Foreshock occurrence before large earthquakes

Rates of foreshock occurrence involving shallow M greater than or equal to 6 and M greater than or equal to 7 mainshocks and M greater than or equal t o 5 foreshocks were measured in two worldwide catalogs over similar to 20-y ear intervals. The overall rates observed are similar to ones measured in p revious worldwide and regional studies when they are normalized for the ran ges of magnitude difference they each span. The observed worldwide rates we re compared to a generic model of earthquake clustering based on patterns o f small and moderate aftershocks in California. The aftershock model was ex tended to the case of moderate foreshocks preceding large mainshocks. Overa ll, the observed worldwide foreshock rates exceed the extended California g eneric model by a factor of similar to 2. Significant differences in foresh ock rate were found among subsets of earthquakes defined by their focal mec hanism and tectonic region, with the rate before thrust events higher and t he rate before strike-slip events lower than the worldwide average. Among t he thrust events, a large majority, composed of events located in shallow s ubduction zones, had a high foreshock rate, while a minority, located in co ntinental thrust belts, had a low rate. These differences may explain why p revious surveys have found worldwide foreshock rates among thrust events in California (especially southern California), while the worldwide observati ons suggests the opposite: California, lacking an active subduction zone in most of its territory, and including a region of mountain-building thrusts in the south, reflects the low rate apparently typical for continental thr usts, while the worldwide observations, dominated by shallow subduction zon e events, are foreshock-rich. If this is so, then the California generic mo del may significantly underestimate the conditional probability for a very large (M greater than or equal to 8) earthquake following a potential (M gr eater than or equal to 7) foreshock in Cascadia. The magnitude differences among the identified foreshock-mainshock pairs in the Harvard catalog are c onsistent with a uniform distribution over the range of observation.