Northern California Seismic Attenuation: 3D Q_P and Q_S Models

The geological complexity of northern California and its accreted terranes produces pronounced regional variations in the attenuation (1/Q) of earthquake waves. Using local earthquakes, 3D Q_P and Q_S models have been obtained for this region that includes the San Andreas fault system, the Great Valley, the Sierra Nevada batholith, and the Gorda subduction volcanic system. Path attenuation t^* values were determined from P and S spectra of 959 spatially distributed earthquakes, magnitude 2.5–6.0, using 1254 stations from Northern California Earthquake Data Center networks and Mendocino and Sierra Nevada temporary arrays. The t^* data were used in Q inversions, using existing 3D velocity models and basic 10 km node spacing. Uneven data coverage was accommodated with linking nodes into larger areas for useful Q images across the 3D volume. Results at shallow depth show very low Q in the Sacramento Delta, the Eureka area, and parts of the San Francisco Bay area. In the brittle crust, low Q tends to be associated with regions of fault zones and microearthquakes. In the ductile lower crust, low Q is observed below fault zones with large cumulative displacement and inferred grain size reduction, with apparent broad ductile shear zones across the fault system in the Coast Ranges. Underlying active arc volcanoes, low Q features are apparent below 20 km depth. Under the Long Valley caldera, two distinct partial melt features at ∼3 and ∼15  km depth are inferred. Moderately high Q is associated with Sierra Nevada and Salinian igneous rocks, whereas the Franciscan subduction complex shows moderately low Q. Prominent high Q relates to the Great Valley ophiolite. The pattern of path‐specific attenuation rate, from 3D Q, is generally similar to the pattern of psuedospectral acceleration residuals for the South Napa earthquake, indicating that it may be beneficial to incorporate 3D Q models into future ground‐motion prediction equations for northern California.

Online Material: Figures of Q_P and Q_S resolution and station delays for t^*, and table of Q_P, V_P, Q_S, and V_S.