Rethinking earthquake prediction

nWe re-examine and summarize what is now possible in predicting earthquakes , what might be accomplished (and hence might be possible in the next few d ecades) and what types of predictions appear to be inherently impossible ba sed on our understanding of earthquakes as complex phenomena. We take predi ctions to involve a variety of time scales from seconds to a few decades. E arthquake warnings and their possible societal uses differ for those time s cales. Earthquake prediction should not be equated solely with short-term p rediction-those with time scales of hours to weeks-nor should it be assumed that only short-term warnings either are or might be useful to society. A variety of "consumers" or stakeholders are likely to take different mitigat ion measures in response to each type of prediction. A series of recent art icles in scientific literature and the media claim that earthquakes cannot be predicted and that exceedingly high accuracy is needed for predictions t o be of societal value, We dispute a number of their key assumptions and co nclusions, including their claim that earthquakes represent a self-organize d critical (SOC) phenomenon, implying a system maintained on the edge of ch aotic behavior at all times. We think this is correct but only in an uninte resting way, that is on global or continental scales. The stresses in the r egions surrounding the rupture zones of individual large earthquakes are re duced below a SOC state at the times of those events and remain so for long periods. As stresses are slowly re-established by tectonic loading, a regi on approaches a SOC state during the last part of the cycle of large earthq uakes. The presence of that state can be regarded as a long-term precursor rather than as an impediment to prediction. We examine other natural proces ses such as volcanic eruptions, severe storms and climate change that, like earthquakes, are also examples of complex processes, each with its own pre dictable, possibly predictable and inherently unpredictable elements. That a natural system is complex does not mean that predictions are not possible for some spatial. temporal and size regimes. Long-term, and perhaps interm ediate-term, predictions for large earthquakes appear to be possible for ve ry active fault segments. Predicting large events more than one cycle into the future appears to be inherently difficult, if not impossible since much of the nonlinearity in the earthquake process occurs at or near the time o f large events. Progress in earthquake science and prediction over the next few decades will require increased monitoring in several active areas.