An earthquake on par with the one that devastated San Francisco in 1906 has a 25% probability of striking the city again within the next 20 years, according to new research.
Reliably forecasting the timing, location, and magnitude of an earthquake has been a difficult problem to crack. Traditionally, scientists have measured how fast strain accumulates in geological faults or studied past earthquakes to determine their pattern of recurrence. A study the U.S. Geological Survey conducted in 2002 used the latter approach to calculate an 18.2% chance that a 7 or greater magnitude earthquake emanating from the San Andreas Fault would hit the San Francisco area between 2002 and 2031. (The 1906 earthquake is estimated to have had a magnitude of 7.9.)
In the new study, physicist John Rundle and geologist Donald Turcotte of the University of California, Davis, and colleagues blended the two approaches. Using a computer simulation called Virtual California, which factors in geological stress accumulation in the San Andreas Fault and nearby faults, the interactions among the faults, and the region's history of earthquake occurrence, the team simulated quake activity in the San Andreas Fault over the next 40,000 years. The simulation predicted 395 repeats of the 1906 earthquake--an average of one every 101 years. Based on this data, the probability of another earthquake of magnitude 7 or greater striking the San Francisco region in the next 20 years is 25%, the team reports online this week in Proceedings of the National Academy of Sciences. That probability rises to 50% in the next 45 years, and to 75% in the next 80 years.
Similar analyses in the future may also give scientists more-detailed information about earthquake activity to come, says Steven Ward, a geophysicist of the University of California, Santa Cruz. For example, he says, computer simulations may one day help estimate how frequently a magnitude 6 quake will precede a magnitude 7 in a given region. And although earthquake prediction remains controversial, Turcotte notes that both the U.S Geological Survey and Virtual California came up with similar forecasts.