San Andreas and San Jacinto Faults Reach Critical Stress Levels
Southern California’s San Andreas and San Jacinto fault systems are currently experiencing their highest levels of tectonic stress in a millennium, according to a recent study. Scientists describe this condition as a “critically loaded state,” which increases the likelihood of a significant earthquake event in the future.
“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,”said Liliane Burkhard, the lead author of the study published in the Journal of Geophysical Research: Solid Earth.
The Role of Cajon Pass
The study highlights Cajon Pass, located at the junction of the San Andreas and San Jacinto faults, as a critical area that could influence seismic activity. This location acts as an “earthquake gate,” potentially blocking or allowing large ruptures to propagate between the two fault systems.
It has been over 100 years since the last major seismic event in this region. During this time, tectonic stress has continued to accumulate, reaching unprecedented levels, which raises the probability of a large earthquake occurring.
“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 explained.
“Right now, with stress at historically high levels across the region and more than 160 years elapsed since the last major rupture, the system is in a critically loaded state.”
Modeling Stress Accumulation and Earthquake Risk
The research team from the University of Hawaii at Mānoa employed a computer model to analyze how stress accumulates and is released along the fault systems. They incorporated 1,000 years of earthquake history into their simulations to estimate current stress levels.
The findings suggest that Cajon Pass could enable a simultaneous rupture of both the San Andreas and San Jacinto faults, potentially causing widespread impacts throughout southern California.
While the study does not predict the timing of such an earthquake, it provides critical insight into the risks involved.
“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.”
Historical Context and Preparedness
Californians have long anticipated the occurrence of the so-called “big one.” The last major destructive earthquake in the state was the 1994 Northridge earthquake in Los Angeles. This disaster struck just before dawn, destroying approximately 87,000 homes and businesses and resulting in over 60 fatalities.
