Three-dimensional seismic velocity structure of the San Francisco Bay area

Seismic travel times from the northern California earthquake catalogue and from the 1991 Bay Area Seismic Imaging Experiment (BASIX) refraction survey were used to obtain a three-dimensional model of the seismic velocity stru cture of the San Francisco Bay area. Nonlinear tomography was used to simul taneously invert for both velocity and hypocenters. The new hypocenter inve rsion algorithm uses finite difference travel times and is an extension of an existing velocity tomography algorithm. Numerous inversions were perform ed with different parameters to test the reliability of the resulting veloc ity model. Most hypocenters were relocated <2 km from their catalogue locat ions. Large lateral velocity variations at shallow (<4 km) depth correlate with known surface geology, including low-velocity Cenozoic sedimentary bas ins, high-velocity Cenozoic volcanic rocks, and outcrop patterns of the maj or Mesozoic geologic terranes. Salinian are rocks have higher velocities th an the Franciscan melange, which in turn are faster than Great Valley Seque nce forearc rocks. The thickess of low-velocity sediment is defined, includ ing >12 km under the Sacramento River Delta, 6 km beneath Livermore Valley, 5 km beneath the Santa Clara Valley, and 4 km beneath eastern San Pablo Ba y. The Great Valley Sequence east of San Francisco Bay is 4-6 km thick. A r elatively high velocity body exists in the upper 10 km beneath the Sonoma v olcanic field, but no evidence for a large intrusion or magma chamber exist s in the crust under The Geysers or the Clear Lake volcanic center. Lateral velocity contrasts indicate that the major strike-slip faults extend subve rtically beneath their surface locations through most of the crust. Strong lateral velocity contrasts of 0.3-0.6 km/s are observed across the San Andr eas Fault in the middle crust and across the Hayward, Rogers Creek, Calaver as, and Greenville Faults at shallow depth. Weaker velocity contrasts (0.1- 0.3 km/s) exist across the San Andreas, Hayward, and Rogers Creek Faults at all other depths. Low spatial resolution evidence in the lower crust sugge sts that the top of high-velocity mafic rocks gets deeper from west to east and may be offset under the major faults. The data suggest that the major strike-slip faults extend subvertically through the middle and perhaps the lower crust and juxtapose differing lithology due to accumulated strike-sli p motion. The extent and physical properties of the major geologic units as constrained by the model should be used to improve studies of seismicity, strong ground motion, and regional stress.