Three-dimensional attenuation variations in southern California

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Journal of Geophysical Research-Solid Earth
Journal Date: 
Dec 10
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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 (approximate to 1.0; Q similar to 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/Q(P) approximate to 1.5 and 1000/Q(S) approximate to 2.3, or Q(P) similar to 700 and Q(S) similar to 400) and beneath the San Gabriel Mountains (1000/Q(P) approximate to 2.2 and 1000/Q(S) approximate to 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 Q(P)/Q(S) (similar to1.5) 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.


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