Simulation of Co-Seismic Off-Fault Stress Effects: Influence of Fault Roughness and Pore Pressure Coupling
Abstract
Aiming at improved safety assessment of geological nuclear waste repositories, we use dynamic 3D earthquake simulations to estimate the potential for co-seismic off-fault distributed fracture slip. Our model comprises a 12.5 x 8.5 km strike-slip fault embedded in a full space continuum where we apply a homogeneous initial stress field. In the reference case (Case 1) the fault is planar and oriented optimally for slip, given the assumed stress field. To examine the potential impact of fault roughness, we also study cases where the fault surface has undulations with self-similar fractal properties. In both the planar and the undulated cases the fault has homogeneous frictional properties. In a set of ten rough fault models (Case 2), the fault friction is equal to that of Case 1, meaning that these models generate lower seismic moments than Case 1. In another set of ten rough fault models (Case 3), the fault dynamic friction is adjusted such that seismic moments on par with that of Case 1 are generated. For the propagation of the earthquake rupture we adopt the linear slip-weakening law and obtain Mw 6.4 in Case 1 and Case 3, and Mw 6.3 in Case 2 (35 % lower moment than Case 1). During rupture we monitor the off-fault stress evolution along the fault plane at 250 m distance and calculate the corresponding evolution of the Coulomb Failure Stress (CFS) on optimally oriented hypothetical fracture planes. For the stress-pore pressure coupling, we assume Skempton's coefficient B = 0.5 as a base case value, but also examine the sensitivity to variations of B. We observe the following: (I) The CFS values, and thus the potential for fracture slip, tend to increase with the distance from the hypocenter. This is in accordance with results by other authors. (II) The highest CFS values are generated by quasi-static stress concentrations around fault edges and around large scale fault bends, where we obtain values of the order of 10 MPa. (III) Locally, fault roughness may have a significant impact. The ratios (max CFS in Case 2) / (max CFS in Case 1) = 1.1 and (max CFS in Case 3) / (max CFS in Case 1) = 1.2 indicate a minor impact. However, at specific locations, CFS in Case 2 and Case 3 may be more than 5 times higher than in Case 1. (IV) The sensitivity to variations of B is modest; (max CFS in Case 1 with B = 0) / (max CFS in Case 1 with B = 1) = 1.15.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.S43B0872F
- Keywords:
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- 3611 Thermodynamics;
- MINERALOGY AND PETROLOGY;
- 5104 Fracture and flow;
- PHYSICAL PROPERTIES OF ROCKS;
- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS