Updated

Updated · UH System Current News · Jun 10

San Andreas, San Jacinto Faults Hit 1,000-Year Stress High as Joint Rupture Risk Rises

Updated

Updated · UH System Current News · Jun 10

San Andreas, San Jacinto Faults Hit 1,000-Year Stress High as Joint Rupture Risk Rises

Q: Seismic stress is at a 1,000-year peak. What decides if a geological 'gate' unleashes a dual-fault megaquake on Southern California?

What decides it is the stress balance and rupture behavior at Cajon Pass, the junction where the San Andreas and San Jacinto faults meet. Researchers describe Cajon Pass as an “earthquake gate” because it can either stop a rupture or let it jump from one fault system to the other, creating a much larger, dual-fault event. The key factor is not just that stress is high, but that it is high on both faults at roughly similar levels. The new modeling shows the San Jacinto-Bernardino section at about 3.6 MPa and the Mojave South section of the San Andreas at about 2.8 MPa, both near or above their highest levels in the past 1,000 years. That kind of matched loading has historically been associated with ruptures that link fault systems rather than die out at the junction. History shows Cajon Pass can behave either way. The 1857 Fort Tejon earthquake stopped there, while the 1812 Wrightwood event appears to have ruptured through it. That suggests the gate’s behavior changes with the evolving stress field over centuries. Other physical conditions also matter, including fault geometry, rock strength, friction, and fluid pressure, because they influence whether slip stays confined or cascades across connected segments. But the study’s main finding is that Southern California is now in a critically loaded state after more than 160 years without a major rupture, making a through-going San Andreas–San Jacinto earthquake more plausible, though not predictable in timing.

3 articles · Updated · UH System Current News · Jun 10

Stress on multiple Southern California segments has reached or exceeded the highest levels of the past 1,000 years, with researchers warning the San Andreas and San Jacinto systems could rupture in a single large event.
A physics-based model built from a 1,000-year earthquake record found more than 160 years have passed since the last major rupture, allowing stress that would normally be released to keep accumulating.
Cajon Pass emerged as a critical “earthquake gate” that can either block a rupture from jumping between the faults or allow it to cascade through both systems when stress levels align.
A combined rupture could be more damaging than a single-fault quake, threatening densely populated corridors including Los Angeles, San Bernardino, Riverside and the Coachella Valley.
The authors said the study does not predict when an earthquake will strike, but could sharpen hazard assessments, infrastructure planning and emergency preparedness in Southern California and other complex fault zones.