Precariously balanced rocks suggest San Jacinto and San Andreas may have ruptured together, raising issues for critical infrastructure resilience planning.
Stacked in gravity-defying arrangements in the western San Bernardino Mountains, granite boulders that should have been toppled long ago by earthquakes are maintaining a stubborn if precarious balance. In puzzling out why these rocks still stand, researchers have uncovered connections between Southern California's San Jacinto and San Andreas faults that could change how the region plans for future earthquakes.
In their study published online on August 5th in Seismological Research Letters (SRL), Lisa Grant Ludwig of University of California, Irvine and colleagues write that the precariously balanced rocks (PBRs) have survived as a result of interactions between the faults that have weakened earthquake ground shaking near the rocks.
One such interaction, the researchers say, might be a rupture that began on the San Andreas Fault but then jumped over to the San Jacinto Fault, near Cajon Pass. "These faults influence each other, and it looks like sometimes they have probably ruptured together in the past," said Grant Ludwig. "We can't say so for sure, but that's what our data point toward, and it's an important possibility that we should think about in doing our earthquake planning."
Cajon Pass is the site of "some very important, lifeline infrastructure like I-15, and we should be considering the possibility that there might be broader disruptions in that area," Grant Ludwig added.
Most of the seismic hazard maps that engineers and others use to guide the design of buildings, aqueducts and other important infrastructure often only account for the ground shaking and other impacts produced by ruptures along one fault, she noted.
"This paper suggests that we might consider the impact of a rupture that involves both the San Jacinto and San Andreas Faults, which has the potential to affect more people than just the San Andreas or just the San Jacinto," Grant Ludwig said.
