Image: Forecast map
This color-coded earthquake forecast map from the U.S. Geological Survey shows the outlook for an earthquake strong enough to knock objects off shelves over the 24 hours ending at 9:22 a.m. PT Thursday. According to the map, the chances are pretty slim.
updated 5/18/2005 5:45:30 PM ET 2005-05-18T21:45:30

California residents wondering if tomorrow’s forecast will be sunny now can find out if there’s also a chance of afternoon tremors.

Scientists launched a Web site Wednesday that calculates the probability of strong ground-shaking at specific locations over a 24-hour period.

The forecast maps, updated hourly, would be most useful after a temblor strong enough to break windows and crack plaster, according to U.S. Geological Survey seismologist Matthew Gerstenberger, who developed the site.

After a big earthquake hits an area, scientists know there will be aftershocks, but they can’t pinpoint when or where. Now residents rattled by a quake can go online and check for the possibility of more jolting in their area.

Details appear in Thursday’s issue of the journal Nature.

The chances of the maps showing when and where a significant earthquake will strike, however, are slim most of the time, scientists say.

“It doesn’t tell us when the ‘Big One’ is coming,” said Lucy Jones, scientist in charge of the USGS office in Pasadena. “It tells us there’s an increased chance of shaking.”

California residents already can view real-time earthquake maps with the click of a mouse, but those are usually posted and updated within minutes of a temblor occurring. Now they can click on real-time, color-coded maps that provide earthquake probabilities in a specific region.

Areas shaded in red represent a high chance of strong shaking within the next 24 hours (less than a 1 in 10 chance) while those in blue represent a very remote chance, say, more than 1 in a million.

“If there’s a red spot, then make sure you’ve done what you need to do in terms of earthquake preparedness,” Gerstenberger said.

While the forecast maps are not a “silver bullet” in quake prediction, they are the first steps in providing the public with more refined quake probabilities, said Tom Jordan, director of the Southern California Earthquake Center.

The earthquake forecast maps are created by considering a variety of factors, including seismic monitoring of the San Andreas Fault and other active faults in California. Scientists also factor in any recent history of small and large temblors and aftershocks on those same faults.

In an accompanying commentary, Duncan Agnew of the Scripps Institution of Oceanography at the University of California, San Diego, noted that the latest forecast maps give earthquake victims a “much more precise answer” about the risk of aftershocks after a strong tremor. Agnew, who was not part of the project, also said he would like to see the same method used in other countries that are vulnerable to earthquakes.

Forecasts are posted at

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