It's been a hundred years since an earthquake and fire destroyed nearly all of San Francisco, but scientists today are still using that earthquake to better understand how to predict future tremors.
With the help of computer models the researchers are using what they know about the 1906 quake as a test for a relatively new approach to earthquake forecasting. One such model has been developed by John Rundle, Director of Computational Science and Engineering at the University of California at Davis. The model, which he calls "Virtual California," takes the information gathered from field research about earthquakes and plugs it into a complex mathematical formula that only a supercomputer can crunch. Then it can be used, according to Rundle, "to produce arbitrarily long simulated histories of earthquakes on into the future."
If that all sounds a bit familiar, that's because it's roughly the same process used to forecast the weather. "What we're talking about here actually is the same type of process for earthquake fault systems," he says.
But weather forecasters need to wait only a few hours to see how well their weather models have worked. How do earthquake researchers test their model to see if it's accurate? They use the model to "predict" past earthquakes. The better the job the computer does mimicking past events, including San Francisco's great quake, the more confidence scientists have in the model for future events. It's a process they call "hindcasting."
Rundle says that, "Once you have such a [computer] model in place, you can then run it into the future." And whatever earthquakes the model generates, "will represent to a good probability what the future earthquakes will be in nature."
Scientists are still working to determine what geologic forces come together to make an earthquake happen at a particular time and place. Quake researchers like Rundle are constantly revising what information goes into the model and what level of importance to give it. While he has used the model to indicate that the area around San Francisco has a 25 percent chance in the next 20 years of having a major earthquake, "We aren't at the point where we have… a hundred percent confidence in these models of simulations," he says.
Rundle adds that not only are researchers able to use this as something to create forecasts, "We are learning a lot about the physics of the way the fault system operates." He says it allows them to experiment with changes in physics, "as if it were a laboratory." Then field researchers can go out and hunt for earthquake clues they might otherwise have not considered.
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