CRUSTAL ANISOTROPY IN THE WALKER LANE AND EASTERN CALIFORNIA SHEAR ZONE

The plate boundary between the North American plate and the Pacific plate consists of an expansive network of faults. One region critical for accommodating transform motion is the Walker Lane and Eastern California Shear Zone (WL-ECSZ), which accommodates roughly 25% of the relative plate boundary motion through a combination of dextral and extensional faulting. Dextral deformation in the region is relatively recent and is thought to have begun between 13 and 4 Ma, with the precise start time varying according to the location within the region. The overall timing of deformation in the WL-ECSZ is thought to be coincident with the migration of southern edge of the plate boundary to the Gulf of California around 13-12 Ma. It has been hypothesized that as the Mendocino Triple Junction propagates northward, the majority of plate boundary motion will shift eastward from the San Andreas fault to the WL-ECSZ.

This study aims to look for evidence of evolving deformation of the WL-ECSZ using observations of seismic anisotropy. More specifically, we use Ps receiver function analysis at 55 long running seismic stations to characterize lateral and vertical variations in anisotropic fabrics in the crust and uppermost mantle throughout the WL-ECSZ. Stations were chosen based on geographic distribution, operation length, and overall data quality using ambient noise probability density functions. Event parameters, distance, and magnitude were tested in order to maximize data coverage while limiting poor quality data. Receiver functions are calculated using events which have a minimum magnitude of 5.8 and have epicentral distances of 30° to 95°. We also apply a harmonic decomposition technique to isolate components of the signal due to dipping structure/anisotropy, from those that are due to horizontally oriented anisotropy. Preliminary results indicate significant anisotropy present in the crust and uppermost mantle with good agreement between some stations, indicating that pervasive deformation may be present. However, a simplified model explaining our observations is still lacking, suggesting complex deformation histories and heterogenous crustal compositions.