Stress levels along two major fault lines in Southern California have climbed to their highest point in over a millennium, according to new research from the University of Hawaii at Manoa and it is raising fresh concerns about seismic risk in one of the most densely populated regions in the United States.
The findings were published in the Journal of Geophysical Research: Solid Earth and highlighted in a university release on June 10.
“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,” lead author Liliane Burkhard said in a statement.
How did scientists reach this conclusion?
Researchers built a physics-based computer model that simulated how stress builds up and gets released along the San Andreas and San Jacinto fault systems, including at a critical junction between the two known as Cajon Pass, a mountain pass located 63 miles northeast of Los Angeles, per People.
To build the model, scientists used 1,000 years of earthquake history gathered from geological evidence, including radiocarbon dating and tree-ring records. They then used the model to estimate how much stress has built up on the faults today.
The study found that 36 earthquakes with magnitudes of 6.4 or higher have hit the Los Angeles region over the past 1,000 years. These included the magnitude 7.9 Fort Tejon earthquake in 1857, the last major quake to strike the area. Today, about 13 million people live in the grekater Los Angeles region.
“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,” Burkhard said.
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How Cajon Pass could influence future earthquakes
One of the study’s most significant findings involves Cajon Pass, which researchers say could act as an “earthquake gate,” sometimes blocking large ruptures from crossing between the two fault systems and sometimes allowing them to pass through, involving both faults in a single event, per People.
Burkhard explained that “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.”
The study found that stress that would normally have been released through large earthquakes has instead continued to build to what researchers describe as “unprecedented levels”. A joint rupture of both the San Andreas and San Jacinto faults simultaneously could be significantly more damaging than a single-fault event, affecting densely populated areas including Los Angeles, San Bernardino, Riverside and the Coachella Valley.
The San Andreas Fault is the primary boundary between the Pacific and North American tectonic plates and stretches approximately 650 miles, with other faults including the Hayward Fault branching off it, per ABC7 News.
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Why scientists say the findings still matter
Scientists say the study is not meant to predict when an earthquake will happen but rather to improve understanding of the risk millions of people face.
“This is not a prediction of when an earthquake will happen,” Burkhard said. “However, studies like this are important contributions to national and global earthquake hazard research in that we are using rigorous, quantitative science to better understand the risk facing millions of people.”
Researchers believe the findings will help improve seismic hazard assessments, emergency preparedness and infrastructure planning.
