Southern California’s most dangerous fault systems are carrying more tectonic stress than at any point in the last millennium, according to research published earlier this month in the Journal of Geophysical Research: Solid Earth.
The San Andreas and San Jacinto faults, which run through some of the region’s most populous areas, have accumulated stress for more than 160 years since the last major rupture. Scientists at the University of Hawaii at Mānoa built a computer model that simulated 1,000 years of earthquake history to measure current stress levels across multiple fault segments.
“Our results show that stress levels on multiple fault segments are now at or above the highest values seen in the past millennium and that the region may be capable of a large through-going rupture involving both fault systems,” study lead author Liliane Burkhard said in a statement.
A key variable is Cajon Pass, the mountain pass where the two fault systems intersect. The research describes it as an “earthquake gate” that can either halt a rupture traveling along one fault or allow it to jump to the other system, producing a much larger combined event.
“The conditions that determine whether the ‘earthquake gate’ at Cajon Pass opens or stays closed appear to be related to how closely the stress levels on the two fault systems are aligned with each other at the time of rupture,” Burkhard said.
The computer model found that stress has now reached levels that could “facilitate a joint rupture of both the San Andreas and San Jacinto faults simultaneously,” which would produce a quake far more destructive than a single-fault event and would affect communities across Southern California.
The study does not forecast when such an earthquake might happen, Burkhard emphasized. “What we can say is that the system is critically stressed, and that physics-based models like this one give us a clearer picture of the range of scenarios we should be prepared for,” she said.
The last major destructive earthquake in the region was the 1994 Northridge quake, which struck before dawn, destroyed roughly 87,000 homes and businesses precipitating broader disaster, and killed more than 60 people. Since then, stress has continued to accumulate on the faults that many seismologists consider the state’s greatest natural hazard.
The research team’s findings represent some of the most detailed stress mapping of the interconnected fault network, using a physics-based modeling approach rather than statistical projections. The scientists said the work is intended to help emergency planners and engineers design scenarios for potential large ruptures, even if the timing of such an event remains unpredictable.