Scientists at the University of Hawaii at Mānoa found that the San Andreas and San Jacinto fault systems near Cajon Pass in Southern California are under tectonic stress not seen in the past 1,000 years. Using computer simulations based on 1,000 years of earthquake history, they estimated stress on important fault segments and described the faults as being in a “critically loaded state.” The findings point to a higher risk of a large earthquake that could impact populated areas including Los Angeles, San Bernardino, Riverside, and the Coachella Valley.
The Cajon Pass junction is key to how a possible fault rupture might act. Located northeast of Los Angeles near San Bernardino, Cajon Pass is where segments of the San Andreas and San Jacinto faults meet. Researchers call this area an “earthquake gate,” which can either stop an earthquake rupture from crossing between faults or allow it to spread across both, causing a bigger seismic event. The study says the gate’s status depends on how close the stress levels on both faults are at the time of rupture.
Stress along these faults has been building for over 160 years since the last major rupture occurred. The computer model measured stress as 2.8 for the Mojave South segment, 1.8 for the North San Bernardino segment of the San Andreas Fault, and 3.6 for the San Jacinto Bernardino segment. That makes the San Jacinto segment the most stressed part at the Cajon Pass fault junction. The close high stress levels increase the chance that the faults could interact, possibly causing a large rupture across both systems.
Historical records show the San Andreas and San Jacinto faults have had at least 36 earthquakes of magnitude 6.4 or higher in the past 1,000 years. Together, they account for about 90% of the North American-Pacific plate slip rate in Southern California, explaining the ongoing strain. The last major quakes in southern California happened over 100 years ago. This differs from the northern San Andreas Fault, where the 1906 San Francisco earthquake took place.
Geophysicist Liliane Burkhard, lead author of the study published in the Journal of Geophysical Research: Solid Earth, said the research does not predict when the next large earthquake will happen but said the high stress levels are a concern. Burkhard said, “The system is critically stressed, and physics-based models like this one give us a clearer picture of the range of scenarios we should be prepared for.”
The Cajon Pass earthquake gate could greatly affect the size of future earthquakes. If the stress on the San Andreas and San Jacinto faults match closely, a rupture could jump between the two faults, causing a longer, more damaging earthquake than one on a single fault. This would threaten urban centers and critical infrastructure across Southern California.
Because the area has a high population and major transportation, energy, and commerce routes, understanding these faults is important. Building codes in Southern California have improved since past decades, but a joint rupture across both faults could still cause widespread damage.
Experts outside the study have noted the importance of these findings. Bill McGuire, Professor Emeritus of Geophysical and Climate Hazards at University College London, welcomed insight on fault interaction. He said, “The idea that the junction at Cajon Pass acts as an ‘earthquake gate’ controlling whether one or both faults rupture during a quake has important impacts for planning future earthquake scenarios.” McGuire also said that large earthquakes are due soon in both northern and southern California fault systems given the stress build-up.
The research stresses the need for continued monitoring and modeling to evaluate earthquake risks in Southern California. It is not possible to predict exactly when a major quake will occur, but the study shows that the faults near Cajon Pass have unprecedented tectonic stress, making a major earthquake more likely than in the past 1,000 years.
The 160 years of stress build-up highlights the need for preparedness among Californians in the region. Combining scientific models and historical earthquake records informs the public about risks, which is important for planning and emergency response in a highly active seismic area of the United States.
