Three-dimensional attenuation variations in southern California

Abstract

Seismic attenuation (1/Q) in southern California varies substantially over distances of tens of kilometers. These variations are apparent in high-frequency spectral decay of body waves. We measure the spectral decay from earthquake seismograms at regional distances by estimating a (frequency-independent) decay parameter t* for both P and S waves, for frequencies of 0.5-25 Hz, and invert the resulting 1138 t* values to show spatial 1/Q variations in the crust and upper mantle. The P and S results agree closely with each other, and both correlate with regional tectonics. The upper crust (0.5-14 km depth) exhibits high 1000/Q (≥ 2.0; Q < 500) in the Los Angeles Basin and Transverse Ranges, compared to low 1000/Q (≈ 1.0; Q ∼ 1000) in the Mojave Desert. High 1/Q of the Los Angeles Basin probably reflects high porosity in basin sediments. The lower crust exhibits areas of high 1/Q beneath the Salton Trough (1000/QP ≈ 1.5 and 1000/QS ≈ 2.3, or QP ∼ 700 and QS ≈ 400) and beneath the San Gabriel Mountains (1000/QP ≈ 2.2 and 1000/QS ≈ 1.7), values 2-4 times larger than other areas. High 1/Q beneath the Salton Trough may reflect elevated temperature or partial melt resulting from active rifting. The high 1/Q beneath the San Gabriel Mountains may represent elevated temperature associated with active mountain building, as seen in similar tectonic settings elsewhere. Attenuation varies less in the mantle, perhaps because resolution is poorer. Relatively high QP/QS (∼1-5) beneath the Salton Trough may similarly indicate high temperatures. These measurements show large variation in 1/Q across the region, relevant to any complete model for ground shaking in southern California. Copyright 2001 by the American Geophysical Union.