Attenuation models (QP and QS) in three dimensions of the southern California crust: Inferred fluid saturation at seismogenic depths

Abstract

We analyze high dynamic range waveform spectra to determine t* values for both P and S waves from earthquakes in southern California. We invert the t* values for three‐dimensional (3‐D) frequency‐independent QP and QS regional models of the crust. The models have 15 km horizontal grid spacing and an average vertical grid spacing of 4 km, down to 22 km depth, and extend from the U.S.‐Mexico border to the Coast Ranges in the south and Sierra Nevada in the north. In general, QP and QS increase rapidly with depth, consistent with crustal densities and velocities. The 3‐D QP and QS models image the major tectonic structures and to a much lesser extent the thermal structure of the southern California crust. The near‐surface low QP and QS zones coincide with major sedimentary basins such as the San Bernardino, Chino, San Gabriel Valley, Los Angeles, Ventura, and Santa Maria basins and the Salton Trough. In contrast, at shallow depths beneath the Peninsular Ranges, southern Mojave Desert, and southern Sierras, we image high QP and QS zones, which correspond to the dense and high‐velocity rocks of the mountain ranges. Several clear transition zones of rapidly varying QP and QS coincide with major late Quaternary faults and connect regions of high and low QP and QS. At midcrustal depths, the QP and QS exhibit modest variation in slightly higher and lower QP or QS zones, which is consistent with reported crustal reflectivity. In general, for the southern California crust, QS/QP is greater than 1.0, suggesting partially fluid‐saturated crust. A few limited regions of QS/QP less than 1.0 correspond to areas mostly outside the major sedimentary basins, including areas around the San Jacinto fault, suggesting a larger reduction in the shear modulus compared to the bulk modulus or almost complete fluid saturation.