Stress- A nd structure-controlled anisotropy in a region of complex faulting-Yuha Desert, California

Abstract

We examine shear velocity anisotropy in the Yuha Desert, California using aftershocks of the 2010 M7.2 El Mayor-Cucapah earthquake. The Yuha Desert is underlain by a complex network of right- A nd left-lateral conjugate faults, some of which experienced triggered slip during the El Mayor-Cucapah earthquake. An automated method that implements multiple measurement windows and a range of bandpass filters is used to estimate the fast direction (φ) and delay time (δt) of the split shear waves. We find an average φ oriented approximately north-south suggesting it is primarily controlled by the regional maximum compressive stress direction. However, the spatial variability in φ reveals that the fault structures that underlie the Yuha Desert also influence the measured splitting parameters. We infer that the northeast- A nd northwest-oriented φ reflect shear fabric subparallel to the conjugate fault structures. We do not observe a simple correlation between δt and hypocentral distance. Instead, the observed spatial variation in δt suggests that near-source variation in anisotropic strength may be equal to or more important than effects local to the station. No temporal variation in splitting parameters is observed during the 70-day period following the main shock. In this region of complex faulting, we observe a spatially variable pattern of anisotropy that is both stress- A nd structure-controlled. This study suggests that shear fabric can form even along short, discontinuous fault strands with minimal offset.