Fault Slip and Seismicity in Geometrically Complex Fault Systems

University Of California-Riverside, Riverside CA

Investigators

Abstract

This project is investigating processes that control slip and earthquake occurrence on geometrically complex faults. Slip of non-planar faults and fault networks involves interactions and processes that do not occur in standard planar fault models. These include coupled interactions among slip, normal stress, and fault strength (as set by the coefficient of friction and normal stress). In addition, to accommodate geometric incompatibilities, yielding and stress relaxation processes must accompany slip of major through-going faults to prevent the development of pathological stress conditions (or in extreme cases fault lock-up). In the brittle seismogenic crust, yielding is accomplished principally through the development of secondary faults. The fractal-like character of fault systems and fault roughness indicates that slight movements of secondary faults at all scales are necessary to accommodate slip of major through-going faults. The yielding is manifest as coseismic off-fault deformation during earthquakes, as aftershocks, and as diffuse background seismicity. The modeling component of this project employs 2D and 3D implementations of the boundary element method to represent stress interactions among major through-going faults. Yielding and stress relaxation off of the major faults are modeled through explicit representations of slip on secondary faults, and as bulk relaxation using an earthquake rate formulation, which is derived from rate- and state-dependent fault constitutive properties. Use of this formulation affords a way to represent seismicity, yielding, and stress relaxation while incorporating time- and stress-dependencies that are consistent with laboratory observations of fault slip and earthquake phenomena, such as aftershocks and foreshocks. The observational component of the study uses a variety of data sets to test the theoretical results and guide model development. These include characteristic decay of the productivity of background seismicity and aftershocks by distance from major faults, partitioning of off-fault seismicity between aftershocks and background earthquakes, and stress heterogeneity and stress rotations as indicated by focal mechanism solutions. This work could have implications for the occurrence of earthquake and thus seismic hazard analysis. The project will also support a promising postdoctoral fellow.