Beneath the bustling and vibrant Southern California, two of the most important fault systems of the region, the San Andreas Fault and the San Jacinto Fault, come together at Cajon Pass. These faults, or fractures, are the venting point of stress that has accumulated in the Earth’s crust, and earthquakes release the pent-up pressure through them. Since the magnitude 7.9 Fort Tejon earthquake in 1857, stress has continued to build up in the area. Scientists warn of the possibility of a major earthquake in the future because of the unusually dormant period the fault segments have experienced, allowing pressure to accumulate.
Cajon Pass is an Earthquake Gate
Massive tectonic plates move past one another and get stuck at faults, allowing pressure to build for years or even centuries before an earthquake hits. 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 examined 1,000 years of earthquake activity along the San Andreas and San Jacinto fault systems to evaluate current stress levels at Cajon Pass.
The findings show that tectonic stress in the region has reached, and even surpassed, the levels seen at any point during the past millennium. The researchers also introduced a new concept to describe Cajon Pass as an “earthquake gate,” a key junction that can provide insights into whether a large earthquake remains confined to a single fault or spreads across both fault systems simultaneously.
Exploring a Millennium of Earthquake Activity
To understand how stress has changed over time, scientists have developed a four-dimensional model that simulates fault behavior in three dimensions and tracks changes over time. Researchers supplied the model with a 1,000-year earthquake history reconstructed from geological evidence, including carbon dating, tree-ring records, and historical observations 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 results are quite concerning. The findings reveal that stress levels across the region are now higher than at any other point during the 1,000-year period examined by the model.
“Earthquake Gate” Explains Earthquake Behavior
Cajon Pass being an “earthquake gate” is a decisive factor. The researchers describe it as a fault junction that can influence whether a rupture (the physical breaking of ground that causes an earthquake) stops on one fault or continues across both the San Andreas and San Jacinto systems.
Looking at past earthquakes, the researchers found examples of both. The Fort Tejon earthquake of 1857 stopped at Cajon Pass and didn’t rupture the San Jacinto Fault. However, the Wrightwood earthquake of 1812 crossed the junction and spread through both fault systems in a singular event. The earthquake gate concept shows how fault junctions work. The Cajon Pass doesn’t simply block or channel earthquakes but responds to stress conditions that can change over centuries.
The key factor here is not just the amount of stress on an individual fault, but also how closely the stress levels of the two fault systems match. When stress has peaked on both faults, conditions become more favorable for a large rupture to cross the junction and propagate through both systems. If the stress levels differ greatly, the ruptures will likely stop at Cajon Pass rather than continue farther.
The model estimates that stress has reached 3.6 MPa (a unit of pressure) in the San Jacinto-Bernardino section, crossing the highest value recorded during the 1,000-year simulation. The neighboring Mojave South section of the San Andreas Fault currently stands at 2.8 MPa. Both sections have accumulated high and relatively similar amounts of stress, a setup that has historically preceded multi-fault ruptures.
The Aftermath of a Multi-Fault Earthquake
An earthquake along the San Andreas Fault is the main event in the 2015 movie San Andreas, starring Dwayne “The Rock” Johnson. While movies often go overboard when depicting devastation, an earthquake that ruptures both the San Andreas and San Jacinto faults through Cajon Pass would be quite severe. Such an event could affect some of the most densely populated regions that are dependent on infrastructure. Greater Los Angeles area, Bernardino, Riverside, and the Coachella Valley will be hit hard. Cajon Pass itself is a major transportation hub with rail lines and energy infrastructure, and an earthquake can jeopardize relief and evacuation.
Right now, one of the greatest problems is predicting when the next major earthquake will occur in this region. However, the findings do not necessarily constitute a predictive scheme. They do, however, emphasize that the stress might have reached a tipping point, and paint a clearer picture of the range of scenarios we should be prepared for. The information is best used for hazard assessment, infrastructure planning, and emergency preparedness, comments Burkhard.
Copyright @smorescience. All rights reserved. Do not copy, cite, publish, or distribute this content without permission.
