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Seismological Investigation of Katla Volcanic System (Iceland): 3D Velocity Structure and Overall Seismicity Pattern
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. (Geophysics)
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The work in this thesis concentrates on Katla volcano in southern Iceland. This is one of Europe’s most active volcanoes and its history tells us that it poses many threats to society, both locally (Iceland) and on a broader scale (Europe). Its geological setting is complex, where the effects of a melting anomaly in the mantle and a changing rift geometry, perturb the classical setting of volcanism in a rifting setting.

The work has focused on two aspects. The first is the varying distribution of physical properties in the subsurface around the volcano. The second is the distribution of microearthquakes around the volcano. The physical properties that we study are the speeds of seismic waves that reflect variations of temperature, composition and fracturing of the rocks. These can, therefore, help us learn about long-term processes in the volcano. The seismicity gives shorter-term information about deformation associated with current processes.

I have applied two tomographic techniques to study Katla’s subsurface to a depth of 5-10 km, namely local-earthquake and ambient-noise tomography. The former makes use of the timing of waves generated by local earthquakes to constrain the earthquakes’ locations and the distribution of wave speed. Here I have concentrated on compressional waves or P waves with a typical frequency content around 10 Hz. With the latter, surface waves are extracted from microseismic noise that is generated far away at sea and their timing is measured to constrain their wave-speed distribution, which then is used to map shear-wave velocity variations. This is done at a typical frequency of 0.3 Hz. I find that the volcano contains rocks of higher velocity than its surroundings, that Katla’s caldera is underlain by low velocities at shallow depth that may be explained by hot or partially molten rocks and that beneath the caldera lies a volume of particularly high velocities that may constitute differentiated cumulates. But, I also find that it is not simple to compare results from such different wave types and discuss a number of complications in that regard.

In addition to the well-known microearthquake distribution in the caldera region of Katla and to its west, we have discovered two additional areas of microearthquake activity on the volcano’s flanks, south and east of the caldera. These point to current activity and are, therefore, of interest from a hazard point of view. However, it is difficult to pinpoint their underlying process. Speculation about possible interpretation leads me to hydrothermal processes or small pockets of melt ascending due to their buoyancy or locally enhancing fluid pressure, thereby lowering the effective stress.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 56 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1430
Keyword [en]
Volcano tomography, volcano seismicity, Katla volcano, Earthquake location
National Category
Geophysics Geosciences, Multidisciplinary
Research subject
Geophysics with specialization in Seismology
Identifiers
URN: urn:nbn:se:uu:diva-303342ISBN: 978-91-554-9696-8 (print)OAI: oai:DiVA.org:uu-303342DiVA: diva2:971569
Public defence
2016-11-10, Axel Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 09:00 (English)
Opponent
Supervisors
Funder
Natural‐Disaster Science
Available from: 2016-10-19 Created: 2016-09-16 Last updated: 2016-11-02
List of papers
1. The Katla volcanic system imaged using local earthquakes recorded with a temporary seismic network
Open this publication in new window or tab >>The Katla volcanic system imaged using local earthquakes recorded with a temporary seismic network
2016 (English)In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 121, no 10, 7230-7251 p.Article in journal (Refereed) Published
Abstract [en]

Katla is one of the most active subglacial volcanoes in Iceland. A temporary seismic network was operated on and around Katla for 2.5 years. A subset of similar to 800 analyzed local earthquakes clustered geographically in four regions: (1) the caldera, (2) the western region, (3) the southern rim, and (4) the eastern rim of the glacier. Based on the frequency content of recorded seismograms, each event was labeled as volcano tectonic (VT), long period (LP), or 'Mixed'. The southern cluster consists of LP events only, and the eastern cluster consists of VT events, while the western cluster is 'Mixed' although primarily LP. The caldera seismicity is confined to a subregion centered in the northeastern part of the caldera above 1 km below sea level (bsl) and gradually deepens away from its center to about 4 km depth. Deeper events are almost all VT, whereas LP events in the center of caldera locate at shallow depths. This is also where the velocities are lowest in the top 3 km of the crust of our 3-D tomographic model. A high-velocity core (similar to 6.5 km/s) is found at 4 km bsl beneath this low-velocity zone. We propose that a "subcaldera" may be developing within the present caldera and suggest a conceptual model for Katla volcano with a thin volume (similar to 1 km thick) that may host hot rhyolitic material in the shallow crust below the relocated seismic activity and above the high-velocity core. We interpret this core to consist of mafic cumulates resulting from fractionation of mafic intrusions and partial melting of subsiding hydrothermally altered rocks.

National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-303338 (URN)10.1002/2016JB013044 (DOI)000388441800015 ()
Funder
Swedish Research Council
Available from: 2016-09-16 Created: 2016-09-16 Last updated: 2017-01-02Bibliographically approved
2. Ambient-noise tomography of Katla volcano, south Iceland, and some ambiguity in its interpretation
Open this publication in new window or tab >>Ambient-noise tomography of Katla volcano, south Iceland, and some ambiguity in its interpretation
(English)Manuscript (preprint) (Other academic)
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-303330 (URN)
Available from: 2016-09-16 Created: 2016-09-16 Last updated: 2016-09-16
3. Long-period seismic events with strikingly regular temporal patterns on Katla volcano’s south flank (Iceland)
Open this publication in new window or tab >>Long-period seismic events with strikingly regular temporal patterns on Katla volcano’s south flank (Iceland)
Show others...
2016 (English)In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 324, 28-40 p.Article in journal (Refereed) Published
Abstract [en]

Katla is a threatening volcano in Iceland, partly covered by the Myrdalsjokull ice cap. The volcano has a large caldera with several active geothermal areas. A peculiar cluster of long-period seismic events started on Katla's south flank in July 2011, during an unrest episode in the caldera that culminated in a glacier outburst. The seismic events were tightly clustered at shallow depth in the Gvendarfell area, 4 km south of the caldera, under a small glacier stream at the southern margin of Myrdalsjokull. No seismic events were known to have occurred in this area before. The most striking feature of this seismic cluster is its temporal pattern, characterized by regular intervals between repeating seismic events, modulated by a seasonal variation. Remarkable is also the stability of both the time and waveform features over a long time period, around 3.5 years. We have not found any comparable examples in the literature. Both volcanic and glacial processes can produce similar waveforms and therefore have to be considered as potential seismic sources. Discerning between these two causes is critical for monitoring glacier-clad volcanoes and has been controversial at Katla. For this new seismic cluster on the south flank, we regard volcano-related processes as more likely than glacial ones for the following reasons: 1) the seismic activity started during an unrest episode involving sudden melting of the glacier and a jokulhlaup; 2) the glacier stream is small and stagnant; 3) the seismicity remains regular and stable for years; 4) there is no apparent correlation with short-term weather changes, such as rainstorms. We suggest that a small, shallow hydrothermal system was activated on Katla's south flank in 2011, either by a minor magmatic injection or by changes of permeability in a local crack system.

Keyword
Katla volcano; Iceland; Long-period earthquakes; Volcanic processes; Glacial processes
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-303335 (URN)10.1016/j.jvolgeores.2016.05.017 (DOI)000387197900004 ()
Available from: 2016-09-16 Created: 2016-09-16 Last updated: 2016-12-13Bibliographically approved
4. A peculiar cluster of microearthquakes on the eastern flank of Katla volcano, southern Iceland
Open this publication in new window or tab >>A peculiar cluster of microearthquakes on the eastern flank of Katla volcano, southern Iceland
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-303337 (URN)
Available from: 2016-09-16 Created: 2016-09-16 Last updated: 2016-09-16

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