for National Geographic News
Saturday's South Asia earthquake serves as a tragic reminder of Earth's unpredictability. Now a computer simulation suggests that the San Francisco Bay Area may experience a large quake within the next two decades.
The computer program, dubbed Virtual California, estimates a one-in-four chance that the Bay Area will be rocked by an earthquake measuring between magnitude 7 and 7.1 sometime in the next 20 years.
"The USGS has estimated a 62 percent chance of a magnitude 6.7 or larger earthquake in the Bay Area within the next 28 years," said John Rundle, director of the Center for Computational Science and Engineering at the University of California, Davis. "So we're not talking about something that's all that different."
Over the next 45 years the projected chance of such a quake rises to 50 percent. In 80 years the odds reach 75 percent, according to the UC Davis-based program.
A quake of that size could be similar to the October 1989 Loma Prieta earthquake, which famously disrupted a World Series baseball game at San Francisco's Candlestick Park. That quake killed 63 people and caused some 5.9 billion U.S. dollars in damages.
Orders of Magnitude
Geologists measure the energy released by an earthquake using a logarithmic scale, which increases by powers of ten. This means a magnitude 7 quake is ten times stronger than a magnitude 6.
Using this scale, even increases of a few tenths of a point can indicate a dramatic difference in quake strength.
"We don't know exactly how big the [infamous] 1906 earthquake was, but I think that the consensus would be an estimate of 7.8," explained Donald Turcotte, a geologist at UC Davis. "Of course, that is much bigger than a 7 on this logarithmic scale."
The probabilities expressed in the Virtual California study fall off quickly after a magnitude 7 event, so that quakes larger than 7.1 are considerably less likely.
In 2000 the Virtual California simulation was used for a combined NASA/U.S. Department of Energy study to identify the California regions with the highest probabilities of near-future quakes.
The simulation's forecast map has proven uncannily accurate in succeeding years.
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