INFLUENCE OF LITHOLOGY ON MESOSCALE AND MICROSCALE FAULT DAMAGE AT RIDGECREST, CALIFORNIA

Fault zones accumulate damage over time due to both seismic and aseismic processes. Damage asymmetry and overall along strike variability in off-fault damage is common along faults, but the degree to which this can be attributed to material properties is uncertain. Our study aims to improve our understanding of the lithological controls on off-fault damage through multiscale field and experimental observations. This study concentrates its findings on fault zones around Ridgecrest, California. Ridgecrest is a unique place to study these processes because numerous faults were activated during the 2019 Ridgecrest earthquake sequence, and the faults cut the Independence dike swarm. At Ridgecrest it is possible to isolate the influence of lithology by analyzing off-fault damage in both the dikes and the surrounding granite and compare it to cumulative displacement as determined by dike offsets and incremental displacements during a single earthquake observed in InSAR. Our study uses ground-based LiDAR and imagery to evaluate fault damage at the outcrop scale. We use experiments and petrography, SEM, and XRD to evaluate deformation at the microscale. We investigate how rock surrounding the faults might deform during earthquakes by performing compressive and tensile loading Split Hopkinson Pressure Bar experiments on rocks from the field area including diabase, granite, aplite, and mylonite. We perform digital image correlation on high speed imagery and compare it with the data from a strain gauge mounted on the sample to track deformation during experiments. Micromechanical analysis of experiments showed that diabase fragments less than granite during dynamic loading. In contrast, foliated rocks fragment less the isotropic crystalline rocks, and fractures follow foliation. Rock types that fragment more during dynamic loading exhibit strain time series that are smooth during failure while failure for rocks that fragment less is marked by abrupt changes in the strain signal. We found that lithology influences fault damage intensity, anisotropy, and spatial distribution at both the mesoscale and microscale. These results provide further insights into how fault damage evolves and influences seismicity in areas with varying lithology.