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Influence of Fault Roughness on 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]

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.

National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-346811OAI: oai:DiVA.org:uu-346811DiVA, id: diva2:1195080
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: 2018-04-27

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