This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
Understanding seismic ground motion in the high frequency range (>1 Hz) is of substantial interest to the engineering community since the resonance frequency of most man-made structures lies within it. In this frequency range, ground motion is relatively incoherent and all frequencies are present, suggesting a source process that excites waves of all wavelengths. This project explores earthquake rupture propagation on rough faults with the working hypothesis that the irregular propagation induced by roughness might be responsible for producing observed levels of high frequency radiation. In particular, natural faults are well described as self-similar fractal surfaces: roughness is present at all scales and the amplitude of the roughness associated with a particular wavelength is proportional to that wavelength. Slip on nonplanar faults perturbs the local stress field, causing local accelerations and decelerations of propagating ruptures and the emission of seismic waves. In addition to the specific focus on high frequency ground motion, the project investigates general characteristics of earthquake dynamics on nonplanar faults.
These research questions will be addressed using direct numerical simulations of dynamic ruptures on nonplanar faults. The faults are governed by rate-and-state friction laws featuring extreme velocity-weakening behavior at coseismic slip rates, as observed in laboratory experiments. The off-fault response is modeled as that of a Drucker-Prager elastic-plastic material (similar to a Mohr-Coloumb material) in either a rate-independent or viscoplastic formulation. The irregular geometries are handled using a coordinate mapping technique and high order finite differences are used to approximate the governing equations numerically. The particular numerical method features several mechanisms (upwinding and the adaptive selection of finite difference stencils) that prevent the development of spurious oscillations that would otherwise contaminate the solution at the high frequencies of interest.
