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

Abstract

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.