Tectonic stress along the San Andreas and San Jacinto fault systems in Southern California has now reached, and in some places exceeded, the highest levels seen in the past 1,000 years, according to research led by Earth scientists at the University of Hawaiʻi at Mānoa. The study, published in Journal of Geophysical Research: Solid Earth , has direct implications for seismic hazard assessments in one of the most densely populated and infrastructure-critical corridors in the U.S.
"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 lead author Liliane Burkhard, research affiliate in the Hawaiʻi Institute of Geophysics and Planetology at the UH Mānoa School of Ocean and Earth Science and Technology and scientist at the University of Bern, Switzerland. "We also found that Cajon Pass may act as an 'earthquake gate': sometimes blocking large ruptures from crossing between the faults, and sometimes allowing them to pass through and involve both systems in a single event."
1,000 years of earthquake history
The researchers built a physics-based computer model that simulates how stress builds up and releases along the southern San Andreas and San Jacinto fault systems, including at Cajon Pass, which is a critical junction between the two fault systems. They fed the model a 1,000-year record of earthquake history of the region reconstructed from geological evidence such as radiocarbon dating of displaced sediments and tree-ring records. By running this simulation forward to the present day, they estimated how much stress has built up.
"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. "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."
Results from this study suggest that the stress that would normally be released in large earthquakes has continued to accumulate and is now at unprecedented levels. Perhaps most importantly, the study showed that Cajon Pass could facilitate a joint rupture of both the San Andreas and San Jacinto faults simultaneously, which is a scenario that could be significantly more damaging than a single-fault event, and one that affects densely populated areas including Los Angeles, San Bernardino, Riverside and the Coachella Valley.
Improving earthquake hazard research
This kind of physics-based stress modeling can help refine seismic hazard assessments and inform infrastructure planning, emergency preparedness, and building codes in the region. Additionally, the modeling framework used in this study is applicable to other complex fault junctions globally, so the researchers are interested in developing it as a reusable tool for multi-fault hazard assessments.
"This is not a prediction of when an earthquake will happen," Burkhard said. "However, studies like this are important contributions to national and global earthquake hazard research in that we are using rigorous, quantitative science to better understand the risk facing millions of people. 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. That information matters for hazard assessments, infrastructure planning, and emergency preparedness."
Additional authors of the study include researchers from Northern Arizona University, University of Bern, U.S. Geological Survey and University of California, San Diego.
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