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

Hauksson, Egill and Haase, Jennifer S. (1997) Three-dimensional V_P and V_P/V_S Velocity Models of the Los Angeles basin and central Transverse Ranges, California. Journal of Geophysical Research B, 102 (B3). pp. 5423-5453. ISSN 0148-0227. doi:10.1029/96JB03219.

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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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Hauksson, Egill0000-0002-6834-5051
Additional Information:© 1997 American Geophysical Union. Received April 10, 1996; revised October 7, 1996; accepted October 16, 1996. S. Hough and L. Jones provided helpful critical reviews. G. Abers, T. Davis, and C. Jones provided constructive JGR reviews. We are grateful to the seismic analysts of Caltech, the USGS, and USC for competent processing of the earthquake data. We thank D. Eberhart-Phillips and C. Thurber for help with the SIMULFS code and R. Clayton, T. Wright, T. Bracher, and S. Chavez for constructive comments. This research was partially supported by USGS grant 1434-94-G-2440, USGS cooperative agreement 1434-95-A-01300, and NSF grant 94-16119 to Caltech. Southern California Earthquake Center publication 325; partial SCEC support from NSF Cooperative Agreement EAR-8920136 and USGS Cooperative Agreement 14-08-0001-A0899. Contribution 5672, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena. Electronic version of the velocity models is available form the first author via e-mail or from the Southern California Earthquake Center, Data Center, anonymous FTP at in /publhauksson.
Group:Seismological Laboratory, Division of Geological and Planetary Sciences
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Southern California Earthquake Center (SCEC)325
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Caltech Division of Geological and Planetary Sciences5672
Issue or Number:B3
Record Number:CaltechAUTHORS:20131126-105839496
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Official Citation:Hauksson, E., and J. S. Haase (1997), Three-dimensional VP and VP/VS Velocity Models of the Los Angeles basin and central Transverse Ranges, California, J. Geophys. Res., 102(B3), 5423–5453, doi:10.1029/96JB03219.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42739
Deposited By: Ruth Sustaita
Deposited On:27 Nov 2013 16:51
Last Modified:10 Nov 2021 16:27

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