Earthquake model reveals: Stress in California at record level

Earthquakes usually occur along fracture zones in the Earth's crust, where large tectonic plates slide past one another and become locked. Stress builds up over long periods of time and is suddenly released in the form of an earthquake. In Southern California, the San Andreas and San Jacinto faults are among the most significant of these zones, accommodating the majority of the plate motion in the region. Where the two fault systems approach each other northeast of Los Angeles lies the Cajon Pass – a tectonically complex junction where a rupture on one fault could potentially cross onto the other. Since the last major earthquake to affect the wider Los Angeles region, the Fort Tejon earthquake of 1857 with a magnitude of 7.9, tectonic stress along the fault segments has built up continuously, which is a prolonged quiet period that has long concerned researchers given the potential for a large future rupture.

In a new study led by Dr. Liliane Burkhard of the Division of Space Research and Planetary Sciences (WP) at the Physics Institute of the University of Bern, an international research team has modeled 1,000 years of earthquake history along the southern San Andreas and San Jacinto fault systems to estimate the present-day stress loading at Cajon Pass. Researchers from the University of Hawaiʻi at Mānoa, the U.S. Geological Survey Earthquake Science Center in Pasadena and the Scripps Institution of Oceanography at UC San Diego were involved. The results show that tectonic stresses in the region have reached and, in some cases, exceeded the highest levels of the last millennium. In the study, the researchers also introduce the concept of Cajon Pass as an "earthquake gate": a junction that controls whether large earthquakes remain confined to a single fault or propagate across both systems simultaneously. The study has just been published in Journal of Geophysical Research: Solid Earth.

Modeling 1,000 years of earthquake history

To investigate how the stress along the San Andreas and San Jacinto faults and at the critical Cajon Pass junction has evolved over time, the research team constructed a physics-based, four-dimensional earthquake cycle model that simulates the processes in three spatial dimensions and over time. The researchers then fed the model with a 1,000-year earthquake record reconstructed from geological evidence such as radiocarbon dating, tree-ring anomalies, and historical documentation of ground ruptures.

"The model tracks how each earthquake changes stress on neighboring fault segments, how stress accumulates during the quiet intervals between events, and how the deeper layers of the crust slowly relax following large ruptures," explains Burkhard. "This simulation allows us to understand how stresses in the fault system build up over centuries,” continues Burkhard. “By running the earthquake history of Southern California as a simulation, we can estimate the extent to which the fault system is already under stress today." The researchers show that stresses in the region are currently at their highest level in the last 1,000 years.

"Earthquake gate" as a decisive key factor
A key finding of the study is that the Cajon Pass can act as a so-called "earthquake gate", a junction that controls whether large ruptures remain confined to a single fault or cross both fault systems. Historical examples of both behaviors exist: The Fort Tejon earthquake of 1857 terminated at Cajon Pass and did not involve the San Jacinto Fault, while the Wrightwood earthquake of 1812 ruptured through the junction and across both systems in a single through-going event. "The earthquake gate concept captures something important about how fault junctions work," explains Burkhard. "Cajon Pass doesn’t simply block or channel earthquakes: It responds to stress conditions, and those conditions change over centuries."

The study also shows that the decisive factor is not only how much stress has built up on a single fault but how aligned the stresses on the two fault systems are. When the stress on both faults rises in concert over time, toward similarly high levels, conditions favor a large joint rupture crossing both systems. When stress levels evolve out of step with each other, ruptures are more likely to terminate at the junction rather than propagate further. Currently, modeled stress has reached 3.6 MPa on the San Jacinto-Bernardino section, exceeding the highest value seen anywhere in the 1,000-year simulation. On the neighboring Mojave South section of the San Andreas fault, it is 2.8 MPa. Both segments are therefore highly and relatively similarly stressed, placing the system in a configuration that historically has preceded joint ruptures. "So not only is it concerning that the stresses are reaching historic highs," says Burkhard, "but also that the relative stress conditions between the two fault systems are approaching the range we associate with major ruptures crossing both faults simultaneously – and that is a scenario with much larger consequences for the region."

Increased risk in densely populated regions
A joint rupture of the San Andreas fault and the San Jacinto fault that crosses the Cajon Pass would be a much more severe event than one that is limited to a single fault. The affected region includes some of the most densely populated, infrastructure-critical corridors in the U.S., including the greater Los Angeles area, San Bernardino, Riverside and the Coachella Valley. Major highways, railroads and energy infrastructure run through the Cajon Pass itself.

"The question of when and how the next major earthquake will occur in this region is one of the most pressing problems in applied geoscience. Our results provide a clearer, physics-based picture of the current stress state of the fault system, and the framework we developed is not just applicable to California, but also for other complex fault junctions worldwide," says Burkhard.

However, Burkhard emphasizes: "The study is not a prediction of when an earthquake will occur. What we can say is that the system is critically stressed and that physics-based models like ours give a clearer picture of the range of scenarios we should be prepared for. This information is important for hazard assessment, infrastructure planning and emergency preparedness."