Seismic Imaging of the Southern California Plate Boundary around the South-Central Transverse Ranges Using Double-Difference Tomography

We derive high-resolution P and S seismic velocities (V-P and V-S) within the South-Central Transverse Ranges section of the San Andreas Fault (SAF), using a new double-difference tomography algorithm incorporating both event-pair and station-pair differential times. The addition of station-pair data allows for better absolute event locations and higher model resolution at shallow depths. Velocities within a 222kmx164km region are inverted using >1,000,000 P and S arrival (picked with an automatic detection algorithm) and differential times from >10,000 local events recorded by >250 stations. Similarly large P and S datasets lead to high-quality V-P/V-S estimates of the region. The resulting models include low velocities along major fault segments and across-fault velocity contrasts. They also show very high V-P/V-S anomalies near shallow damaged rock, whereas fault zones exhibit either low (<1.73) or high (>1.73) V-P/V-S characteristics at greater depth. The variations in amplitude of these anomalies along the SAF through San Gorgonio Pass (SGP) suggest abrupt west-to-east changes in elastic properties. Moreover, their geometries imply near-vertical SAF segments northwest of SGP and northeast dipping faults southeast of that area. The SAF near Coachella Valley is estimated to dip by 57 degrees. Regional-scale low and high V-P/V-S values are related to relative abundances of crystalline or metamorphic rocks. Near-fault V-P/V-S anomalies at depth are likely associated with changes in wet crack geometries. The obtained results can improve future calculations of seismic motion from large earthquakes in the area and related seismic hazard estimates.