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.