Variation of energy release, rate of occurrence and b-value of earthquakes in the Main Seismic Region, New Zealand

Abstract Time variations in the rate of occurrence, b -value and cumulative energy release have been calculated for earthquakes in the Main Seismic Region (MSR) of New Zealand from 1950 to 1977, and energy release also estimated over the longer period from 1840 to 1977. Although there were no clear premonitory changes in these parameters prior to the magnitude 7.1 earthquake at Inangahua in May, 1968, this event was followed by a marked increase in both the rate of occurrence and b -value of shallow earthquakes, other than aftershocks, throughout the MSR. There was also an increase in the rate of deep shocks. Despite the increase in numbers of events the rate of energy release since the Inangahua earthquake is much less than in the preceding 16 years. These observations seem to require more extensive stress relief than can be accounted for by Inangahua event in itself. The overall rate of energy release for shallow shocks, 1950–1977, is only about 12% of the average rate for the longer period 1840–1977, thus continuing a period of relative quiescence that has existed since the very active period 1929–1934. A similar long quiescent period followed another active period from 1843 to 1855. The energy release during the two active periods is approximately the same. If this long-term pattern of energy release is extrapolated the years from about 2000 could mark a change to another active period.