Daily Earthquake Forecasts for California Now Online

Stefan Lovgren
for National Geographic News
May 18, 2005
Starting today, Californians will be able to check the Internet for their daily earthquake forecast.

A map, which can be viewed at, shows, in color coding, the probability that a location will be hit by severe shaking over the next 24 hours.

Large earthquakes are notoriously difficult to predict. They don't occur often enough for scientists to gather data and there is an inherently random aspect as to how quakes occur. The new model primarily forecasts the additional ground shaking that follows an earthquake.

To build their model, the scientists combined long-established earthquake probability models with information about California's faults, such as the San Andreas.

"The breakthrough is getting something like this to work in an automated sense, in real time, and to have it out there running on its own," said Matt Gerstenberger, a geophysicist with the U.S. Geological Survey in Pasadena, California.

Gerstenberger led the research, which will be published tomorrow in the academic journal Nature.

Background Check

Earthquake forecasts are usually very vague. The simplest model assumes randomness—it says the probability of an earthquake of a particular size occurring in any given location is always the same.

One model, known as the Gutenberg-Richter law, states that small earthquakes occur much more often than large ones. Another model, known as the Omori law, shows that big earthquakes are followed by smaller aftershocks, which then decrease with time.

"Everything that we do is based on these two ideas that are well established," Gerstenberger said.

He and his colleagues used so-called background hazard maps that show California's earthquake risk over 30 years. These maps are based on the behavior of the region's fault lines. The scientists then translated those 30-year probability levels into 24-hour probability levels.

The next step involved factoring in the anticipated add-on effects of any medium or large earthquakes that have occurred in the region in the past few years.

The final result is a map of earthquake risk for any given location in California. The model shows the probability of earth movement that would be strong enough to throw objects off shelves. (The model does not calculate an earthquake's magnitude, which measures the energy contained in a quake.)

"Our ability to predict earthquakes remains very limited, but this research is a big step forward," said Thomas Jordan, director of the Southern California Earthquake Center at the University of Southern California in Los Angeles. "[I]t combines what we know about short-term and long-term earthquake probabilities to provide what is, for now, the best earthquake forecast for California."

Red Spots

The U.S. Geological Survey will continually update the 24-hour forecasts on the Web site.

The color coding of the map, showing the probability of severe shaking, goes from blue (small probability) to red (great probability). But most of the information in the maps has to do with aftershock sequences.

"You're not going to see red spots unless there's already been a large event," Gerstenberger said.

He hopes the forecast will be used as an educational tool.

"I don't see this as a weather forecast where people will look at it every day," he said. "But people can look at it and get a general idea of how the hazard works."

"By putting it in a visual format, we can remind the public that these things can happen," he added. "If they're somewhere with high probability, they may want to make sure they've done the standard things to be prepared for a quake."

The model could have important public-sector implications in the aftermath of a large earthquake, helping local authorities to coordinate rescue activities, for example.

In the future the tool may also be used outside California, in other earthquake-prone areas around the world.

"The model incorporates what we currently understand about earthquake forecasting," said Jordan, the Southern California Earthquake Center director. "Because it has been verified against data, it provides a new, more rigorous framework for assessing other earthquake-prediction experiments."

"In the scientific race to improve our ability to predict earthquake[s], it's the horse to beat," he added.

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