SOUTHERN CALIFORNIA — Tectonic stress on the San Andreas and San Jacinto fault systems has reached its highest level in 1,000 years, according to a peer-reviewed study. The study, which analyzed the fault systems, was published in the Journal of Geophysical Research: Solid Earth.

"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 Liliane Burkhard, planetary geologist and study lead author. The research does not provide a specific timeline or probability forecast for a future major earthquake.

Researchers reconstructed 1,000 years of regional seismic activity using tree-ring records and age data from displaced sediments. Scientists at the University of Hawaii at Mānoa and the University of Bern developed a computer model to simulate tectonic stress accumulation and release.

"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," said Burkhard. Cajon Pass is located at the geological junction where the San Andreas and San Jacinto faults meet. The faults have produced 36 earthquakes measuring magnitude 6.4 or greater over the last millennium. The 1857 earthquake on the San Andreas fault involved a 205-mile horizontal slip between Parkfield and Cajon Pass.

"What we can say is that the system is critically stressed, and that physics-based models like this one give us a clearer picture of the range of scenarios we should be prepared for," said Burkhard. The 1994 Northridge earthquake in Los Angeles destroyed 87,000 structures and resulted in more than 60 fatalities.