The most significant seismic event in California history, a magnitude 7.9 earthquake, occurred in 1857 and ruptured approximately 325 miles along the San Andreas Fault. The final quake, dubbed the “Big One,” killed two people and caused shaking that lasted between one and three minutes.
Since then, California’s population has exploded and it has experienced many large earthquakes, including one that is far more deadly and equally strong. But Californians are also familiar with the existential fear that another Big One is looming.
Researchers have been warning for years that another major earthquake will strike Southern California. However, I don’t know when that will be.
Recent research shows that the red flags will continue to increase in 2026. The San Andreas and San Jacinto fault systems have reached their highest stress levels in 1,000 years, according to a study conducted at the University of Hawaii at Manoa.
The San Andreas Fault stretches across the state, passing through San Francisco and San Bernardino, and the San Jacinto Fault is in Southern California, running through Riverside, San Diego, and Imperial counties.
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Why the San Andreas Fault is so dangerous
According to Jonathan Stewart, a professor of earthquake engineering at the University of California, Los Angeles, the San Andreas Fault ruptures about every 150 years.
But more than 300 years have passed since the last “major destruction” south of the 1857 Fort Tejon earthquake, Stewart said. “As far as we know, no major cracks have occurred since about 1690. It’s been well over 150 years now, so we’re seeing quite a bit of accumulation. That doesn’t mean it’s going to happen right away, it just means more stress is building up.”
When an earthquake occurs, it can cause major damage. One big concern is the state’s water system.
“An earthquake like this would collapse most, if not all, of the major aqueducts that bring water to Southern California,” Stewart said. “After this earthquake, most people won’t be affected by collapsed buildings, but everyone will be affected by water issues.”
Study highlights long-standing risks
To study the probability of an earthquake occurring, the researchers built a physics-based simulation and fed it with the region’s earthquake history. In doing so, they were able to estimate how much stress is building up along the Southern San Andreas and San Jacinto fault systems.
This result suggests that a catastrophe may be on the horizon.
“Currently, with historically high levels of stress across the region and more than 160 years since the last major rupture, the system is under critical stress,” said lead author Lillian Burkhardt.
Most importantly, Cajon Pass, located at the junction of the San Andreas and San Jacinto faults, could facilitate joint rupture of the two faults. The study said this scenario would likely cause “much more damage” than a single failure.
How is earthquake safety considered?
Seismic hazard assessments are critical to the safety of California’s millions of residents living in densely populated and earthquake-prone areas.
According to the U.S. Geological Survey, seismic hazard maps use information about past faults, seismic wave behavior, and near-surface conditions at a particular location to show the relative risk associated with an earthquake.
The larger the value, the stronger the earthquake shaking. A maximum ground speed of 269 cm/s corresponds to extreme shaking and significant damage. For reference, the highest ground velocity recorded during the 1999 Taiwan Chi-Chi earthquake reached 318 cm/s and had a magnitude of 6.7.
That earthquake killed more than 2,000 people and caused about $14 billion in damage.
How do fault lines rupture?
The San Andreas and San Jacinto faults are strike-slip faults that typically cause horizontal displacement. Both faults constitute the geological boundary between the Pacific and North American plates.
When a large earthquake occurs on the San Andreas Fault, surface rupture can occur as the fault penetrates to the surface. However, most earthquakes do not cause surface rupture, according to the USGS.
In the event of a large-scale rupture, strong shaking can cause severe damage to areas near faults or built on soft or moist soil, and the shaking can be amplified. Fractures can directly offset roads, buildings, and other structures that span the fault trace.
While this study helps explain the risks associated with the California fault line, Burkhardt stressed that it should not serve as a prediction.
“This does not predict when an earthquake will occur,” Burkhardt said. “However, studies like this one make an important contribution to national and global earthquake hazard research in that they use rigorous, quantitative science to better understand the risks faced by millions of people.”
(This story has been updated to add new information.)
Contributor: Brandi D. Addison, USA TODAY NETWORK
