USGS: California Has 99.7 Percent Chance of Big Earthquake in Next 30 Years

Published April 15, 2008

| Associated Press

California — the land of sun, beaches and earthquakes — faces an almost certain risk of being rocked by a strong temblor by 2037, scientists said Monday in the first statewide forecast of the seismic threat.

New calculations reveal there is a 99.7 percent chance a magnitude 6.7 quake or larger will hit the Golden State in the next 30 years.

The odds of such an event are higher in Southern California than Northern California, 97 percent versus 93 percent.

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The last time a jolt this size rattled California was the 1994 Northridge disaster, which measured 6.7 on the Richter scale, killed 72 people, injured more than 9,000 and caused $25 billion in damage.

"It basically guarantees it's going to happen," said Ned Field, a seismologist with the U.S. Geological Survey in Pasadena and lead author of the report.

[That in itself's kind of a no-brainer. There have been at least five major California earthquakes since 1970. The strongest, the 1993 Landers quake, measured 7.3 but occurred in a remote area, as did the 7.1 Hector Mine quake in the Mojave Desert. The 1989 Loma Prieta quake in the Bay Area, which killed 67 people, also measured 7.1.]

Despite the new results, scientists still cannot predict exactly where in the state such a quake will occur or when. But they say it should be a wake-up call for residents to prepare for a natural disaster in earthquake country.

"A big earthquake can happen tomorrow or it can happen 10 years from now," said Tom Jordan, director of the Southern California Earthquake Center at the University of Southern California, who was part of the research.

California is one of the most seismically active regions in the world. More than 300 faults crisscross the state, which sits atop two of the Earth'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.

Knowing the likelihood of a strong earthquake is the first step in allowing scientists to draw up hazard maps that show the severity of ground shaking to an area.

The information can also help with updating building codes and emergency plans and setting earthquake insurance rates.

The latest analysis is the first comprehensive effort by the USGS, SCEC and California Geological Survey to calculate earthquake probabilities for the entire state using newly available data.

Previous quake probabilities focused on specific regions and used various methodologies that made it difficult to compare.

In the study, researchers computed the likelihood of a fault rupture using new information about where past quakes have struck, location of hard-to-spot faults and their slip rates as well as satellite-based GPS data of the Earth's crustal movement.

Scientists determined a Northridge-size shock occurs on average once every five years. The chance of a temblor that size striking the Los Angeles Basin is 67 percent compared to 63 percent for the San Francisco Bay Area.

The San Francisco figure is similar to a 2003 analysis that put the probability at 62 percent. There is no past comparison that exists for Los Angeles.

Given California's seismic history, the new results should come as no surprise, said David Schwartz, a USGS geologist in Menlo Park who was not part of the study.

Researchers also calculated the statewide probabilities for larger temblors over the same time period. Among their findings: There is a 94 percent chance of a magnitude 7 shock or higher; a 46 percent chance of a magnitude 7.5 and a 4.5 percent chance of a magnitude 8.

Of all the faults in the state, the southern San Andreas, which runs from Parkfield to the Salton Sea, appears most primed to break, scientists found.

There is a 59 percent chance in the next three decades that a Northridge-size quake will occur on the fault compared to 21 percent for the northern section.

The northern San Andreas produced the 1906 San Francisco earthquake, a recent disaster in geologic time compared to the southernmost segment, which has not popped in more than three centuries.

Scientists are also concerned about the Hayward and San Jacinto faults, which have a 31 percent chance of producing a Northridge-size temblor in the next 30 years.

The Hayward fault runs through densely populated cities in the San Francisco Bay Area. The San Jacinto fault bisects the fast-growing city of San Bernardino.

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