INVESTIGATIONS
The emphasis of the Southern California Earthquake Center on predicting ground motions of scenario earthquakes for high risk areas makes an accurate 3-D velocity model for southern California more important than ever. In particular the majority of waveforms from any large earthquake are recorded at regional distances and not in the near-field. Hence, mapping the 3-D shapes of regional refractors becomes very important.
RESULTS: Hadley and Kanamori Refractors in 3D
The one dimensional velocity model by Hadley and Kanamori (1977) of southern California has prominent refractors of 6.3 and 6.7 km/s at 5.5 and 16.0 km depth. To explore the spatial extent of these refractors, we use a modified Hadley and Kanamori model with a surface layer of 10% lower velocity, and minor gradients at the refractor boundaries as a starting model.
We use P and S arrival times from 12,000 earthquakes and timed explosions, recorded by the Southern California Seismographic Network, to invert for the three-dimensional P- velocity (VP) and the P and S-velocity ratio (VP/VS ) in southern California. 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, and 20 km spacing of horizontal grid nodes. Layers of grid nodes are placed at depths of 1, 4.5, 6.5, 10, 15.5, 16.5, and 20 km. The data variance decreased significantly in the gradational inversion. Ample data from recent major 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 final 3-D model that has an upper refractor of 6.0 km/s ranges in depth from about 5 km to 12 km below the deepest basins (Figure 1). The 6.7 km/s refractor is present beneath the Peninsular Ranges, the Tehachapi Mountains, and Imperial Valley. The Mojave Desert and the Transverse Ranges lack the 6.7 km/s refractor and thus have lower VP velocities than predicted by the Hadley and Kanamori model. The high VP/VS ratios in the near surface are consistent with high pore fluid pressures in the basin sediments. At depth beneath the Ventura basin and Imperial Valley, the high VP/VS and high VP suggest the presence of ophiolitic assemblages or mid-Miocene volcanics. We relocate the southern California earthquake catalog of more than 250,000 earthquakes to compare patterns of earthquake distribution with mapped tectonic structures.
Publications
Hauksson, E., Exploring the Three-Dimensional Aspects of the Refractors in the Hadley and Kanamori (1977) Velocity Model of Southern California, (abstract), 9th Annual IRIS Workshop, Beaver Run, CO, June 8-12, 1997.
Hauksson, E., Improved Three-Dimensional Vp And Vp/Vs Velocity Models For Southern California, (abstract), 1997 Annual Meeting, Southern Calif. Earthquake Center, Costa Mesa, CA, October 4-7, 1997.
Hauksson, E., Improved Three-Dimensional Vp And Vp/Vs Velocity Models For Southern California And Relocation Of The Seismicity, (abstract), 1997 Annual Fall Meeting, San Francisco, CA, December 8-12, 1997.
