Does the spatial distribution of smaller earthquakes delineate areas where larger earthquakes are likely to occur?

In the development of the latest generation of National Seismic Hazard Maps, one of the component models used the spatial distribution of smaller earthquakes in the eastern United States to forecast the locations of larger earthquakes (Frankel, 1995). Variations of this hypothesis, that smaller earthquakes indicate where larger earthquakes are likely to occur, are found throughout earthquake studies. In a previous study (Kafka and Walcott, 1998), we tested this hypothesis for earthquakes in the northeastern United States (NEUS) to see how well the spatial distribution of smaller earthquakes recorded by seismic networks in the NEUS "forecasts" the locations of larger earthquakes that have already occurred. The essence of our procedure is to systematically analyze how often previously occurring smaller earthquakes occurred in the vicinity of larger earthquakes. The purpose of this study is to extend that investigation to other areas of the world to obtain a more global perspective on this issue. Here we report on an extension of that investigation to the southeastern United States, the New Madrid seismic zone, southern California, northern California, Israel, Turkey, and the entire eastern United States. Our results to date do, in fact, suggest that (in a variety of tectonic environments) the spatial distribution of smaller earthquakes delineates areas in which larger earthquakes are likely to occur. In a number of cases where locations of larger earthquakes were not forecast based on this approach, we suspect that the misses are, at least in part, due to incompleteness and quirks in the earthquake catalogs.