively. The magnitudes estimated using the tau(c) method exhibited more scatter with higher SDs than those using the vertical components using the tau(max)(p) approach. Our results also indicate that using the horizontal components produces larger SDs, which may be attributed to the larger site effects; however, the horizontal components can be used as auxiliary available data to provide more constrained information for a multilevels pilot alarm system and to reduce the number of missed or false alarms. The main uncertainties in the proposed magnitude-scaling relations result from the absence of any large earthquakes and poor station distributions in the study area. However, the results presented in this study can be used as a pilot onsite earthquake EWS in the Tehran region.

  • 33.
    Hensch, Martin
    et al.
    Nordic Volcanological Center, University of Iceland.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Árnadóttir, Thóra
    Nordic Volcanological Center, University of Iceland.
    Brandsdottir, Bryndis
    Nordic Volcanological Center, University of Iceland.
    Temporal stress changes associated with the 2008 May 29 Mw 6 earthquake doublet in the western South Iceland Seismic Zone2016In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 204, no 1, p. 544-554Article in journal (Refereed)
    Abstract [en]

    On 2008 May 29, two magnitude Mw ~ 6 earthquakes occurred on two adjacent N-S faults in the western South Iceland Seismic Zone. The first main shock was followed approximately 3 s later by the rupture on a parallel fault, about 5 km to the west. An intense aftershock sequence was mostly confined to the western fault and an E-W aligned zone, extending west of the main shock region into the Reykjanes oblique rift. In this study, a total of 325 well-constrained focal mechanisms were obtained using data from the permanent Icelandic SIL seismic network and a temporary network promptly installed in the source region following the main shocks, which allowed a high-resolution stress inversion in short time intervals during the aftershock period. More than 800 additional focal mechanisms for the time period 2001-2009, obtained from the permanent SIL network, were analysed to study stress changes associated with the main shocks. Results reveal a coseismic counter-clockwise rotation of the maximum horizontal stress of 11 +/- 10 degrees ( 95 per cent confidence level) in the main rupture region. From previous fault models obtained by inversion of geodetic data, we estimate a stress drop of about half of the background shear stress on the western fault. With a stress drop of 8-10 MPa, the pre-event shear stress is estimated to 16-20 MPa. The apparent weakness of the western fault may be caused by fault properties, pore fluid pressure and the vicinity of the fault to the western rift zone, but may also be due to the dynamic stress increase on the western fault by the rupture on the eastern fault. Further, a coseismic change of the stress regime-from normal faulting to strike-slip faulting-was observed at the northern end of the western fault. This change could be caused by stress heterogeneities, but may also be due to a southward shift in the location of the aftershocks as compared to prior events.

  • 34.
    Hieronymus, Christoph
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Goes, S.
    Complex cratonic seismic structure from thermal models of the lithosphere: effects of variations in deep radiogenic heating2010In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 180, no 3, p. 999-1012Article in journal (Refereed)
    Abstract [en]

    Cratons are the long-term tectonically stable cores of the continents. Despite their thermal stability they display substantial seismic complexity with lateral and vertical lithospheric anomalies of up to several percent in both V(S) and V(P). Although some of these anomalies have been correlated with compositional variations, others are too large to be explained with any common mantle lithosphere compositions ranging from fertile peridotites to highly melt-depleted dunites, under the assumption that thermal perturbations are negligible. To test whether temperature anomalies could contribute to seismic complexity, we performed a set of 2-D thermal calculations for a range of cratonic tectonic models and converted them into seismic structure, accounting for variations in phase and elastic and anelastic response to pressure and temperature. With the long thermal equilibration time in cratonic settings, even relatively mild variations in concentrations of radioactive elements can leave long-lasting lithospheric thermal anomalies of 100-300 degrees C. Concentrations of radioactive elements decrease with increasing melt depletion ( or decreasing metasomatic refertilization), resulting in lower temperatures and increased seismic velocities. This thermal seismic effect enhances the intrinsic velocity-increasing compositional seismic signature of melt depletion. The joint thermochemical effects can leave cratonic seismic anomalies of up to 3-4.5 per cent in V(S) and up to 2.5-4 per cent in V(P), with gradients sometimes as sharp as a few kilometre in width. Thus the variations in major and minor element mantle lithosphere composition commonly seen in mantle samples can account for much of the variability in imaged seismic structure of cratonic lithosphere.

  • 35.
    Hieronymus, Christoph
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Goes, Saskia
    Department of Earth Science and Engineering, Imperial College London, London, UK.
    Complex cratonic seismic structure from thermal models of the lithosphere: effects of variations in deep radiogenic heating2010In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 180, no 3, p. 999-1012Article in journal (Refereed)
    Abstract [en]

    Cratons are the long-term tectonically stable cores of the continents. Despite their thermal stability they display substantial seismic complexity with lateral and vertical lithospheric anomalies of up to several percent in both  VS and  VP . Although some of these anomalies have been correlated with compositional variations, others are too large to be explained with any common mantle lithosphere compositions ranging from fertile peridotites to highly melt-depleted dunites, under the assumption that thermal perturbations are negligible. To test whether temperature anomalies could contribute to seismic complexity, we performed a set of 2-D thermal calculations for a range of cratonic tectonic models and converted them into seismic structure, accounting for variations in phase and elastic and anelastic response to pressure and temperature. With the long thermal equilibration time in cratonic settings, even relatively mild variations in concentrations of radioactive elements can leave long-lasting lithospheric thermal anomalies of 100–300 °C. Concentrations of radioactive elements decrease with increasing melt depletion (or decreasing metasomatic refertilization), resulting in lower temperatures and increased seismic velocities. This thermal seismic effect enhances the intrinsic velocity-increasing compositional seismic signature of melt depletion. The joint thermochemical effects can leave cratonic seismic anomalies of up to 3–4.5 per cent in  VS and up to 2.5–4 per cent in  VP , with gradients sometimes as sharp as a few kilometre in width. Thus the variations in major and minor element mantle lithosphere composition commonly seen in mantle samples can account for much of the variability in imaged seismic structure of cratonic lithosphere.

  • 36.
    Huang, Fei
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Han, Li
    CNOOC Res Inst, Overseas Evaluat Ctr, Taiyanggong South St 6, Beijing 100028, Peoples R China.
    Kempka, Thomas
    GFZ German Res Ctr Geosci, Helmholtz Ctr Potsdam, D-14473 Potsdam, Germany.
    Lüth, Stefan
    GFZ German Res Ctr Geosci, Helmholtz Ctr Potsdam, D-14473 Potsdam, Germany.
    Zhang, Fengjiao
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Jilin Univ, Coll Geoexplorat Sci & Technol, Xi Min Zhu St 938, Changchun 130026, Peoples R China.
    Quantitative evaluation of thin-layer thickness and CO2 mass utilizing seismic complex decomposition at the Ketzin CO2 storage site, Germany2016In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 207, no 1, p. 160-173Article in journal (Refereed)
    Abstract [en]

    Determining thin layer thickness is very important for reservoir characterization and CO2 quantification. Given its high time-frequency resolution and robustness, the complex spectral decomposition method was applied on time-lapse 3D seismic data from the Ketzin pilot site for CO2 storage to evaluate the frequency-dependent characteristics of thin layers at the injection level. Higher temporal resolution and more stratigraphic details are seen in the all-frequency and monochromatic reflectivity amplitude sections obtained by complex spectral decomposition compared to the stacked sections. The mapped geologic discontinuities within the reservoir are consistent with the preferred orientation of CO2 propagation. Tuning frequency mapping shows the thicknesses of the reservoir sandstone and gaseous CO2 is consistent with the measured thickness of the sandstone unit from well logging. An attempt to discriminate between pressure effects and CO2 saturation using the extracted tuning frequency indicates that CO2 saturation is the main contributor to the amplitude anomaly at the Ketzin site. On the basis of determined thickness of gaseous CO2 in the reservoir, quantitative analysis of the amount of CO2 was performed and shows a discrepancy between the injected and calculated CO2 mass. This may be explained by several uncertainties, like structural reservoir heterogeneity, a limited understanding of the complex subsurface conditions, error of determined tuning frequency, the presence of ambient noise and ongoing CO2 dissolution.

  • 37.
    Hübert, Juliane
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    García Juanatey, María de los Ángeles
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggavson, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    The upper crustal 3-D resistivity structure of the Kristineberg area, Skellefte district, northern Sweden revealed by magnetotelluric data2013In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 192, no 2, p. 500-513Article in journal (Refereed)
    Abstract [en]

    A 3-D model of the crustal electrical resistivity was constructed from the inversion of magnetotelluric data in the Kristineberg area, Skellefte district, the location of one of Sweden's most successful mining activities. Forward modelling of vertical magnetic transfer data supports our model which was derived from the magnetotelluric impedance only. The dominant features in the 3-D model are the strong conductors at various depth levels and resistive bodies of variable thickness occurring in the shallower subsurface. The deepest conductor, previously associated with the Skellefte crustal conductivity anomaly, is imaged in the southern part of the area as a north-dipping feature starting at similar to 4 km depth. Several shallow conductors are attributed to graphite in the black shales defining the contact between the metasedimentary rocks and the underlying metavolcanic rocks. Furthermore, an elongated intermediate depth conductor is possibly associated with alteration zones in the metavolcanic rocks that host the ore occurrences. The most prominent crustal resistors occur in the southern and northern part of the area, where their lateral extent on the surface coincides with the late-orogenic Revsund type intrusions. To the east, a resistive feature can be correlated to the early-orogenic Viterliden intrusion. The 3-D model is compared with two previous 2-D inversion models along two perpendicular profiles. The main electrical features are confirmed with the new model and previous uncertainties regarding 3-D effects, caused by off-profile conductors, can be better assessed in 3-D, although the resolution is lower due to a coarser model discretization. The comparison with seismic sections along two north-south profiles reveals structural correspondence between electrical features, zones of different reflectivity and geological units.

  • 38. Ivandic, Monika
    et al.
    Grevemeyer, Ingo
    Bialas, Joerg
    Petersen, C. Joerg
    Serpentinization in the trench-outer rise region offshore of Nicaragua: constraints from seismic refraction and wide angle data2010In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246XArticle in journal (Refereed)
  • 39.
    Jokat, Wilfried
    et al.
    Ayred Wegener Institute for Polar and Marine Research.
    Weigelt, Estella
    Ayred Wegener Institute for Polar and Marine Research.
    Kristoffersen, Yngve
    University of Bergen, Institut for Solid Earth Physics.
    Rasmussen, Thorkild Maack
    University of Uppsala.
    Schöne, Tilo
    University of Uppsala.
    New geophysical results from the south‐western Eurasian Basin (Morris Jesup Rise, Gakkel Ridge, Yermak Plateau) and the Fram Strait1995In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 123, no 2, p. 601-610Article in journal (Refereed)
  • 40.
    Jokat, Wilfried
    et al.
    Ayred Wegener Institute for Polar and Marine Research.
    Weigelt, Estella
    Ayred Wegener Institute for Polar and Marine Research.
    Kristoffersen, Yngve
    University of Bergen, Institut for Solid Earth Physics.
    Rasmussen, Thorkild Maack
    University of Uppsala.
    Schöne, Tilo
    University of Uppsala.
    New insights into the evolution of the Lomonosov Ridge and the Eurasian Basin1995In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 122, no 2, p. 378--392Article in journal (Refereed)
  • 41. Jordan, T.A.
    et al.
    Neale, R.F.
    Leat, P.T.
    Vaughan, A.P.M.
    Flowerdew, M.J,
    Riley, T.R.
    Whitehouse, Martin J.
    Swedish Museum of Natural History, Department of Geology.
    Ferraccioli, F.
    Structure and evolution of Cenozoic arc magmatism on the Antarctic Peninsula; a high resolution aeromagnetic perspective2014In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 198, p. 1758-1774Article in journal (Refereed)
  • 42.
    Joud, Mehdi S. Shafiei
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Bagherbandi, Mohammad
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning. University of Gävle, Sweden.
    Use of GRACE data to detect the present land uplift rate in Fennoscandia2017In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 209, no 2, p. 909-922Article in journal (Refereed)
    Abstract [en]

    After more than 13 yr of GRACE monthly data, the determined secular trend of gravity field variation can be used to study the regions of glacial isostatic adjustment (GIA). Here we focus on Fennoscandia where long-term terrestrial and high-quality GPS data are available, and we study the monthly GRACE data from three analysis centres. We present a new approximate formula to convert the secular trend of the GRACE gravity change to the land uplift rate without making assumptions of the ice load history. The question is whether the GRACEderived land uplift rate by our method is related to GIA. A suitable post-processing method for the GRACE data is selected based on weighted RMS differences with the GPS data. The study reveals that none of the assumed periodic changes of the GRACE gravity field is significant in the estimation of the secular trend, and they can, therefore, be neglected. Finally, the GRACEderived land uplift rates are obtained using the selected post-processing method, and they are compared with GPS land uplift rate data. The GPS stations with significant differences were marked using a statistical significance test. The smallest rms difference (1.0 mm a-1) was obtained by using GRACE data from the University of Texas.

  • 43.
    Joud S., Mehdi
    et al.
    KTH.
    Sjöberg, Lars E.
    KTH.
    Bagherbandi, Mohammad
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Industrial Development, IT and Land Management, Land management, GIS. 1Division of Geodesy and Satellite Positioning, Royal Institute of Technology (KTH), Stockholm, Sweden.
    Use of GRACE Data to Detect the Present Land Uplift Rate in Fennoscandia2017In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 209, no 2, p. 909-922Article in journal (Refereed)
    Abstract [en]

    After more than 13 years of GRACE monthly data, the determined secular trend of gravity field variation can be used to study the regions of glacial isostatic adjustment (GIA). Here we focus on Fennoscandia where long-term terrestrial and high-quality GPS data are available, and we study the monthly GRACE data from three analysis centres. We present a new approximate formula to convert the secular trend of the GRACE gravity change to the land uplift rate without making assumptions of the ice load history. The question is whether the GRACE-derived land uplift rate by our method is related to GIA. A suitable post-processing method for the GRACE data is selected based on weighted RMS differences with the GPS data. The study reveals that none of the assumed periodic changes of the GRACE gravity field is significant in the estimation of the secular trend, and they can, therefore, be neglected. Finally, the GRACE-derived land uplift rates are obtained using the selected post-processing method, and they are compared with GPS land uplift rate data. The GPS stations with significant differences were marked using a statistical significance test. The smallest RMS difference (1.0 mm/a) was obtained by using GRACE data from the University of Texas.

  • 44.
    Ju, Ma
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering. Department of Mining Engineering, North China University of Science and Technology, Tangshan.
    Dineva, Savka
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering.
    Cesca, Simone
    GFZ German Research Centre for Geosciences Potsdam, Germany.
    Heimann, Sebastian
    GFZ German Research Centre for Geosciences Potsdam, Germany.
    Moment tensor inversion with three-dimensional sensor configuration of mining induced seismicity (Kiruna mine, Sweden)2018In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 213, no 3, p. 2147-2160Article in journal (Refereed)
    Abstract [en]

    Mining induced seismicity is an undesired consequence of mining operations, which poses significant hazard to miners and infrastructures and requires an accurate analysis of the rupture process. Seismic moment tensors of mining-induced events help to understand the nature of mining-induced seismicity by providing information about the relationship between the mining, stress redistribution and instabilities in the rock mass. In this work, we adapt and test a waveform-based inversion method on high frequency data recorded by a dense underground seismic system in one of the largest underground mines in the world (Kiruna mine, Sweden). A stable algorithm for moment tensor inversion for comparatively small mining induced earthquakes, resolving both the double-couple and full moment tensor with high frequency data, is very challenging. Moreover, the application to underground mining system requires accounting for the 3-D geometry of the monitoring system. We construct a Green's function database using a homogeneous velocity model, but assuming a 3-D distribution of potential sources and receivers. We first perform a set of moment tensor inversions using synthetic data to test the effects of different factors on moment tensor inversion stability and source parameters accuracy, including the network spatial coverage, the number of sensors and the signal-tonoise ratio. The influence of the accuracy of the input source parameters on the inversion results is also tested. Those tests show that an accurate selection of the inversion parameters allows resolving the moment tensor also in the presence of realistic seismic noise conditions. Finally, the moment tensor inversion methodology is applied to eight events chosen from mining block #33/34 at Kiruna mine. Source parameters including scalar moment, magnitude, double-couple, compensated linear vector dipole and isotropic contributions as well as the strike, dip and rake configurations of the double-couple term were obtained. The orientations of the nodal planes of the double-couple component in most cases vary from NNW to NNE with a dip along the ore body or in the opposite direction.

  • 45.
    Juhlin, C.
    et al.
    Uppsala University.
    Elming, Sten-åke
    Luleå tekniska universitet, Geovetenskap och miljöteknik.
    Mellqvist, C.
    SGAB Analytica.
    Öhlander, Björn
    Luleå tekniska universitet, Geovetenskap och miljöteknik.
    Weihed, Pär
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences. Luleå tekniska universitet.
    Wikström, A.
    Geological Survey of Sweden.
    Crustal reflectivity near the Archean-Proterozoic boundary in northern Sweden and implications for the tectonic evolution of the area2002In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 150, no 1, p. 180-197Article in journal (Refereed)
    Abstract [en]

    Sm–Nd isotope ratios of 1.9–1.8 Ga granitoids delineate the Archaean–Proterozoic boundary in northern Sweden, an important feature in the Fennoscandian Shield. The boundary strikes approximately WNW–ESE and is defined as a c. 20 km wide zone with juvenile Palaeoproterozoic rocks to the SSW and Archaean and Proterozoic rocks, derived to a large extent from Archaean sources, to the NNE. It therefore constitutes the strongly reworked margin of the old Archaean craton. Extrapolation of the boundary offshore into the Bothnian Bay and correlation with the marine reflection seismic BABEL Lines 2 and 3/4 indicates that the boundary dips to the south-southwest, consistent with interpretation of the Sm–Nd data. In order to tie the BABEL results with onshore surface geology and obtain detailed images of the uppermost crust a short (30 km of subsurface coverage) pilot profile was acquired in the Luleå area of northern Sweden during August 1999. The profile consisted of a high-resolution shallow component (1 kg shots) and a lower-resolution deep component (12 kg shots). Both components image most of the reflective crust, with the deep component providing a better image below 10 s. Comparison of signal penetration curves with data acquired over the Trans-Scandinavian Igneous Belt (a large batholith) indicate the transparent nature of the crust there to be caused by geological factors, not acquisition parameters. Lower crustal reflectivity patterns on the Luleå test profile are similar to those observed on the BABEL lines, suggesting the same lower crust onshore as offshore. Interpreted Archaean reflective upper crust in the NE extends below more transparent Proterozoic crust in the SW. This transparent crust contains a number of high-amplitude reflectors that may represent shear zones and/or mafic rock within granite intrusions. A marked boundary in the magnetic field in the SW has been interpreted as being the result of a gently west-dipping contact zone between meta-sediments and felsic volcanic rocks, however, the seismic data indicate a near-vertical structure in this area. By correlating the onshore and offshore seismic data we have better defined the location of the Archaean–Proterozoic boundary on the BABEL profiles. Our new interpretation of the crustal structure along the northern part of the BABEL Line 2 shows a more bi-vergent geometry than previous interpretations. Comparison of the re-interpreted crustal structure in northern Sweden with that found in the Middle Urals shows several similarities, in particular the accretion of a series of arcs to a stable craton. Based on this similarity and geological data, we deduce that a continental arc accreted to the southwestern margin of the Archaean craton at c. 1.87 Ga. Shortly thereafter, the Skellefte island arc underthrust the continental arc owing to a collision further to the southwest resulting in the bi-vergent crustal structure observed today.

  • 46.
    Juhlin, C.
    et al.
    Uppsala University.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Mellqvist, C.
    SGAB Analytica.
    Öhlander, Björn
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Weihed, Pär
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Wikström, A.
    Geological Survey of Sweden.
    Crustal reflectivity near the Archean-Proterozoic boundary in northern Sweden and implications for the tectonic evolution of the area2002In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 150, no 1, p. 180-197Article in journal (Refereed)
    Abstract [en]

    Sm–Nd isotope ratios of 1.9–1.8 Ga granitoids delineate the Archaean–Proterozoic boundary in northern Sweden, an important feature in the Fennoscandian Shield. The boundary strikes approximately WNW–ESE and is defined as a c. 20 km wide zone with juvenile Palaeoproterozoic rocks to the SSW and Archaean and Proterozoic rocks, derived to a large extent from Archaean sources, to the NNE. It therefore constitutes the strongly reworked margin of the old Archaean craton. Extrapolation of the boundary offshore into the Bothnian Bay and correlation with the marine reflection seismic BABEL Lines 2 and 3/4 indicates that the boundary dips to the south-southwest, consistent with interpretation of the Sm–Nd data. In order to tie the BABEL results with onshore surface geology and obtain detailed images of the uppermost crust a short (30 km of subsurface coverage) pilot profile was acquired in the Luleå area of northern Sweden during August 1999. The profile consisted of a high-resolution shallow component (1 kg shots) and a lower-resolution deep component (12 kg shots). Both components image most of the reflective crust, with the deep component providing a better image below 10 s. Comparison of signal penetration curves with data acquired over the Trans-Scandinavian Igneous Belt (a large batholith) indicate the transparent nature of the crust there to be caused by geological factors, not acquisition parameters. Lower crustal reflectivity patterns on the Luleå test profile are similar to those observed on the BABEL lines, suggesting the same lower crust onshore as offshore. Interpreted Archaean reflective upper crust in the NE extends below more transparent Proterozoic crust in the SW. This transparent crust contains a number of high-amplitude reflectors that may represent shear zones and/or mafic rock within granite intrusions. A marked boundary in the magnetic field in the SW has been interpreted as being the result of a gently west-dipping contact zone between meta-sediments and felsic volcanic rocks, however, the seismic data indicate a near-vertical structure in this area. By correlating the onshore and offshore seismic data we have better defined the location of the Archaean–Proterozoic boundary on the BABEL profiles. Our new interpretation of the crustal structure along the northern part of the BABEL Line 2 shows a more bi-vergent geometry than previous interpretations. Comparison of the re-interpreted crustal structure in northern Sweden with that found in the Middle Urals shows several similarities, in particular the accretion of a series of arcs to a stable craton. Based on this similarity and geological data, we deduce that a continental arc accreted to the southwestern margin of the Archaean craton at c. 1.87 Ga. Shortly thereafter, the Skellefte island arc underthrust the continental arc owing to a collision further to the southwest resulting in the bi-vergent crustal structure observed today.

  • 47.
    Kalscheuer, Thomas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Blake, Sarah
    Podgorski, Joel E.
    Wagner, Frederic
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Green, Alan G.
    Maurer, Hansruedi
    Jones, Alan G.
    Muller, Mark
    Ntibinyane, Ongkopotse
    Tshoso, Gomotsang
    Joint inversions of three types of electromagnetic data explicitly constrained by seismic observations: results from the central Okavango Delta, Botswana2015In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 202, no 3, p. 1429-1452Article in journal (Refereed)
    Abstract [en]

    The Okavango Delta of northern Botswana is one of the world’s largest inland deltas or megafans. To obtain information on the character of sediments and basement depths, audio-magnetotelluric (AMT), controlled-source audiomagnetotelluric (CSAMT) and central-loop transient electromagnetic (TEM) data were collected on the largest island within the delta. The data were inverted individually and jointly for 1-D models of electric resistivity. Distortion effects in the AMT and CSAMT data were accounted for by including galvanic distortion tensors as free parameters in the inversions. By employing Marquardt–Levenberg inversion, we found that a 3-layer model comprising a resistive layer overlying sequentially a conductive layer and a deeper resistive layer was sufficient to explain all of the electromagnetic data. However, the top of the basal resistive layer from electromagnetic-only inversions was much shallower than the well-determined basement depth observed in high-quality seismic reflection images and seismic refraction velocity tomograms. To resolve this discrepancy, we jointly inverted the electromagnetic data for 4-layer models by including seismic depths to an interface between sedimentary units and to basement as explicit a priori constraints. We have also estimated the interconnected porosities, clay contents and pore-fluid resistivities of the sedimentary units from their electrical resistivities and seismic P-wave velocities using appropriate petrophysical models. In the interpretation of our preferred model, a shallow ∼40 m thick freshwater sandy aquifer with 85–100 Ohmm resistivity, 10–32 per cent interconnected porosity and <13 per cent clay content overlies a 105–115 m thick conductive sequence of clay and intercalated salt-water-saturated sands with 15–20 Ohmm total resistivity, 1−27 per cent interconnected porosity and 15–60 per cent clay content. A third ∼60 m thick sandy layer with 40–50 Ohmm resistivity, 10–33 per cent interconnected porosity and <15 per cent clay content is underlain by the basement with 3200–4000 Ohmm total resistivity. According to an interpretation of helicopter TEM data that cover the entire Okavango Delta and borehole logs, the second and third layers may represent lacustrine sediments from Paleo Lake Makgadikgadi and a moderately resistive freshwater aquifer comprising sediments of the recently proposed Paleo Okavango Megafan, respectively.

  • 48.
    Kalscheuer, Thomas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Garcia Juanatey, Maria de los Angeles
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Meqbel, Naser
    College of Oceanic and Atmospheric Sciences, Oregon State University, USA.
    Pedersen, Laust B.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Non-linear model error and resolution properties from two-dimensional single and joint inversions of direct current resistivity and radiomagnetotelluric data2010In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 182, no 3, p. 1174-1188Article in journal (Refereed)
    Abstract [en]

    P>For the first time, a comparative analysis of the resolution and variance properties of 2-D models of electrical resistivity derived from single and joint inversions of dc resistivity (DCR) and radiomagnetotelluric (RMT) measurements is presented. DCR and RMT data are inverted with a smoothness-constrained 2-D scheme. Model resolution, model variance and data resolution analyses are performed both with a classical linearized scheme that employs the smoothness-constrained generalized inverse and a non-linear truncated singular value decomposition (TSVD). In the latter method, the model regularization used in the inversion is avoided and non-linear semi-axes give an approximate description of the non-linear confidence surface in the directions of the model eigenvectors. Hence, this method analyses the constraints that can be provided by the data. Model error estimates are checked against improved and independent estimates of model variability from most-squares inversions. For single and joint inverse models of synthetic data sets, the smoothness-constrained scheme suggests relatively small model errors (typically up to 30 to 40 per cent) and resolving kernels that are spread over several cells in the vicinity of the investigated cell. Linearized smoothness-constrained errors are in good agreement with the corresponding most-squares errors. The variability of the RMT model as estimated from non-linear semi-axes is confirmed by TSVD-based most-squares inversions for most model cells within the depth range of investigation. In contrast to this, most-squares errors of the DCR model are consistently larger than errors estimated from non-linear semi-axes except for the smallest truncation levels. The model analyses confirm previous studies that DCR data can constrain resistive and conductive structures equally well while RMT data provide superior constraints for conductive structures. The joint inversion can improve error and resolution of structures which are within the depth ranges of exploration of both methods. In such parts of the model which are outside the depth range of exploration for one method, error and resolution of the joint inverse model are close to those of the best single inversion result subject to an appropriate weighting of the different data sets.

  • 49.
    Kalscheuer, Thomas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    A non-linear truncated SVD variance and resolution analysis of two-dimensional magnetotelluric models2007In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 169, no 2, p. 435-447Article in journal (Refereed)
    Abstract [en]

    A novel approach to assess variance and resolution properties of 2-D models of electrical resistivity derived from magnetotelluric measurements is presented. Based on a truncated singular value decomposition (TSVD) scheme on a local subspace, it partly takes the non-linearity of the inverse problem into account. The TSVD resolution and variance analysis is performed on a single cell at a time. A variance threshold is selected and the resulting model resolution is determined. As an improvement over existing schemes, non-linear semi-axes are introduced to describe the non-linear confidence surface in the directions of the model eigenvectors and they replace the inverse singular values entering into the standard expression of model variances. The model variance of the cell considered is estimated from the sum of squares of the non-linear semi-axes up to the given variance threshold. This, in turn, gives the truncation level of the TSVD and the row of the model resolution matrix belonging to the considered cell can be computed from the model eigenvectors of the TSVD. The information contained in the resolution matrix is condensed to easily comprehensible measures like the centre of resolution and horizontal and vertical resolution lengths.The validity of our non-linear model variance and resolution estimates is tested with a most-squares technique which gives an improved estimate of model variability.A synthetic model with a conductive block in a homogenous half-space is analysed. TSVD analyses for model cells on the upper edge of the block and outside the block illustrate how the truncation process works. Typically, the linear and non-linear semi-axes are almost equal up to a certain singular value number, after which the non-linear semi-axes increase much less than the linear semi-axes. This important result indicates that the resolution of 2-D magnetotelluric models is significantly better than previously suggested by linear schemes for the computation of model variance and resolution properties. A field example from the Skediga area (Sweden) shows that the electrical resistivity distribution of sand and gravel formations which are only laterally bounded by conductive clay lenses is relatively well resolved whereas there is little resolution for the transition between the sand and gravel layer and the basement under a clay cover.

  • 50.
    Kalscheuer, Thomas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Siripunvaraporn, Weerachai
    Department of Physics, Faculty of Science, Mahidol University, Rama VI Rd., Rachatawee, Bangkok 10400, Thailand.
    Radiomagnetotelluric two-dimensional forward and inverse modelling accounting for displacement currents2008In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 175, no 2, p. 486-514Article in journal (Refereed)
    Abstract [en]

    Electromagnetic surface measurements with the radiomagnetotelluric (RMT) method in the frequency range between 10 and 300 kHz are typically interpreted in the quasi-static approximation, that is, assuming displacement currents are negligible. In this paper, the dielectric effect of displacement currents on RMT responses over resistive subsurface models is studied with a two-dimensional (2-D) forward and inverse scheme, that can operate both in the quasi-static approximation and including displacement currents.Forward computations of simple models exemplify how responses that allow for displacements currents deviate from responses computed in the quasi-static approximation. The differences become most obvious for highly resistive subsurface models of about 3000 Ohm*m and more and at high frequencies. For such cases, the apparent resistivities and phases of the transverse magnetic (TM) and transverse electric (TE) modes are significantly smaller than in the quasi-static approximation. Along profiles traversing 2-D subsurface models, sign reversals in the real part of the vertical magnetic transfer function (VMT) are often more pronounced than in the quasi-static approximation. On both sides of such sign reversals, the responses computed including displacement currents are larger than typical measurement errors.The 2-D inversion of synthetic data computed including displacement currents demonstrates that serious misinterpretations in the form of artefacts in inverse models can be made if displacement currents are neglected during the inversion. Hence, the inclusion of the dielectric effect is a crucial improvement over existing quasi-static 2-D inverse schemes. Synthetic data from a 2-D model with constant dielectric permittivity and a conductive block buried in a highly resistive layer which in turn is underlain by a conductive layer are inverted. In the quasi-static inverse model, the depth to the conductive structures is overestimated, artefactual resistors appear on both sides of the conductive block, and a spurious conductive layer is imaged at the surface.High-frequency RMT field data from Ävrö, Sweden, are re-interpreted using the newly developed 2-D inversion scheme which includes displacement currents. In contrast to previous quasi-static modelling, the new inverse models have electrical resistivity values comparable to a normal-resistivity borehole log and boundaries between resistive and conductive structures which correlate with the positions of seismic reflectors.

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