Imaging P and S Attenuation in the Sacramento-San Joaquin Delta Region, Northern California

Abstract

We obtain 3D Q P and Q S 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 with an exponent 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 seismic 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, whereas 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. Online Material: Distribution of earthquake magnitude, figures of t * values, 3D initial regional velocity and Q models, station terms, and Q P inversion with 1D initial Q P model.