San Andreas and San Jacinto faults show stress levels highest in a millennium, study finds
A new physics-based model indicates that tectonic stress along Southern California's San Andreas and San Jacinto fault systems has reached its highest level in 1,000 years, raising concerns for seismic hazard assessments.
University of Hawai‘i at Mānoa researchers have determined that tectonic stress on the San Andreas and San Jacinto fault systems in Southern California is at or above the highest levels recorded in the past millennium. The findings, published in the Journal of Geophysical Research: Solid Earth, stem from a computer model that simulates stress accumulation and release over a 1,000-year period using geological data such as radiocarbon-dated sediments and tree-ring records.
“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,” said lead author Liliane Burkhard in a press release.
The model highlights Cajon Pass, a junction between the two fault systems, as a potential “earthquake gate” that can either block or permit a joint rupture.
“The conditions that determine whether the ‘earthquake gate’ at Cajon Pass opens or stays closed appear to be related to how closely the stress levels on the two fault systems are aligned with each other at the time of rupture,” Burkhard explained.
With more than 160 years since the last major rupture, the researchers say the fault system is in a critically loaded state, meaning stress that would normally be released in large earthquakes has continued to accumulate. The study suggests a simultaneous rupture of both faults could affect densely populated areas including Los Angeles, San Bernardino, Riverside and the Coachella Valley, potentially causing greater damage than a single-fault event.
The authors stress that the work does not predict when an earthquake will occur, but it provides quantitative insight useful for updating hazard assessments, infrastructure planning, building codes and emergency preparedness. The modeling framework may also be adapted for other complex fault junctions worldwide.