Where--and when--will the next Northridge-like quake occur?
Seismologists meeting here say the good news is that there is only a 10% chance of that happening in any specific locale over the next half-century. The bad news is that 7 of every 10 Californians are at risk of it happening where they live.
Seismic hazard maps for California released Wednesday show that about 70% of the state's population lives in areas where there is at least a 10% probability of destructive shaking in the next half century.
Destructive shaking--defined as at least 40% of the force of gravity--is the level at which the most vulnerable buildings will suffer damage, said state geologist James Davis at the annual meeting of the American Geophysical Union.
Davis said such shaking was common during the 1994 Northridge earthquake, where there were also many instrumental shaking readings of 60% or 80% of the force of gravity, and where a few readings reached as high as 180% of the force of gravity.
The seismic hazard maps were prepared jointly by the U.S. Geological Survey and the state Division of Mines and Geology. The most seismically risky areas in the state include a band as wide as 60 miles straddling the San Andreas fault through most of its length, as well as bands straddling faults in the San Gabriel and Santa Susana mountains in Los Angeles and Ventura counties and the San Jacinto fault in the Inland Empire.
Also at relatively high risk are offshore faults affecting Humboldt County on the North Coast, the Calaveras and Hayward faults in the Bay Area, and scattered parts of the Eastern Sierra.
Included in the least risky areas are most of the Central Valley. Sacramento has the least risk of destructive earthquakes of any big city in the state, and San Diego is well below the norm.
Parts of the city of Los Angeles are less risky than areas of the San Fernando Valley, and most of Orange County is less risky than Los Angeles County.
"We can have more confidence in these maps than in earlier ones," said Davis, who nonetheless conceded, along with other scientists at a news conference here, that there is always a chance of a much greater quake occurring than is indicated on the maps.
Davis and Arthur D. Frankel, chief of the mapping project for the Geological Survey, said the maps will be used by public and private agencies interested in estimating quake risks for the purpose of planning mitigation efforts, enacting building codes or, perhaps, setting insurance rates.
However, in an exhaustive chart listing groups that might make use of the maps, the new California Earthquake Authority, which began selling state residential earthquake insurance on Dec. 1, was not mentioned.
Study of the maps showed, in fact, that homeowners in some of the areas rated most risky in California are getting earthquake insurance under the new state system at some of the lowest rates.
Eureka on the North Coast, Palmdale astride the San Andreas fault and Palm Springs, for example, are in areas shown as among the most risky, yet homeowners there are being charged rates in many cases only a fraction of those being charged in equally risky parts of Los Angeles and Ventura counties and the San Francisco Bay Area.
Davis said that the Division of Mines and Geology is undertaking a study for the earthquake authority, and some of its rates may be revised.
But at the same time, he said it is his understanding that the authority has set its rates based not only on risk of a quake, but on its estimated damage claims in specific areas should a quake occur.
Greg Butler, chief executive officer of the new authority, said Wednesday that in calculating its initial rates, the authority was paying attention to how many customers it had in an area, rather than just the risk in that area.
"What our modeling suggests is what the losses will be to the California Earthquake Authority should a damaging quake occur," Butler said.
"So density is a factor. One house falling down in Eureka is different than 1,000 houses falling down in San Francisco."
At the same time, Butler fuzzed the issue somewhat when he added, "If there are changes in the scientific information that is considered credible, we will make changes in our [pricing] model."
The seismic hazard maps released Wednesday, while stating risk in terms of 50 years, were calculated on a prorated assumption of all quakes expected to occur within a 475-year-period, scientists said.
At an earlier presentation, a scientist with the Division of Mines and Geology, Chris Cramer, warned there could be variability of perhaps 25% in all risks shown on the maps.
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The state Division of Mines and Geology and the U.S. Geological Survey have released this map estimating that there is a 10% probability of shaking reaching the levels shown within the next 50 years. The minimum level at which the most vulnerable buildings suffer structural damage is 40% the force of gravity. In the Northridge quake, shaking levels reached a high of 180% of the force of gravity. The dark red portions on the map are the most hazardous, with a 10% likelihood of shaking reaching more than 60% the force of gravity over the next half- century. In dark blue areas there is a 10% probability within that time of shaking amounting to only 10% to 20% of the force of gravity.
Percentage of the force of gravity
Source: California Department of Conservation, Division of Mines and Geology