Recurrent earthquake sources

Abstract

The incidence of high seismicity at particular localities in New Zealand (excluding the North Island region of Quaternary volcanism) is of three kinds; mainshock-aftershock sequences, swarms, and recurrent concentrations of shallow earthquake activity. Recurrent seismicity is named a Recurrent Earthquake Source after the two distinguishing features of continuous activity and concentration in space. Characteristic features of this seismicity are distinctive at the regional seismic and microseismic level. Statistical studies show the data distribution is significantly different from surrounding areas. Large earthquakes above ML 7.0 have been recorded close to these areas and simple mainshock-aftershock sequences and swarms occur as part of the recurrent seismicity. All recurrent sources studied here are located in the overlying or top of the underthrust plate and activity may be related to the plate boundary. Recurrent earthquake sources have been positively identified at two localities, Palliser Bay and Wanganui. Activity near Masterton appears to be a recurrent earthquake source but further work on microearthquakes is necessary; Hanmer Springs does not display the typical features. The principal occurrence of persistent activity is near Wanganui, where located earthquakes have occurred at a fairly steady rate of about 6 per year since 1964, the largest magnitude being ML 5.0. Recurrent seismicity at Wanganui has withstood the test of time and has been observed at the regional and microseismic level. Recurrent seismicity observed at Palliser Bay since 1978 is limited to microearthquakes and has an irregular rate of occurrence with an average of 6 events per year. The activity occupies a very small area of about 23 km2 and at a depth of 28 km lies in the subducted plate. The difference in magnitude range and areal extent well reflects the different scale of Wanganui activity in comparison to Palliser Bay where the largest magnitude is 4%