A Statistical Survey of Earthquakes in the Main Seismic Region of New Zealand

Abstract

Earthquake data from the main seismic region of New Zealand have been examined for trends in the total recorded activity and in its distribution over the region. During the years 1942–61 there has been a continuing increase in the numbers of deep-focus shocks with depths of over 100 km. A more than four-fold increase remains after corrections have been made for changes in the recording network. At the same time there has been a decrease in shallow activity (depths of 100 km or less), chiefly in the numbers of shocks belonging to swarms or aftershock sequences. Despite these changes, the total rate of earthquake occurrence has remained approximately constant. The latitude of the observed mean epicentre has shown a northerly drift of 8·6 km per year over the period. The three major factors contributing to this drift are the changes in the recording network, the increase in (low-latitude) deep activity, and a smaller drift within the shallow component alone. There is evidence that groups of related deep-focus shocks occur, and that for both deep and shallow shocks, grouping is a more pervasive phenomenon than is commonly supposed.