Monday, April 20, 2015

Earthquake fault heightens California tsunami threat, experts say

 
[LA Times] The earthquake fault cuts through the heart of Ventura's quaint downtown, past the ornate hilltop City Hall and historic Spanish-era mission before heading into the Pacific Ocean.
For decades, some seismic experts believed the Ventura fault posed only a moderate threat and was incapable of producing a major temblor.
But research in recent years shows that the fault is extremely dangerous, capable of producing an earthquake as large as magnitude 8 as well as severe tsunamis that until now experts didn't believe were possible from a Southern California quake.
Such a big earthquake on the fault estimated to occur every 400 to 2,400 years, experts said. The last sizable quake on the Ventura area hit about 800 years ago. Large temblors occur on this fault less frequently than on the San Andreas fault, which has long been considered the state's most dangerous.
 The California Geological Survey is studying whether it needs to revise tsunami hazard maps because of the researchers' findings. One study found that the inundation would be "severe right along the coast" but didn't call for redrawing evacuation zones farther inland.
"We're not done looking at it," said Rick Wilson, a California Geological Survey senior engineering geologist. "If new information comes forward, we'll change the lines to make sure the communities are as safe as possible."
Scientists have long known the San Andreas fault was capable of producing an 8.0 quake. But because that fault is so far inland in Southern California, the San Andreas does not pose a tsunami risk. The worst tsunami risk comes from a mega-quake from Alaska, which would give Californians hours of time to evacuate.
A huge quake 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," said Tom Jordan, director of the Southern California Earthquake Center and USC earth sciences professor, who was not involved in the research.
Scientists from Harvard University, USC, the U.S. Geological Survey and San Diego State used underground oil data, cutting-edge earth imaging and research on ancient beach mapping to form their conclusions, which were published last year in the Bulletin of the Seismological Society of America.
One key finding is that the fault now appears to be connected to a network of others that stretch from the Santa Barbara coast and into eastern Ventura County.
A major earthquake on the Ventura fault could then cause shaking along nearby faults to the east, along the foothill suburbs of Los Angeles and San Bernardino counties. Read More