Southern California's Fault Lines Hit a 1,000-Year Stress Peak and Scientists Are Alarmed

A Ticking Clock Beneath Los Angeles

The San Andreas and San Jacinto fault systems are at their highest levels of tectonic stress in 1,000 years, raising the threat of a major earthquake that could devastate Southern California, a new study finds. The research, published in the peer-reviewed Journal of Geophysical Research: Solid Earth, relies on sophisticated computer modeling to track tectonic strain accumulation across the southern San Andreas and San Jacinto fault networks over the past ten centuries.

A collaborative team from the University of Hawaiʻi at Mānoa, the University of Bern, the U.S. Geological Survey (USGS), and UC San Diego discovered that stress along key segments has either matched or blown past the highest thresholds recorded in the model's 1,000-year history. That pressure is estimated to be 2.8 MPa on the Mojave South segment and 3.6 MPa on the San Jacinto Bernardino segment. To put that in perspective, in the past 160 years, both of these fault lines have been scarily silent.

The "Earthquake Gate" at Cajon Pass

Cajon Pass lies at the critical point where the Mojave South and North San Bernardino segments of the San Andreas Fault, and the San Jacinto Bernardino segment of the San Jacinto Fault meet, together forming a critical fault junction northeast of Los Angeles. Researchers have given this junction a striking new label. "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," said lead author Liliane Burkhard.

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. History, in other words, shows both outcomes are possible. 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 — and when the stress on both faults rises in concert toward similarly high levels, conditions favor a large joint rupture crossing both systems.

What a Combined Rupture Could Mean

The study finds that current stress levels on both faults are aligned in the configuration historically associated with cross-fault propagation. The alignment of high stress on both faults increases the likelihood of a joint rupture, which could result in a larger, more damaging earthquake affecting densely populated regions. Those densely populated areas include Los Angeles County, Ventura County, Orange County, San Diego County, the Palm Springs–Indio metropolitan area, and even as far south as Tijuana.

In the last 1,000 years, the San Andreas Fault and the San Jacinto Fault have hosted at least 36 earthquakes of magnitude 6.4 or larger and accommodate approximately 90% of the North American–Pacific plate slip rate in Southern California. Southern California's last "big one" was a magnitude 7.9 event in 1857, when a 205-mile segment of the San Andreas fault slipped horizontally between Parkfield and Cajon Pass. That was nearly 170 years ago — and the earth has been storing energy ever since.

Not a Prediction, But a Warning

"This is not a prediction of when an earthquake will happen," Burkhard says. "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." While the study does not predict when the next major earthquake will strike, it highlights the dangerous reality: stress continues to build across one of the most complex and consequential fault junctions in North America.

The USGS National Seismic Hazard Model — the scientific foundation for California's seismic building codes and insurance rate structures — is updated periodically to incorporate new research, and the physics-based stress figures in the Burkhard study are exactly the type of input that drives such updates. As one researcher put it, "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." The ground beneath Southern California may be silent for now — but the science is anything but reassuring.