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Post by Watchman on Jun 22, 2006 13:12:36 GMT -5
Expert believes Los Angeles area at risk for massive earthquake
LONDON - The southern end of the San Andreas fault near Los Angeles, which has been still for more than two centuries, is under immense stress and could produce a massive earthquake at any moment, a scientist said Wednesday.
Yuri Fialko, of the Scripps Institution of Oceanography at La Jolla, Calif., said that given average annual movement rates in other areas of the fault, there could be enough pent-up energy in the southern end to trigger a cataclysmic jolt of up to 32 feet (10 meters).
"The observed strain rates confirm that the southern section of the San Andreas fault may be approaching the end of the interseismic phase of the earthquake cycle," he wrote in the journal Nature.
A sudden lateral movement of 23 to 32 feet (7 to 10 meters) would be among the largest ever recorded.
According to the U.S. Geological Survey, the earthquake that destroyed San Francisco in 1906 was produced by a sudden movement of the northern end of the fault of up to 21 feet (6.4 meters).
Fialko said there had been no recorded movement at the southern end of the fault — the 800-mile-long (1,280-kilometer-long) geological meeting point of the Pacific and the North American tectonic plates — since the dawn of European settlement in the area.
He said this lack of movement for 250 years correlated with the predicted gaps between major earthquakes at the southern end of the fault of between 200 and 300 years.
Elsewhere on the fault, there were average slippage rates up to a couple of centimeters a year that prevented the build-up of explosive pressure deep underground.
When these became blocked and then suddenly broke free, they produced tremors or earthquakes of varying intensity depending on the movement that had taken place before and the duration of the blockage.
USGS says the most recent major earthquakes in the northern and central zones of the San Andreas fault were in 1857 and 1906.
Three possible explanations Fialko said there were three possible explanations for the lack of observed movement in the southern section: creepage under the surface that had no external manifestation, a scenario in which the section simply doesn't not move as much as the rest of the section, or a major blockage.
"Except for the first possibility above, the continued quiescence increases the likelihood of a future event," he wrote.
Making calculations based on a wide range of land and satellite observations, he discounted the idea of creepage and warned of impending disaster.
"Regardless of fault geometry and mechanical properties of the ambient crust, results presented in this study lend support to intermediate-term forecasts of a high probability of major earthquakes on the southern SAF system," Fialko said.
Copyright 2006 Reuters Limited. All rights reserved.
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Post by Watchman on Sept 12, 2007 10:29:28 GMT -5
Are a Series of California Mini-Quakes Gearing Up for the “Big One”?
Earthquake_3 A series of seven quakes has rattled southern California over the past few weeks. The event has led many to question if this could be the build up preceding the dreaded “Big One” that scientists have been warning is on its way.
The experts say there is about 300 years of pent-up seismic stress about to explode along southern California fault lines. The future event has been predicted to perhaps match or exceed the 1906 earthquake that reduced the San Francisco Bay Area to piles of rubble. Those who live in the area are understandably eager to know when it’s coming, but geologists say there’s no way to pin down a day, nor know of these recent quakes are a good or bad sign.
“If you had asked me two weeks ago, I would have said a 6.0 earthquake could hit today, or it might hit in 100 years,” said Patrick Abbott, a geologist with San Diego State University. “My answer today would be the same. This [series of quakes event] sheds no light. We just don't have short-term ability to predict.”
The quakes frightened residents but did no damage. However, they did serve as a wake-up call for those who forget they’re living on a major fault line—one that is due for serious geological activity. While scientists can’t say exactly when it’s coming, they are certain that it is on its way.
"It is fully charged for the next big event," said Yuri Fialko, a geophysicist at the Scripps Institution of Oceanography in La Jolla, California. " It could be tomorrow or 20 years from now, but it appears unlikely the fault can take another few hundred years of slow strain accumulation."
Quake clusters are unusual, but there have been other previous occurrences in the San Diego area. Abbott said a similar cluster occurred around San Diego Bay in 1985 and 1986. Graham Kent, a seismologist at the Scripps Institution of Oceanography, said there is little chance that the cluster of quakes could be foreshocks, precursors of a larger quake.
“Even if something is a foreshock, that doesn't mean the next one will be much bigger,” Kent said. “Typically, what you see is the first one is bigger, then the magnitude goes down.”
But even if the series of quakes isn’t indicative of a bigger event, Kent said the possibility for the Big One striking is greater than most people realize, and many residents have not bothered to prepare their homes or store emergency supplies.
“I don't think the average person in San Diego County is anywhere near as prepared as they should be,” Kent said. “People should go through their houses and figure out what will get thrown off the walls.”
The state's Earthquake Authority provides residential earthquake insurance and encourages Californians to reduce their risk, said Tim Richardson, acting chief executive officer. But the vast majority of homeowners in California do not have quake insurance. Richardson said he doesn’t expect the recent round of tremors to spur people to make changes either. He says only death and damage will motivate people to change.
“Anything short of that doesn't seem to get people to do anything.”
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