Map Forecasts Earthquakes

18 May 2005 (All day)

What's shakin'? On this color-coded map of earthquake risk, the warmer the colors, the hotter the chance of a damaging quake.

Starting tonight, Californians will be able to check their local earthquake forecast, thanks to a new online map that continually updates the chances of strong shaking throughout the state.

The map is the brainchild of seismologist Matt Gerstenberger of the U.S. Geological Survey (USGS) in Pasadena and colleagues at USGS in Menlo Park and at the Institute of Geophysics in Zurich, Switzerland. The researchers were able to set up the autonomous forecasting map thanks to advances in computer automation and in calculating earthquake distribution over small distances.

Though the map looks bland--most of the state is softly colored in low-risk blues and greens--there's a lot going on beneath the surface. The map uses all the known faults in the state and records of past earthquakes to generate a baseline historical level of risk. Whenever an earthquake occurs, a computer runs through mathematical relationships that estimate size (big quakes are exponentially less likely than small quakes) and frequency (there tend to be half as many aftershocks 2 days after an earthquake than there are 1 day afterward, only a third as many aftershocks 3 days afterward, etc.). After it calculates how many aftershocks are likely to happen, where they are likely to occur, and how big they are likely to be, the computer redrafts the probabilities on the map accordingly. Colors indicate risk and run from blue to red: blue being a one in a million chance of level 6 shaking (shaking strong enough to break windows and crack plaster); red, higher than one in 10.

The map will be most useful, the researchers say, in the aftermath of large quakes, which are often followed by aftershocks. Usually less powerful than the original quake, aftershocks can still cause substantial damage, and advance warning can give areas at risk time to prepare.

"It's the state of the art," in earthquake forecasting, says Thomas Heaton, a seismologist at the California Institute of Technology in Pasadena. He says related tools that display the areas most shaken by recent quakes are already well-used by emergency personnel, but how the public will use the probability map isn't clear. Efforts to create an 'early warning' system that announces quakes over the radio (radio waves travel faster than earthquakes) are ongoing, but at present, "this is as good as it gets."

Related sites
The map
USGS Earthquake site

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