Southern San Andreas and San Jacinto Faults Reach Highest Stress in 1,000 Years

Southern portions of the San Andreas fault and adjacent segments of the San Jacinto fault have reached their highest recorded stress levels in 1,000 years, according to a study that reconstructed a millennium of seismic activity. 6 megapascals, above its prior peak from nearly 50 years ago. 8 megapascals, exceeding its own record from a decade ago.

The two fault systems meet at the Cajon Pass, which researchers describe as an earthquake gate that can either stop or transmit large ruptures. Earthquakes crossed the pass in the past when stress levels on both sides were similarly high, a configuration the study says the region is now approaching. 8 megapascals.

3 megapascals. 8 and affect areas from north of Los Angeles through San Bernardino, Riverside and the Coachella Valley at once. Liliane Burkhard, the study's lead author and geophysicist at the University of Bern, Switzerland, said a joint rupture would affect a far larger area than a single-fault event.

She added that city managers and emergency responders should plan for such joint ruptures as a realistic possibility. Matthew Weingarten, a geologist at San Diego State University who was not involved in the study, said the physics-based estimate shows the system at a 1,000-year high.

He noted the insight is that the balance of stress across the junction may determine whether the next earthquake stays contained or grows larger.

Stress has been accumulating for more than a century along the southern San Andreas and San Jacinto systems less than 60 miles northeast of downtown Los Angeles. The Pacific and North American tectonic plates slide past each other a few centimeters each year while other zones remain locked. 7 or higher earthquake along the southern San Andreas in upcoming decades.