SAN BERNARDINO, Calif., June 24 (Inland Empire News Online) — Tectonic stress along Southern California’s San Andreas and San Jacinto fault systems has reached, and in some places exceeded, the highest levels seen in the past 1,000 years, according to a peer-reviewed study led by University of Hawaiʻi at Mānoa earth scientists.
The research, published in the Journal of Geophysical Research: Solid Earth, was led by Liliane Burkhard, a research affiliate in UH Mānoa’s Hawaiʻi Institute of Geophysics and Planetology and a scientist at the University of Bern in Switzerland. The authors said the findings have direct implications for seismic hazard assessments in a densely populated and infrastructure-critical corridor in Southern California.
According to the study, modeled stress reached 3.6 megapascals on the San Jacinto-Bernardino section and 2.8 megapascals on the neighboring Mojave South section of the San Andreas Fault. The researchers said those levels were the highest seen anywhere in their 1,000-year reconstruction.
For the Inland Empire, the study singled out Cajon Pass as a location that could help link the San Andreas and San Jacinto systems in a joint rupture. The authors said such a through-going event could be significantly more damaging than a rupture on a single fault.
“Our results show that stress levels on multiple fault segments are now at or above the highest values seen in the past millennium and that the region may be capable of a large through-going rupture involving both fault systems,” Burkhard said in the university release describing the study.
The release said the combined-fault scenario could affect densely populated areas including Los Angeles, San Bernardino, Riverside and the Coachella Valley. The study framed those communities as part of the area that would be most exposed if a large rupture were to involve both fault systems.
Burkhard’s team said the San Andreas-San Jacinto system is now in a critically loaded state. The university release said more than 160 years have passed since the last major rupture, adding to the concern the researchers attached to the current stress levels.
“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 release quoted Burkhard as saying.
The study’s main finding is based on a model of stress through time, not on a direct measurement of when the next earthquake will happen. The researchers said the concern is the amount of stress now stored across multiple fault segments, and how that may affect the chances of a larger rupture involving more than one fault.
The paper’s publication in a peer-reviewed journal means the work was reviewed by other scientists before it was published. The university said the study was led by scientists at UH Mānoa, with Burkhard listed in the Hawaiʻi Institute of Geophysics and Planetology and the University of Bern.
The journal article was published in Journal of Geophysical Research: Solid Earth, and the university release said the findings are especially important because the fault system runs through a corridor that includes major population centers and critical infrastructure in Southern California.
The study’s location-specific warning points directly to the Inland Empire, where San Bernardino and Riverside are named in the release and where Cajon Pass was identified as a place that could help a rupture spread between the two fault systems. The researchers said that kind of joint rupture would be more damaging than a single-fault event, making the current stress state a key issue for regional earthquake planning.
The source materials did not identify the last major rupture by date or name, but the researchers said the long gap since that event leaves the fault system critically loaded. They said the region has now reached stress levels that match or exceed the highest values seen in the past 1,000 years.
