Helmholtz Centre Potsdam
GFZ German Research Centre for Geosciences
Abstract (EDOC: 15264)
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 gathers of the Los Angeles Region Seismic Experiment, Phase II (LARSE II), and we developed refraction velocity models from these picks using two different inversion algorithms. Vp/Vs ratios were calculated from the resulting P- and S-wave models where both models are constrained by ray coverage. The two P-wave velocity models are compared to each other and to results from forward modeling. Generally, the P-wave inverse and forward models agree well for velocities lower than 5.0 km/s but only broadly agree with each other for velocities above 5.0 km/s. Similarly, the S-wave inverse models agree well with each other for velocities lower than 2.5 km/s but only broadly agree for velocities higher than 2.5 km/s. The most prominent structures in our S-wave models are two north-dipping low-velocity zones in the Central Transverse Ranges that we interpret as faults. These low-velocity zones differ somewhat between the two inversion models, but the Vp/Vs models (one model for each technique) show these features to be remarkably similar. Interestingly, both Vp/Vs models have several features that are not visible in either the P- or S-wave models alone. Two of these features (relatively high Vp/Vs ratios) occur in the vicinity of wells that bottom in "granitic" rocks and we interpret these high Vp/Vs ratios to indicate that the granitic rocks are highly fractured or even brecciated. Finally, to evaluate the Southern California Earthquake Center (SCEC) Community Velocity Model (CVM), which predicts Vs based on the Vp model, we compare data from our Vp and Vp/Vs models to empirical formulas that relate P- to S-wave velocities (see Brocher, 2005). These empirical curves provide an adequate average relationship between Vp and Vs, but our model Vs varies as much as ± 20 % for Vp ≤ 5.0 km/s and as much as ± 35 % for Vp ≥ 5.0 km/s. This large variation in the predicted S- wave velocity demonstrates the value of determining Vs independently from Vp.
(2010): Detailed P- and S-wave Velocity Models Along the LARSE II Transect, Southern California. Bulletin of the Seismological Society of America, 100, 6, 3194-3212.