EVIDENCE FROM PRECISE EARTHQUAKE HYPOCENTERS FOR SEGMENTATION OF THE SAN-ANDREAS FAULT IN SAN-GORGONIO PASS

We use precise hypocenter patterns and focal mechanisms to investigate the presence or absence of a continuous strike-slip fault at depth co nnecting the San Bernardino strand of the San Andreas fault with the C oachella Valley segment of the Banning fault. We inverted 560,000 arri val times from 23,000 earthquakes (1981-1993) for high-quality hypocen ters and three-dimensional P wave velocity structure in a 1 degrees by 2 degrees area centered on the San Gorgonio Pass. Cross-sectional plo ts of relocated earthquakes reveal an abrupt 5 to 7 km high step in th e maximum depth of hypocenters, The step riser defines a near-vertical , locally curved surface that extends westerly more than 60 km from th e Coachella Valley segment of the San Andreas fault to the San Jacinto fault. A hypothetical continuous vertical San Andreas fault through S an Gorgonio Pass would cross the step at an oblique angle. We suggest that the step is the expression of the contact between different basem ent rock types juxtaposed by large-scale right-slip motion on the ance stral San Andreas fault. South of the step in Peninsular Ranges type b asement (intrusives), brittle failure occurs down to about 20-km depth , while north of the step in San Bernardino type basement (Pelona schi st), brittle failure occurs to only about 13-km depth. The step provid es a piercing plane that should be offset about 3 km right laterally b y an active, continuous, vertical San Andreas fault. Within the resolu tion of our mapping the step is not offset in this manner, implying ei ther that there has not been a throughgoing vertical fault at depth, t hat a throughgoing fault has not experienced enough slip to offset the step, or that a throughgoing fault is not vertical and dips north ove r the top of the step, Hypocentral patterns and focal mechanisms indic ate distributed deformation (thrust, normal, and strike-slip faulting) over a large volume in the San Gorgonio Pass region; there is no evid ence of hypocenter or slip vector alignments that would indicate a thr oughgoing, continuous, near-vertical San Andreas fault. In summary, we find no evidence indicating a continuous fault at seismogenic depth c onnecting the San Bernardino strand and Coachella Valley segment of th e San Andreas fault zone. We speculate that this is because the 3 km o f right slip on the San Bernardino stand of the San Andreas fault and Coachella Valley segment of the Banning fault has not been sufficient to form a single new structure through the 15- to 20-km gap between th e two previously unconnected segments. This implies that large earthqu ake rupture on the San Andreas fault may be inhibited from propagating through San Gorgonio Pass, thus limiting the maximum magnitudes on th e southern San Andreas fault.