Seismic Velocity Structures in the Southern California Plate Boundary Environment from Double-difference Tomography

We present tomographic images of crustal structures in the southern California plate boundary area, with a focus on the San Jacinto Fault Zone (SJFZ), based on double-difference inversions of earthquake arrival times. Absolute arrival times of over 359,410 P- and S- wave phase picks for 5493 earthquakes recorded at 139 stations in Southern California are used. In addition to the absolute event-station traveltimes, the double-difference inversions utilize differential travel times for phases from nearby event pairs. Starting with a layered 1D model, and continuing in later iterations with various updated initial models, we invert the data for Vp and Vs in a 270 km long, 180 km wide and 35 km deep volume around the SJFZ using a space-variable grid with higher density around the SJFZ. The examined volume stretches from Cajon Pass to the northernmost Imperial Fault Zone and includes portions of the southern San Andreas Fault (SAF), the Elsinore Fault, and the Brawley Seismic Zone in the Salton Trough. Because differential traveltimes are most sensitive to near-source structures, we obtain high resolution around the earthquake sources. After 24 iterations we improve the average travel time misfit by a factor of 20. Though ray coverage is limited at shallow depths, we obtain high-resolution images of seismic velocities from 3 to 16 km throughout much of the study area. Our final velocity model shows clear velocity contrasts across the SJFZ and the southern SAF as well as zones of low-velocity and anomalous Vp/Vs ratios associated with various fault strands and sedimentary basins. The NE side of the SJFZ has generally higher velocities than the SW block, and the contrasts of Vp are generally higher (up to 20%) than the contrasts of Vs (up to 15%). Near the San Jacinto valley to the NW, and the SE end of the SJFZ close to the Salton trough, the polarity of the velocity contrast in the shallow crust is reversed. For the southern SAF the NE side is generally the block with lower seismic velocities. The velocity reductions in fault zone regions are generally highest in geometrically complex areas (up to 25% in the top few km), and are higher for Vs than for Vp. The imaged features have important implications for various aspects of earthquake and crustal dynamics in the region.