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

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.