Seismic velocity structures in the southern California plate-boundary environment from double-difference tomography

Abstract

SUMMARY 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 247 472 P and 105 448 S wave phase picks for 5493 earthquakes recorded at 139 stations in southern California are used. Starting with a layered 1-D model, and continuing in later iterations with various updated initial models, we invert the data for Vp and Vs in a 270 km long, 105 km wide and 35 km deep volume around the SJFZ using a spatially 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 used in the double-difference inversions are most sensitive to near-source structures, we obtain high resolution around the earthquake sources. After 30 iterations we improve the average traveltime misfit by a factor of 16. Though ray coverage is limited at shallow depths, we obtain detailed images of seismic velocities from 3 to 20 km throughout much of the study area. Our final velocity results show zones of low-velocity and anomalous Vp/Vs ratios associated with various fault strands and sedimentary basins, along with clear velocity contrasts across the SJFZ and the southern SAF. The velocity reductions in fault zone regions are generally highest in geometrically complex areas (up to 30–50 per cent in the top few kilometres), are higher for Vs than for Vp, and follow a flower-type pattern with depth. In the central section of the SJFZ, from the San Jacinto valley to the trifurcation area, the northeast side of the fault has generally higher seismic velocities than the southwest block. The obtained contrasts of Vp are more persistent and higher (up to 20 per cent) than the contrasts of Vs (up to 15 per cent), although the differences may stem (at least partially) from the higher resolution of Vp images. In the SJFZ sections to the northwest and southeast, there are patches with reversed velocities contrasts especially in the shallow crust near the San Jacinto Valley and other basins. Along the Banning fault there is no clear velocity contrast. For the southern SAF, the northeast side has generally lower seismic velocities in the seismogenic zone with patches of contrast reversals in the shallow crust. In the Brawley Seismic Zone, the northeast side has somewhat lower velocities in the ∼20 km section near the southern SAF and higher velocities farther to the southwest. The imaged features have important implications for various aspects of earthquake and crustal dynamics in the region.