Southern California’s Next Major Quake May Depend on an ‘Earthquake Gate,’ Study Finds

A mountain pass near Los Angeles may play a much bigger role in Southern California’s earthquake future than previously understood.

According to a new study, a stretch of terrain northeast of Los Angeles known as the Cajon Pass could act as an “earthquake gate,” which is a place that can decide whether a rupture remains confined to a single fault system or spreads into a larger event involving multiple faults.

The idea offers a new way to assess the risk of earthquakes in Southern California.

Instead of considering major faults as isolated features that rupture independently, the research suggests that interactions between them could define the size and reach of future earthquakes.

For the study, which was published in the Journal of Geophysical Research: Solid Earth, researchers reconstructed 1,000 years of the region’s seismic history using historical records, geological evidence, radiocarbon dating, and tree-ring records.

Most Important Fault Systems in the Region

The study focused on two of the region’s most important fault systems: the San Andreas Fault and the San Jacinto Fault.

Both fault systems accommodate the slow movement between tectonic plates beneath Southern California and have produced significant earthquakes throughout history.

According to the U.S. Geological Survey (USGS), the San Andreas Fault is the region’s most famous fault and can produce earthquakes as large as magnitude 8.

While the San Andreas fault is well known, the San Jacinto Fault is the most active and has generated some of the region’s largest earthquakes, including the magnitude 7.9 earthquake of 1857, which was felt in Los Angeles, according to the USGS.

While the new study does not predict a repeat of such an intense earthquake, it suggests that the movements of the fault systems can influence how future earthquakes occur.

Researchers reconstructed how stress has built up and shifted across these faults over the last millennium.

The resulting model helped them to understand how one earthquake may influence the next, sometimes centuries later.

One region was identified as the most concerning.

Critical Junction

The Cajon Pass emerged as a critical junction between the two fault systems.

Researchers called this area an “earthquake gate” because conditions there could either prevent a rupture from spreading or allow it to jump from one fault network to another.

If the “gate” remains closed, an earthquake may rupture along one fault before stopping. On the other hand, if the gate were to open, the rupture could cascade into neighboring faults, creating a larger and more complex event.

The findings, however, do not mean that Southern California will experience a major earthquake. The study does not attempt to predict when the next large event might occur.

Instead, the study offers insight into the current state of the region’s fault system.

Peak Stress Levels

According to the research, stress levels along the San Andreas and San Jacinto faults are among the highest observed during the study’s 1,000-year reconstruction.

Researchers found that both fault systems were carrying elevated stress simultaneously, a combination that was historically associated with earthquakes involving interactions between the two.

For Southern California residents, the findings are less of a warning of what might happen and more of a new way to understand the hazards that have been shaping the region for 1,000 years.

Scientists are not predicting the next major earthquake. Instead, they are suggesting that where an earthquake originates may just be part of the bigger picture.

A mountain pass most drivers cross without a second thought might determine where the rupture ends.