Crustal P-wave Velocity Structure of Southern California Revealed by First P and PmP Traveltime Data

We construct a new crustal P-wave velocity model of southern California by jointly inverting arrival times of first P and Moho reflected PmP waves. An eikonal equation-based tomographic technique is applied, which allows us to accurately trace the ray paths for both the first P and PmP waves. To increase the number of available PmP arrivals, we use a new semi-automatic PmP picking workflow. As a result, a total of 29,512 robust PmP arrivals are successfully recognized, forming the largest earthquake-sourced PmP dataset in southern California to date. Since the PmP waves are particularly sensitive to velocity structures in the middle to lower crust, including PmP arrival times in seismic imaging greatly increases the resolution in the middle and lower crust. In the upper crust, our model reveals similar features as most previous models in the region. While in the middle and lower crust, our model clearly delineates an east-dipping mafic middle-lower crust of the west Peninsular Range, with a dipping angle smaller than 45°. Furthermore, we identify distinct low-velocity anomalies beneath the Eastern California Shear Zone (ECSZ) and south of the Coso volcanic field (CVF). We interpret the low velocities beneath the ECSZ as the presence of interconnected high-pressure fluids, which may not only trigger regional earthquakes but also play a role in the shear zone's long-term development. Meanwhile, the low velocities south of the CVF may indicate a southward shift of the magma source in the lower crust, probably in the form of a basaltic magma reservoir that connects with an upper mantle upwelling. This finding contradicts many previous studies that assumed a direct underneath magma origin beneath the CVF. Our new velocity model may provide important constraints on the lower crust deformation and dynamics in southern California, and may further shed light on the interplay between the lower crust and the overlying brittle upper crust or the underlying uppermost mantle.