3D Structure of Southern California from TriNet Surface Wave Data

The three-dimensional S-wave velocity structure was constructed from Rayleigh and Love wave phase velocities, measured using teleseismic TriNet data. Two station method was used for the phase velocity measurement, using spectral fitting technique through perturbation of S-wave velocity structure, specifically developed for this project. The total number of paths are about 3500 for Rayleigh waves and 1500 for Love waves. The S-wave velocity maps show some large-scale features that are much more distinct than previous tomographic results in Southern California. There is a clear seismic velocity contrast between the Pacific plate and the North American plate. The North American plate side is systematically slower than the Pacific plate side and this velocity contrast extends at least down to the depth of about 60 km. There is no question that this contrast exceeds the Moho. In the northern part, this contrast in the upper mantle is fairly vertical (abrupt) and is almost exactly under the San Andreas Fault. In the southern part, the contrast is under the Elsinore and the San Jacinto Fault area, somewhat west of the San Andreas Fault. Under the Eastern California Shear Zone, there are distinct crustal slow velocity anomalies, suggesting higher temperature and lower viscosity in the lower crust. In the upper mantle, in addition to the well-known fast velocity anomaly under the Transverse Ranges, there is another distinct fast velocity anomaly under the peninsular and coastal ranges toward south. This fast velocity anomaly appears to be just as large as the one under the transverse ranges. Also slow velocity anomalies are found under the Salton Trough and under the Southern Sierra region near the Coso area.