TELESEISMIC TOMOGRAPHY OF THE LOMA-PRIETA EARTHQUAKE REGION, CALIFORNIA - IMPLICATIONS FOR STRAIN PARTITIONING

From teleseismic travel times we derive three-dimensional velocity mod els of the upper 71 km in the 1989 Loma Prieta earthquake region, cent ral California. Shallow crustal structure is consistent with local-ear thquake tomography. Horizontal velocity gradients at all depths sugges t that the San Andreas fault was a deep shear locus, at least at one t ime. A large low-velocity feature near the Moho beneath Loma Prieta pr obably 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 ab out 45 km. Cooling of this portion of the upper mantle [Furlong et at, 1989] could have frozen in displacements in this region within a few million years after passage of the Mendocino Triple Junction. These re sults are consistent with Furlong et al.'s model.