A Bimaterial Interface Along the Northern San Jacinto Fault Through Cajon Pass

We investigate the existence of bimaterial interfaces along the San Jacinto fault zone (SJFZ) and the San Andreas fault (SAF) around their intersection at Cajon Pass (CP) to clarify if large earthquakes can rupture simultaneously parts of both faults. Analysis of fault zone head waves reveals a bimaterial interface along the northern SJFZ with a >7.5% velocity contrast (slower northeast side) that extends continuously through and perhaps slightly beyond CP. Other seismological studies show an opposite sense of velocity contrast across the nearby SAF. These observations, combined with expected properties of bimaterial ruptures, suggest that large earthquakes on the northern SJFZ are likely to propagate to the southeast and hence unlikely to trigger large SAF events. However, large earthquakes propagating on the SAF near CP can trigger continuing rupture on the SJFZ to the southeast. This inference is consistent with ruptures of large paleoseismic earthquakes and directivities of smaller events.