Imaging P and S attenuation in the Sacramento-San Joaquin Delta region, northern California

Bulletin of the Seismological Society of America
By: , and 

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Abstract

We obtain 3-D Qp and Qs models for the Delta region of the Sacramento and San Joaquin Rivers, a large fluvial-agricultural portion of the Great Valley located between the Sierra Nevada batholith and the San Francisco Bay - Coast Ranges region of active faulting. Path attenuation t* values have been obtained for P and S data from 124 distributed earthquakes, with a longer variable window for S based on the energy integral. We use frequency dependence of 0.5 consistent with other studies, and weakly favored by the t* S data. A regional initial model was obtained by solving for Q as a function of velocity. In the final model, the Great Valley basin has low Q with very low Q (<50) for the shallowest portion of the Delta. There is an underlying strong Q contrast to the ophiolite basement which is thickest with highest Q under the Sacramento basin, and a change in structure is apparent across the Suisun Bay as a transition to thinner ophiolite. Moderately low Q is found in the upper crust west of the Delta region along the faults in the eastern North Bay Area, while, moderately high Q is found south of the Delta, implying potentially stronger ground motion for earthquake sources to the south. Very low Q values in the shallow crust along parts of the major fault zones may relate to sediment and abundant microfractures. In the lower crust below the San Andreas and Calaveras-Hayward-Rodgers Creek fault zones, the observed low Q is consistent with grain-size reduction in ductile shear zones and is lowest under the San Andreas which has large cumulative strain. Similarly moderately low Q in the ductile lower crust of the Bay Area block between the major fault zones implies a broad distributed shear zone.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Imaging P and S attenuation in the Sacramento-San Joaquin Delta region, northern California
Series title Bulletin of the Seismological Society of America
DOI 10.1785/0120130336
Volume 104
Issue 5
Year Published 2014
Language English
Publisher Seismological Society of America
Contributing office(s) Earthquake Science Center
Description 15 p.
First page 2322
Last page 2336
Country United States
State California
Other Geospatial Sacramento-San Joaquin Delta Region