Crustal thickness, seismicity, and P-wave velocity structure in southern California from data recorded by the Southern California Seismic Network

This thesis describes three projects that use some of the vast amount of data that has been collected by the Caltech/USGS Southern California Seismic Network (SCSN) over the last 25 years to investigate seismicity patterns and crustal structure in southern California. This data includes both waveforms and analyst-picked arrival times from over 300,000 local seismic events recorded at over 300 stations.; In Chapter 2 we apply a stacking procedure to waveforms from local events that enables us to measure the times of P_mP (a phase not visible on most individual seismograms) relative to the first arriving P wave and thus to estimate the variations in crustal thickness across southern California. We find an average crustal thickness of 28 km, with thinner crust (<20 km) in the Salton Trough and offshore and thicker crust under the San Gabriel Mountains (33 km) and the southern Sierra Nevada (36 km).; Chapter 3 describes a method of jointly relocating seismicity which is efficient enough to applied to very large numbers of events and greatly improves the accuracy of the relative locations of nearby events. A key feature of this method is that we apply empirical corrections for three-dimensional structure by computing station timing corrections that continuously vary as a function of source position. We apply this method to the existing SCSN P and S picks to relocate nearly 300,000 events in southern California. Our locations exhibit much less scatter, particularly in depth, than those of the SCSN catalog and a greater tendency to align into linear and planar features suggestive of fault structures.; The final chapter is devoted to progress made toward removing a deficiency of the relocation scheme of the previous chapter—that it does not improve the accuracy of the absolute locations as it does the relative locations. To do this we are constructing a model of the three-dimensional velocity variations in the southern California crust. We present our first-order P-wave velocity model and discuss various features of the model and improvements that might be made before using it in a large-scale relocation effort.