Seismic stress is building along major fault lines near Los Angeles, raising concerns among researchers about the possibility of a significant earthquake in the coming years. A recent study published in Advancing Earth and Space Sciences highlights increasing tectonic pressure on key segments of the San Andreas and San Jacinto fault systems, some reaching stress levels not observed in at least a millennium.
The last major earthquake to impact the Los Angeles area directly was the magnitude 7.9 Fort Tejon quake on January 9, 1857. This event ruptured a 225-mile section of the southern San Andreas Fault but did not extend beyond the Cajon Pass region. Since then, that southern segment has remained largely quiet, while stress has continued to accumulate on other portions of both the San Andreas and the nearby San Jacinto faults.
The study’s authors examined 1,000 years of paleoseismic data, which reconstruct historical fault activity through geological records. Their analysis revealed pronounced stress accumulation along the Mojave South segment of the San Andreas Fault and the San Jacinto Bernardino segment of the San Jacinto Fault. These two major strike-slip fault systems run parallel east of Los Angeles and converge near Cajon Pass, a geological feature the researchers describe as an “earthquake gate.”
Liliane Burkhard, lead author of the study and a researcher at the University of Bern, explained the concept of the earthquake gate as a junction where seismic ruptures can potentially transfer from one fault to the other depending on their stress conditions. When stress levels on both faults are closely aligned, ruptures can propagate across the pass, resulting in larger, more extensive earthquakes. However, when stress levels differ, ruptures tend to stop at the junction.
Historical records show differing behavior at the Cajon Pass “gate.” The 1857 Fort Tejon quake stopped short of crossing the pass, but the 1812 Wrightwood earthquake did rupture both faults. Current data indicate that stress levels on these faults are both high and converging, suggesting the possibility of a joint rupture similar to or surpassing past events.
While the study emphasizes that the exact timing and scale of a future earthquake remain unpredictable, the elevated stress conditions imply increased seismic hazard in the region. Burkhard cautioned that although there is no definitive stress threshold that triggers earthquakes like flipping a switch, the current state of stress falls within or above historically significant ranges.
Harold Tobin, director of the Pacific Northwest Seismic Network and a professor at the University of Washington who was not involved in the study, supported the findings. Tobin noted that although 1,000 years of paleoseismic data is relatively short for earthquake studies globally, it is sufficiently comprehensive to offer meaningful insights for Southern California’s fault systems. He added that the complexity of these interacting faults complicates efforts to predict when a major quake might occur, contrasting with more regular seismic patterns observed in simpler fault systems or subduction zones elsewhere.
As Southern California continues to develop and its population grows, monitoring these stress accumulations remains critical for preparedness and risk mitigation. The study underscores the need for continued research and vigilance regarding seismic hazards in the Los Angeles metropolitan area.