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浪迹天涯，追月寻梦
LOS ANGELES - Californiafaces an almost certain risk of being rocked by a strong earthquake by2037, scientists said Monday in the first statewide temblor forecast.
Newcalculations reveal there is a 99.7 percent chance a magnitude 6.7quake or larger will strike in the next 30 years. The odds of such anevent are higher in Southern California than Northern California, 97percent versus 93 percent.
"It basically guarantees it's going to happen," said Ned Field, a geophysicist with the U.S. Geological Survey in Pasadena and lead author of the report.
The 1994 Northridge earthquake under Los Angeles'San Fernando Valley was magnitude 6.7. It killed 72 people, injuredmore than 9,000 and caused $25 billion in damage in the metropolitanarea.
The damage created by an earthquake depends greatly on where it hits. A 7.1 quake — much stronger than Northridge — hit the Mojave Desert in 1999 but caused only a few injuries and no deaths.
California is one of the world's most seismically active regions.More than 300 faults crisscross the state, which sits atop two ofEarth's major tectonic plates, the Pacific and North American plates.About 10,000 quakes each year rattle Southern California alone,although most of them are too small to be felt.
The analysis is the first comprehensive effort by the USGS, Southern California Earthquake Center and California Geological Surveyto calculate earthquake probabilities for the entire state using newlyavailable data. Previous quake probabilities focused on specificregions and used various methodologies that made it difficult tocompare.
For example, a 2003 report found the San Francisco Bay Areafaced a 62 percent chance of being struck by a magnitude 6.7 quake by2032. The new study increased the likelihood slightly to 63 percent by2037. For the Los Angeles Basin, the probability is higher at 67percent. There is no past comparison for the Los Angeles area.
Scientists still cannot predict exactly where in the state such aquake will occur or when. But they say the analysis should be a wake-upcall for residents to prepare for a natural disaster in earthquakecountry.
Knowing the likelihood of a strong earthquake is the first step inallowing scientists to draw up hazard maps that show the potentialseverity of ground shaking in an area. The information can also helpwith updating building codes and emergency plans and setting earthquake insurance rates.
"A big earthquake can happen tomorrow or it can happen 10 years fromnow," said Tom Jordan, director of the earthquake center, which isheadquartered at the University of Southern California.
Researchers also calculated the statewide probabilities for largertemblors over the same time period. Among their findings: There is a 94percent chance of a magnitude 7 shock or larger; a 46 percent chance ofa magnitude 7.5 and a 4.5 percent chance of a magnitude 8.
The odds are higher that a magnitude 7.5 quake will hit Southern California than Northern California — 37 percent versus 15 percent.
Of all the faults in the state, the southern San Andreas, which runs from Parkfield in central California southeast to the Salton Sea,appears most primed to break, scientists found. There is a 59 percentchance in the next three decades that a Northridge-size quake willoccur on the fault compared to 21 percent for the northern section.
The northern San Andreas produced the 1906 San Francisco earthquake, but the southernmost segment has not popped in more than three centuries.
Scientists are also concerned about the Hayward and San Jacintofaults, which have a 31 percent chance of producing a Northridge-sizetemblor in the next 30 years. The Hayward fault runs through denselypopulated cities in the San Francisco Bay Area. The San Jacinto fault bisects the fast-growing city of San Bernardino east of Los Angeles. （The article comes from Yahoo news）
