Abstract

Since the processes that cause earthquakes in the eastern United States are poorly understood, it is not yet possible to predict where and when future earthquakes are likely to occur in this region. One approach to forecasting future locations of large earthquakes in this region is that used as part of the methodology for developing the latest generation of National Seismic Hazard Maps. This approach involves using the spatial distribution of small earthquakes (m ≥ 3) to forecast the locations of large (m ≥ 5) earthquakes. The hypothesis that underlies this approach is that the smaller earthquakes are delineating locations that are capable of generating larger earthquakes. We were curious to see how well such an approach would work for a scaled-down simulation in which the spatial distribution of even smaller earthquakes (m ≥ 2) recorded by seismic networks in the northeastern United States and adjacent areas is used to “forecast” the locations of the larger earthquakes (m ≥ 4) that have already occurred. We use such a scaled-down simulation to investigate the extent to which, in this situation, the spatial distribution of small earthquakes forecasts the locations of larger earthquakes in an intraplate environment. The essence of our procedure is to analyze systematically how often previously occurring small earthquakes occurred in the vicinity of large earthquakes. We devised three methods of summarizing and reporting whether small earthquakes tended to occur in the epicentral regions of future large earthquakes. We found that, on average, large earthquakes occurred in the vicinity of small earthquakes more frequently than would be expected for a random distribution of large earthquakes. There are, however, a significant number of large earthquakes that would not have been forecast based on the distribution of small earthquakes. This suggests that, if we are going to use this approach, we will have to be satisfied with a significant number of “misses” in our ability to forecast locations of large earthquakes. Perhaps this level of ability to forecast locations of large earthquakes is a realistic estimate of the level of success we can expect to achieve given the current state of knowledge of the mechanisms that cause earthquakes in this region.

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