Teleseismic tomography of the Loma Prieta Earthquake Region, California: Implications for strain partitioning

From teleseismic travel times we derive three-dimensional velocity models of the upper 71 km in the 1989 Loma Prieta earthquake region, central California. Shallow crustal structure is consistent with local-earthquake tomography. Horizontal velocity gradients at all depths suggest that the San Andreas fault was a deep shear locus, at least at one time. A large low-velocity feature near the Moho beneath Loma Prieta probably is caused by a crustal root. Two low-velocity features at about 45–70 km depth are offset right-laterally along the San Andreas by about 45 km. Cooling of this portion of the upper mantle [Furlong et al., 1989] could have frozen in displacements in this region within a few million years after passage of the Mendocino Triple Junction. These results are consistent with Furlong et al.'s model.