Bimaterial interfaces in the south San Andreas Fault with opposite velocity contrasts NW and SE from San Gorgonio Pass

Analysis of fault zone head waves indicates the existence of two deep large‐scale bimaterial interfaces in the structure of the southern San Andreas Fault (SAF) with opposite sense of velocity contrast. One interface extends from the Cajon Pass area to the San Gorgonio Pass (SGP) region and is associated with a slower seismic velocity on the southwest side of the SAF. The second interface extends from the SGP region to the Coachella Valley and involves a slower seismic velocity on the northeast side. Tomographic imaging of alternating velocity structures supports the velocity contrast reversal across the SAF. The observations and expected properties of bimaterial ruptures suggest that earthquakes nucleating slightly to the NW and SE of SGP tend to propagate along the SAF in the opposite along‐strike directions. This is consistent with geological and seismological evidence in the Mojave and southernmost sections of the SAF.