Three-dimensional V_P and V_P/V_S Velocity Models of the Los Angeles basin and central Transverse Ranges, California

Abstract

We use P and S arrival times from 5225 earthquakes and 53 explosions, recorded by the Southern California Seismographic Network, to invert for the three-dimensional P- velocity (V_P) and the P and S-velocity ratio (V_P/ V_S) in the central Transverse Ranges and the Los Angeles basin. To model long-wavelength features of the velocity structure, we invert for the models by interpolating models determined successively from a sparse, medium, to a dense grid, with 40, 20, and 10 km spacing of horizontal grid nodes. Layers of grid nodes are placed at depths of 1, 4, 6, 8, 12, 16, and 20 km. The data variance decreased about 80% in the gradational inversion. Ample data from the 1994 Northridge and other earthquake sequences, the rich background seismicity, and the dense station distribution along with controlled sources made the model well resolved, except along the edges, to the southwest in the offshore region, and at depths greater than 20 km. The V_P model images the shape of the Los Angeles and east Ventura basins down to depths of 8 and 12 km. Three low-velocity areas at 1 km depth in the Los Angeles basin that coincide with recent sediment depositional areas are also imaged. The north edge of the Peninsular Ranges, the Santa Monica, and the San Gabriel Mountains, form discontinuous high-velocity ridges extending to depths of 20 km. The high V_P/V_S ratios in the near surface are consistent with high pore fluid pressures in the basin sediments. At depth beneath the east Ventura basin the high V_P/V_S and high V_P suggest the presence of ophiolitic assemblages or mid-Miocene volcanics. In contrast, a body with normal V_P and low V_P/V_S that is identified in the upper crust beneath Santa Monica Bay may be a fragment of quartz-rich continental crust. The V_P model of both the Ventura and the Los Angeles basins shows features that suggest deformation of the hanging wall or basin closure. At seismogenic depths of 16 km the hypocenters of moderate-sized and large earthquakes are located within or adjacent to high-V_P-velocity bodies. In most cases these high-velocity bodies form the upper block, consistent with shortening of the lower crust as described in thick-skinned tectonic interpretations.

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