Foreshocks of shallow New Zealand earthquakes

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Abstract
A foreshock is defined as an earthquake occurring within 30 days before, and within 50 km of, a main shock. This is based on a 4-fold increase in the average rate of activity in this interval, which is too small an increase to be of itself useful for prediction purposes. This foreshock activity is not confined to the area defined by the subsequent aftershocks. Many, but probably not all, earthquakes of ML⩾6 in New Zealand have foreshocks. The largest fore shock is often about 2 magnitude units smaller than the main shock. There is no readily discernible pattern in the spatial distribution of main shocks with and without foreshocks.

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