Ventura fault could cause damaging earthquake, new research discovers

People living in coastal Southern California continue to face the greatest earthquake risk across the nation and, historically, much of that risk has been assumed by the San Andreas Fault. But a new study has identified a new source of a potential magnitude 8.0 quake.

The Ventura-Pitas Point fault, an onshore and offshore fault extending eastward from Santa Barbara to Ventura, has the potential to cause greater damage and stronger shaking during an earthquake than previously expected, according to a team of geophysics researchers, including UC Riverside Geology Professor Gareth Funning.

When identified as an active and potentially dangerous fault in the 1980s, experts widely believed the Ventura fault was capable of producing little more than moderate shaking and a minor temblor. This thought stemmed from confusion over the fault’s geometry. Researchers initially believed it to be steeply planar, extending to a depth of about 13 miles underground.

However, a 2014 study conducted by scientists from Harvard University, the University of Southern California, San Diego State University and the U.S. Geological Survey suggested the fault has a “ramp-flat geometry,” which forms a step-like pattern, consisting of a flat section situated between two ramps tilted across more resistant rock units.

The study also discovered that, in a worst-case scenario, the Ventura fault could potentially connect to a network of nearby faults including the San Cayetano Fault, Lion Fault and Red Mountain Fault.

If accurate, this research suggested the Ventura fault is capable of producing as large as a magnitude 8.0 earthquake as well as severe tsunamis, previously believed to be impossible from a Southern California earthquake.

Receiving funding through a five-year extension granted to the Southern California Earthquake Center (SCEC) — a publicly funded research consortium of which UCR is a core member — the research team, including Funning and headed by the research letter’s lead author Scott Marshall, an associate geology professor at Appalachian State University, used computer modeling to test the competing models of the fault’s geometry.

Their findings favor the “ramp-flat” model, providing further evidence that the Ventura fault is extremely dangerous.

Research on the faultline is still ongoing, but Funning hopes that the results of more follow-up studies such as theirs will bring researchers closer to a scientific consensus on the faultline.

“We are learning more details about dangerous faults like the Ventura-Pitas Point fault all the time. While some might say these developments are frightening, I would argue that it’s important to know what we’re up against,” shared Funning.

Funning acknowledges the likelihood of a massive quake from the Ventura fault in the coming years remain ambiguous. The site “has had four earthquakes in the last 7000 years,” according to Funning and last ruptured somewhere between 700 and 1,000 years ago, according the the aforementioned 2014 study.

The study published by Marshall’s team suggests the fault’s recurrence interval rests somewhere between 500 and 2,000 years. “It is definitely a possibility that another one could occur in our lifetimes,” Funning said.

Going forward, Funning states the team plans to continue research within the region, citing discoveries of “human activity” by their GPS data while in the field. “We saw several areas where the land was subsiding (sinking),” shared Funning. “Our hunch is that it is due to groundwater pumping — if you pump water out of the ground and it isn’t replenished, it’s quite likely it’ll sink.”

Funning joined the UCR faculty in 2007 and is a member of the Earthquake Processes group in the Department of Earth Sciences. He hopes research such as this can serve to caution the community to help them better prepare for quakes of potentially damaging magnitude.

“This information can be used to make better assessments of the hazards that future earthquakes may bring to the region, and filters down to local and state governments, agencies and utilities so that they can prepare for what’s coming,” Funning said. “I think that at least should be reassuring.”

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