Stress along the San Andreas and San Jacinto fault systems in Southern California has reached, and in some places exceeded, the highest levels seen in the past 1,000 years, according to new research led by the University of Hawaiʻi at Mānoa.
The study, published in the Journal of Geophysical Research: Solid Earth, looked at how stress has built up and released along two of California’s most important fault systems.
Lead author Liliane Burkhard said the findings suggest the region may be capable of a large rupture involving both the San Andreas and San Jacinto faults.
Researchers built a physics-based computer model to simulate stress across the southern San Andreas and San Jacinto fault systems. The model included Cajon Pass, a key area where the two systems interact.
The team fed the model with about 1,000 years of earthquake history reconstructed from geological evidence. That included records from displaced sediments, radiocarbon dating, and tree-ring data.
By running the model forward to today, researchers estimated that stress has continued to build instead of being released by a major rupture.
The study found that Cajon Pass may act like an “earthquake gate.” In some cases, it may stop a rupture from moving between faults. In other cases, it may allow a rupture to pass through both systems in one event.
That matters because a joint rupture could be more damaging than an earthquake on just one fault. It could affect major population centers including Los Angeles, San Bernardino, Riverside, and the Coachella Valley.
Burkhard said more than 160 years have passed since the last major rupture in the region. She described the system as being in a critically loaded state.
Scientists stressed that the study does not predict when an earthquake will happen. Earthquakes cannot be forecast with exact dates or times.
Instead, the findings help researchers understand what kinds of earthquake scenarios Southern California should prepare for. The study may improve hazard assessments, emergency planning, infrastructure decisions, and building codes.
The U.S. Geological Survey Earthquake Hazards Program says earthquake science focuses on hazards, risk, monitoring, and preparedness rather than exact prediction.
The San Andreas Fault is the main boundary between the Pacific and North American tectonic plates. The San Jacinto system runs through parts of Southern California and is one of the region’s most active fault zones.
Because millions of people live near these faults, even a scientific update can carry major public interest. Roads, water systems, power lines, hospitals, schools, and homes could all be affected by a large earthquake.
The new research does not mean a major quake is about to happen today or tomorrow. But it does show why scientists continue to study the fault system closely, and why emergency preparation remains important across Southern California.