Low-velocity fault zones (LVFZs) around many active faults with a reduction in seismic wave velocities of several to several tens percent relative to wall rocks have been detected by fault zone trapped waves and geodetic observations of the coseismic deformation. These LVFZs are mechanically distinct from the ambient crustal rock, as evidenced by their responses to nearby coseismic ruptures. A temporal reversal of the healing process of the 1992 Landers fault zone due to the nearby coseismic rupture of the 1999 Hector Mine earthquake and a net decrease in seismic velocities of at least 2.5% within the San Andreas fault zone due to the coseismic rupture of the 2004 M6 Parkfield earthquake have been documented. Displacements of several millimeters to several centimeters across several LVFZs due to nearby coseismic ruptures have also been observed by geodetic surveys. Earthquake source models used in previous studies to examine these observations of LVFZs are either dislocation (kinematic slip) models or point source models, which cannot accurately characterize the near-source dynamic stress and deformation fields. This project utilizes the most advanced spontaneous dynamic rupture model to investigate responses of LVFZs to nearby coseismic ruptures. Spontaneous dynamic rupture models build upon physical principles such as elasticity, elastoplasticity, and friction laws and provide physical insights to observations from real earthquakes. A finite element method (FEM) code for simulating spontaneous dynamic ruptures along geometrically complex faults and wave propagation in complex velocity structures has been verified in a community-wide code validation effort and is used in this study. Recent modifications to the FEM, including addition of elastoplastic off-fault response to dynamic ruptures, allow the PIs to more accurately calculate dynamic stress field and to better examine responses of LVFZs to coseismic ruptures. This project also investigates how a LVFZ surrounding a rupturing fault affects spontaneous dynamic rupture on the fault and near-field ground motion. With elastoplastic off-fault response included in spontaneous dynamic rupture models, the PIs examine interactions of inhomogeneous material distribution, off-fault damage and spontaneous dynamic rupture and effects on near-field ground motion. This project integrates observations of LVFZs with models of spontaneous dynamic ruptures. Compared with kinematic or point source models, spontaneous dynamic rupture models provide more accurate quantification of how LVFZs respond to earthquake coseismic ruptures, which has significant implications for improving our understanding of fault zone mechanics and stress and strain transfer in fault systems. Observations of LVFZs provide constraints to parameters in dynamic rupture models, including velocity structure of LVFZs and material strength (i.e., cohesion and internal friction). These observation-constrained model parameters can be used to support studies of other significant questions, such as prediction of strong ground motion at critical facility sites and in earthquake-prone areas. Results from this study are expected to promote the integration of observation and theory in earthquake sciences, and to impact both observation-oriented and theory-oriented communities of earthquake sciences.
