U.S. sets up earthquake site to keep Californians posted

The forecast Wednesday called for mostly stable ground in California.

The U.S. Geological Survey began publishing a public Web site Wednesday that displays the statistical probabilities of a modest earthquake occurring in California over the next 24 hours. Lowest- risk areas appear in blue on the map, highest in red. Nothing on the map Wednesday was red or even yellow, the next-highest level of concern.

The forecasts, based on data from past earthquakes, do not provide predictions or warnings of impending large quakes. Rather, they show the likelihood and distribution of aftershocks from an earthquake that has already occurred. Scientists have long observed recurring patterns in the distribution of the sizes of aftershocks and in how the frequency of aftershocks diminishes over time.

"It gives more solid numbers than what we've had in the past," said Dr. Matthew C. Gerstenberger, a scientist at the Geological Survey and the lead author of a paper describing the system that appears in Thursday's issue of the journal Nature.

The forecasting model does not include any geological information -- like earthquake faults -- or look at factors like bursts of electromagnetic radiation that some scientists say precede earthquakes.

Even when a smaller, foreshock earthquake precedes a larger, destructive one -- which occurs for about half of the large quakes in California -- the forecasts would not provide a meaningful warning. Although the foreshock would raise the earthquake probability in the surrounding area, the degree of alarm would be no greater than when only smaller aftershocks occurred.

Dr. Lucile M. Jones, the Geological Survey's scientist in charge for Southern California and another author of the Nature paper, said the forecasts would be of most use in the aftermath of a large temblor. "After an earthquake, people need to be making decisions," Jones said. "Do I go into this building and get my papers?"

She said she hoped the site would help people understand earthquake risks and dispel rumors like those that came after a magnitude 7.3 earthquake that shook Landers, about 100 miles east of Los Angeles, in 1992. According to the rumors, the Geological Survey scientists knew that the shaking had weakened the nearby San Andreas Fault and that a much larger quake was imminent but were keeping quiet to prevent panic. No larger quake occurred.

Jones said the Web site would tell people: "This is the best we got. This is what we know."

The forecast for Thursday, which is at pasadena.wr.usgs.gov/step/ and is updated hourly, will probably call for continued ground stability, with most of the map bathed in blue, indicating a probability of an earthquake causing significant shaking at less than one in 10,000.

"I don't see it necessarily as a weather forecast map," Gerstenberger said. "I don't expect people to look at it every day."

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