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A large Ventura Fault quake could trigger a tsunami

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EarthquakesA large Ventura Fault quake could trigger a tsunami

Published 22 April 2015

Earthquake experts had not foreseen the 2011 magnitude-9 Japan earthquake occurring where it did, so soon after the disaster, scientists in Southern California began asking themselves, “What are the big things we’re missing?” For decades, seismic experts believed the Ventura fault posed only a minor to moderate threat, but new research suggests that a magnitude-8 earthquake could occur on the fault roughly every 400 to 2,400 years. The newly discovered risk may even be more damaging than a large earthquake occurring on the San Andreas Fault, which has long been considered the state’s most dangerous. Unlike the Ventura fault, the San Andreas Fault is so far inland in Southern California, that it does not pose a tsunami risk. A large earthquake on the Ventura fault, however, could create a tsunami that would begin “in the Santa Barbara Channel area, and would affect the coastline … of Santa Barbara, Carpinteria, down through the Santa Monica area and further south.”

Earthquake experts had not foreseen the 2011 magnitude-9 Japan earthquake occurring where it did, so soon after the disaster, scientists in Southern California began asking themselves, “What are the big things we’re missing?” That earthquake “informed all of us that, basically, we should really question our assumptions more,” said U.S. Geological Survey geophysicist Ken Hudnut. “We know it in our minds as scientists that events could occur that are sort of off the charts and blow away all of our prior assumptions — and usually that’s because we never saw anything like that happen in historical time.”

Scientists from Harvard University, University of Southern California (USC), the U.S. Geological Survey, and San Diego State University (SDSU) published last year in the Bulletin of the Seismological Society of America, research that suggests that a large earthquake on the Ventura fault could create a tsunami that would begin “in the Santa Barbara Channel area, and would affect the coastline … of Santa Barbara, Carpinteria, down through the Santa Monica area and further south.”

For decades, seismic experts believed the Ventura fault posed only a minor to moderate threat, but new research suggests that a magnitude-8 earthquake could occur on the fault roughly every 400 to 2,400 years. The newly discovered risk may even be more damaging than a large earthquake occurring on the San Andreas Fault, which has long been considered the state’s most dangerous. Unlike the Ventura fault, the San Andreas Fault is so far inland in Southern California, that it does not pose a tsunami risk.

According to the Los Angeles Times, the new study found that the Ventura fault appears to be connected to a network of other faults that stretch from the Santa Barbara coast into eastern Ventura County. A major quake on the Ventura fault could then cause tremors along nearby faults reaching the suburbs of Los Angeles and San Bernardino counties. “The Ventura fault goes right through downtown Ventura, so … clearly there would be a lot of damage,” said Tom Jordan, director of the Southern California Earthquake Center and USC earth sciences professor, who was not involved in the research. “But the shaking of an event of this magnitude would affect an area for hundreds of kilometers around it. So it’s not just right there in Ventura — it’s the whole, effectively, coastal Southern California from Santa Barbara to L.A.,” Jordan said.

Scientists first noticed signs of large earthquakes along the Ventura fault when they observed old sections of shoreline west of Ventura. The Pitas Point and Punta Gorda areas were “yanked out of the surf zone by large uplift events,” growing between sixteen feet to twenty-six feet suddenly, said SDSU geology professor Thomas Rockwell. Earth movements of that magnitude could only be explained by a mega-quake. “You need really large faults to get large earthquakes,” Rockwell said.

Still scientists were unsure what fault produced the earthquake because there were no signs of large faults nearby. To get a better understanding of their observation, scientists created an image of the underground Ventura fault, based on old oil well data and seismic reflection. They soon discovered that the fault extends much deeper than experts had originally thought, said Judith Hubbard, the study’s lead author, who worked on it as she obtained a geology doctorate at Harvard University. Scientists then found that deep underground, the Ventura fault could connect with other faults, and they could join together to create a mega-earthquake. “The only way you can get such large earthquakes on the Ventura-Pitas Point faults is to have it rupture simultaneously, with mechanically interlinked faults to the east or the west,” said USC earth sciences professor James Dolan, a co-author of the study.

That scenario is conceivable but rare, Dolan said, adding that the resulting quake would produce thirty-two to 126 times more energy than the 1994 magnitude-6.7. Northridge earthquake.

The California Geological Survey is studying whether it should revise tsunami hazard maps because of the research findings. “We’re not done looking at it,” said Rick Wilson, a senior engineering geologist. “If new information comes forward, we’ll change the lines to make sure the communities are as safe as possible.”

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