Characterizing Seismic Properties of the Sacramento-San Joaquin River Delta, California

The Sacramento-San Joaquin River Delta is a component of the major basins east of the California Coast Ranges, and it contains levees that may fail in major earthquakes. The largest magnitude earthquake sources may occur on Bay Area faults, over 50-km away from the Delta, and thus regional attenuation is important to consider. Regionally, long-term seismic networks record numerous earthquakes and this data has been used for several regional-scale 3-D velocity studies. However there is little knowledge of 3-D Q structure. A small number (7-10) of broadband instruments have been deployed in the Delta since 2007, by the USGS (Fletcher and Sell, 2009) for site response studies. The Delta itself is relatively aseismic, yet the Delta array has provided useful records of regional earthquakes. We are using the permanent and temporary data to improve the velocity structure and to obtain the 3-D Q structure. For P-arrival windows, t* (t-star) is obtained, including site response, and then 3-D Q is computed. Stations across the Delta array indicate varied attenuation with apparent low Q in the central Delta. Velocity results are being used to refine the basin shape. Ref: Fletcher, J. B., R. Sell, Site response and slow basin waves in the Sacramento/San Joaquin Delta, Am. Geophys. U., Fall Meeting 2009, abstract #S42B-01, 2009.