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Meanwhile, in 1983, scientists with the U.S. Geological Survey predicted that a moderate earthquake was due to strike near Parkfield, California. The prediction was based on the observation that earthquakes with magnitudes of about 6.0 had occurred there in 1857, 1881, 1901, 1922, 1934, and 1966. On average, this is every 22 years, so the prediction was made for 1988 plus or minus five years. A large research effort was mounted to monitor the region. When the quake did not hit by 1993, the prediction was cancelled. The Parkfield incident contributed to an erosion of faith in specific earthquake prediction. But many seismologists are still hopeful about general earthquake forecasting. Instead of predicting specific events over short time scales (hours to days), the scientists hope to forecast the probability of earthquakes over longer periods. In 1988, the USGS said there was a high probability of a major earthquake in the Santa Cruz Mountains region sometime in the following 30 years. Sure enough, the Loma Prieta quake rumbled through the region in 1989. These probability forecasts are based on the idea that a fault builds up strain until it reaches a critical point, when it is released as an earthquake. Then the whole process starts over. The cycle won't necessarily be perfectly regular, like the ticking of a clock. But it will repeat in a predictable way, say proponents of forecasting. According to this theory, faults that haven't had a quake in the longest time are most at risk. This theory has been conventional wisdom for thirty years. However, it did come under attack recently when UCLA seismologist David Jackson and colleague Yan Kagan scrutinized a global set of forecasts made in 1979. They found that areas thought to be at low risk of earthquakes -- the ones that had recently had quakes -- actually experienced five times as many shocks as perceived high-risk areas. The seismology community is still debating the issue. Still, the dream of quake prediction is very much alive and well in some quarters. A group of Greek scientists claim to be able to predict specific earthquakes based on changes in electrical activity in the ground and say that they have done so three times in the last decade, although their methods and evidence have been sharply criticized. And regardless of the ongoing controversy surrounding the Greek predictions, many seismologists believe the problem is not the inherent unpredictability of earthquakes, but our ignorance of the processes that trigger them. In any case, as long as earthquakes remain deadly and destructive, we will dream of being able to foresee the next "big one" coming before it strikes.
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