Publication Date:
2010-12-01
Description:
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 (sub P) /V (sub S) ) where there is coincident P- and S-wave ray coverage. We compare the two V (sub P) inverse velocity models to each other and to results from forward modeling, and we compare the V (sub S) inverse models. The V (sub S) and V (sub P) /V (sub S) models differ in structural details from the V (sub P) models. In particular, dipping, tabular zones of low V (sub S) , or high V (sub P) /V (sub 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 (sub S) versus V (sub 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 (sub P) =3 to 5 km/s (or V (sub S) approximately 1.25 to 2.9 km/s), where our refraction V (sub S) is lower by a few tenths of a kilometer per second from V (sub S) based on direct measurements. Our new V (sub S) -V (sub P) curve may be useful for modeling the lower limit of V (sub S) from a V (sub P) model in calculating strong motions from scenario earthquakes.
Print ISSN:
0037-1106
Electronic ISSN:
1943-3573
Topics:
Geosciences
,
Physics
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