Foreshock probabilities in New Zealand

Abstract

An event that is not already part of an aftershock sequence is considered a foreshock if it is followed by an equal or larger earthquake within 5 days and 30 km. The likelihood that an apparently isolated event (shallower than 40 km and greater than magnitude 5.0) in New Zealand is a foreshock averages 4.5 ± 0.7%. If the mainshock is required to have a magnitude at least one unit greater than the foreshock, the probability drops to 0.8 ± 0.3%. Little difference in foreshock probability is noticeable between two different aftershock-removal windows, between different magnitude ranges, or between different time periods. However, events deeper than 40 km yield probabilities that depend on magnitude and also on aftershock-removal parameters, suggesting that clustering of deep earthquakes occurs via a different process from that in shallow earthquakes. For shallow earthquakes, the results are consistent with a model in which foreshocks are manifestations of the same process as aftershocks.