Detailed P- and S-Wave Velocity Models along the LARSE II Transect, Southern California

Abstract

Abstract Structural details of the crust determined from P -wave velocity models can be improved with S -wave velocity models, and S -wave velocities are needed for model-based predictions of strong ground motion in southern California. We picked P - and S -wave travel times for refracted phases from explosive-source shots of the Los Angeles Region Seismic Experiment, Phase II (LARSE II); we developed refraction velocity models from these picks using two different inversion algorithms. For each inversion technique, we calculated ratios of P - to S -wave velocities ( V P / V S ) where there is coincident P - and S -wave ray coverage. We compare the two V P inverse velocity models to each other and to results from forward modeling, and we compare the V S inverse models. The V S and V P / V S models differ in structural details from the V P models. In particular, dipping, tabular zones of low V S , or high V P / V S , appear to define two fault zones in the central Transverse Ranges that could be parts of a positive flower structure to the San Andreas fault. These two zones are marginally resolved, but their presence in two independent models lends them some credibility. A plot of V S versus V P differs from recently published plots that are based on direct laboratory or down-hole sonic measurements. The difference in plots is most prominent in the range of V P =3 to 5 km/s (or V S ∼1.25 to 2.9 km/s), where our refraction V S is lower by a few tenths of a kilometer per second from V S based on direct measurements. Our new V S - V P curve may be useful for modeling the lower limit of V S from a V P model in calculating strong motions from scenario earthquakes.