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How does the Fault Rupture Model affect Simulated Co-Seismic Near-Fault Stress Evolution?
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Clay Technology AB.ORCID iD: 0000-0002-3300-2104
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.ORCID iD: 0000-0002-0789-5949
Clay Technology AB.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

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 Vr and strength reduction time interval Δtred. 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 Δtred and Vr implies that there will always be locations on the rupture plane where these values are incompatible. We also see that variationsin Δtred and Vr have a significant impact on the results. Particularly, Vr 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 Vr 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 Vr is important for the secondary stress effects at a near-fault position passed by the rupture, but also the integrated Vr-history up to that position.

National Category
Geophysics
Research subject
Geophysics with specialization in Seismology
Identifiers
URN: urn:nbn:se:uu:diva-347597OAI: oai:DiVA.org:uu-347597DiVA, id: diva2:1195289
Available from: 2018-04-04 Created: 2018-04-04 Last updated: 2018-04-04
In thesis
1. Simulating Earthquake Rupture and Near-Fault Fracture Response
Open this publication in new window or tab >>Simulating Earthquake Rupture and Near-Fault Fracture Response
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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 Mw 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 ~Mw 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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 67
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1661
National Category
Geophysics
Research subject
Geophysics with specialization in Seismology
Identifiers
urn:nbn:se:uu:diva-347600 (URN)978-91-513-0312-3 (ISBN)
Public defence
2018-05-28, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2018-04-27 Created: 2018-04-04 Last updated: 2018-10-08

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