The southern San Andreas has a smaller, neighboring fault to its west

Meet the San Andreas Fault’s newfound neighbor. Mapping deformations deep underground along the shoreline of a Southern California lake called the Salton Sea, seismologists discovered a fault that runs parallel to San Andreas’ southern end.

The newly identified fault, dubbed the Salton Trough Fault, shakes up assessments of the potential for damaging earthquakes in the region, according to Valerie Sahakian, a seismologist at the U.S. Geological Survey in Menlo Park, Calif. She and colleagues report the discovery of the fault in the October Bulletin of the Seismological Society of America.

The Salton Trough Fault may accommodate some of the strain from the larger San Andreas system to the east, which has gone about 300 years without a major temblor. But the fault could also trigger, or be triggered by, a quake along the San Andreas, the researchers say. It could also amplify the effects of a San Andreas quake by causing its seismic waves to reverberate. Future assessments of the region’s vulnerability, the researchers write, should consider these possibilities.