Sweden is presently a low seismicity area where most earthquakes are small and pose no serious threat to constructions. For the long-term perspectives of safety assessments of geological repositories for spent nuclear fuel, however, the effects of large earthquakes have to be considered. For the Swedish nuclear waste storage concept, seismically induced secondary fracture shear displacements across waste canister positions could pose a long-term seismic risk to the repository.

In this thesis, I present earthquake simulations with which I study the potential for near-fault secondary fracture shear displacements. As a measure I use the Coulomb Failure Stress (CFS), but also calculate explicit fracture displacements. I account for both the dynamic and quasi-static stress perturbations generated during the earthquake. As numerical tool I use the 3DEC code, whose performance I validate using Stokes closed-form solution and the Compsyn code as benchmarks. In a model of a M_{w} 6.4 earthquake, I investigate how fault roughness, the fault rupture propagation model and rupture velocity may impact the near-fault CFS evolution. I find that fault roughness can reduce the amount of fault slip by tens of percent, but also increase the near-fault CFS with similar amounts locally. Furthermore, different fault rupture models generate similar CFS levels. I also find that the secondary stresses scale with rupture velocity.

In a model based on data from the Forsmark nuclear waste repository site, and assuming stress conditions prevailing at the end of a glaciation, I simulate several high stress drop ~M_{w} 5.6 earthquake scenarios on the gently dipping fault zone ZFMA2 and calculate secondary fracture displacements on 300 m diameter planar fractures. Less than 1% of the fractures at the shortest distance from ZFMA2 generate displacements exceeding the 50 mm criterion established by the Swedish Nuclear Fuel and Waste Management Co. Given the high stress drops and the assumption of fracture planarity, I consider the calculated displacements to represent upper bound estimates of possible secondary displacements at Forsmark. Hence, the results should strengthen the confidence in the safety assessment of the nuclear waste repository at the Forsmark site.

To assess the long‐term safety of a deep repository of spent nuclear fuel, upper bound estimates of seismically induced secondary fracture shear displacements are needed. For this purpose, we analyze a model including an earthquake fault, which is surrounded by a number of smaller discontinuities representing fractures on which secondary displacements may be induced. Initial stresses are applied and a rupture is initiated at a predefined hypocenter and propagated at a specified rupture speed. During rupture we monitor shear displacements taking place on the nearby fracture planes in response to static as well as dynamic effects. As a numerical tool, we use the 3Dimensional Distinct Element Code (3DEC) because it has the capability to handle numerous discontinuities with different orientations and at different locations simultaneously. In tests performed to benchmark the capability of our method to generate and propagate seismic waves, 3DEC generates results in good agreement with results from both Stokes solution and the Compsyn code package. In a preliminary application of our method to the nuclear waste repository site at Forsmark, southern Sweden, we assume end‐glacial stress conditions and rupture on a shallow, gently dipping, highly prestressed fault with low residual strength. The rupture generates nearly complete stress drop and an M_{w} 5.6 event on the 12 km^{2} rupture area. Of the 1584 secondary fractures (150 m radius), with a wide range of orientations and locations relative to the fault, a majority move less than 5 mm. The maximum shear displacement is some tens of millimeters at 200 m fault‐fracture distance.

The dynamic and static stress perturbations generated in an earthquake affect the stability of faults and fractures in the vicinity of the rupture. Estimates of co-seismic near-fault stress effects can be made using numerical simulations. Here, we study the co-seismic stress evolution close to an earthquake using two different models to simulate the rupture. One model is the linear slip-weakening (SW) model, where a spontaneous earthquake rupture is simulated. We compare this to a constant rupture velocity time-weakening (TW) model, which we implement in four different instances of rupture velocity V_{r} and strength reduction time interval Δt_{red}. We evaluate the near-fault stress effects using the Coulomb Failure Stress (CFS), which we calculate from the stress evolution at various positions relative to the rupture plane. The results show that the TW method is capable of generating similar secondary effects as those generated by the SW model. However, the assumption of constant values of Δt_{red} and V_{r} implies that there will always be locations on the rupture plane where these values are incompatible. We also see that variationsin Δt_{red} and V_{r} have a significant impact on the results. Particularly, V_{r }is important for how the stresses around the rupture front are superimposed, and is thus important for the temporal evolution and spatial distribution of CFS around the fault. Lower V_{r} tends to generate a gentler near-fault stress evolution and lower peak CFS values. The results also indicate that not only the momentary value of V_{r} is important for the secondary stress effects at a near-fault position passed by the rupture, but also the integrated V_{r}-history up to that position.

5.

Fälth, Billy

et al.

Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Clay Technology AB.

Lund, Björn

Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.

Co-seismic displacements on fractures and faults close to large earthquakes may not contribute significantly to the shaking hazard for surface infrastructures. However, for deep geological nuclear waste repositories, such secondary displacements could, if large enough, damage intersected waste containers and constitute a significant long-term safety concern. To study how the potential for such displacements may depend on the earthquake rupture evolution, we simulate dynamic earthquake ruptures, and calculate the co-seismic evolution of Coulomb Failure Stress (CFS) on hypothetical fracture planes in the near-fault continuum. Poroelastic coupling is accounted for via Skempton’s coefficient B. We study three cases: (1) A planar fault with homogeneous properties. (2) A planar fault where the dynamic friction increases gradually along the fault edge to obtain a gentler rupture arrest. (3) An undulated fault with fractal properties. For Case 3, we consider ten different fault surface realizations. Since the undulations reduce fault slip, we also run models with adjusted dynamic friction coefficients, such that they generate seismic moments on par with that of Case 1. We observe the following: (i) The initial stress field, rather than the co-seismic stress effects, is the dominating influence on the fracture orientations that obtain the highest CFS values. (ii) Lower slip gradients and less fault slip in Case 2 reduce the maximum CFS by 10-15% relative to the reference case. (iii) Fault roughness may increase CFS locally by tens of percent. (iv) Given our reference value of B=0.5, B-value variations of ±0.5 would give CFS variations of ±20%, at most.