Attenuation models (Q_P and Q_S) 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 Q_P and Q_S 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 Q_P and Q_S models image the major tectonic structures and to a much lesser extent the thermal structure of the southern California crust. The near-surface low Q_P and Q_S 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 Q_P and Q_S zones, which correspond to the dense and high-velocity rocks of the mountain ranges. Several clear transition zones of rapidly varying Q_P and Q_S coincide with major late Quaternary faults and connect regions of high and low Q_P and Q_S. At midcrustal depths, the Q_P and Q_S 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, Q_S/Q_P is greater than 1.0, suggesting partially fluid-saturated crust. A few limited regions of Q_S/Q_P 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.