Spatial and Temporal Variations of Upper Crustal Anisotropy Along the San Jacinto Fault Zone in Southern California: Constraints from Shear Wave Splitting Analysis

To discern spatial and possibly temporal variations of upper crustal anisotropy in the section of the San Jacinto Fault Zone (SJFZ) where the 03/11/2013 earthquake swamp occurred, we conduct systematic shear wave splitting (SWS) measurements using high quality local S-wave data recorded by three stations for the period of 2000 to 2017. An automatic data selection and splitting measurement procedure is firstly applied, and the resulting splitting measurements are then manually screened to ensure the reliability of the results. Strong spatial variations in crustal anisotropy are revealed by the 929 pairs of resulting high-quality splitting parameters (fast polarization orientations and splitting delay times), as reflected by the clear dependence of the resulting fast orientations on the location and geometry of the raypaths. For raypaths travelling through a particular section of the fault zone, the fast orientations are NW-SE which is parallel to the fault and may reflect fluid-fill fractures; on the other hand, most of the other raypaths show N-S fast orientations which are consistent with the direction of the regional maximum compressive stress. While a significant change in normalized splitting time before and after the largest earthquake in the sequence is revealed, probably indicating variations of the anisotropy strength, we have found that an apparent temporal variation of the fast orientations is actually the result of the occurrence of a large number of earthquakes in an area with different fast orientations.