New Zealand seismicity 1964–87: An interpretation

Abstract

The years 1964-87 represent a relatively quiet period in the historical record of large earthquakes in New Zealand. Nevertheless, over 7000 earthquakes of magnitude M_L ≥ 4.0 were located with the National Seismograph Network during this period, and these provide important information on the structure and tectonics of the country. Events deeper than 40 km reveal the morphology of the subducted plates in Fiordland and at the Hikurangi Margin. When combined with microearthquake results in the central South Island, these deeper earthquakes suggest that plate convergence is accommodated along a common northeasterly trend throughout New Zealand. Variations in the slope of the deeper limit of seismicity and in the depth of maximum earthquake activity in the dipping seismic zone at the Hikurangi Margin provide evidence for lateral segmentation of the subducted plate. Earthquakes shallower than 40 km lack a clear association with major late Quaternary fault traces. However, the distribution of some of these events can be related to the large earthquake cycle. For example, the concentration of shallow seismicity along the east coast of the North Island between Northern Wairarapa and Gisborne can be interpreted as relaxation in the subducted plate following the 1931 Hawke's Bay and 1932 Wairoa earthquakes. There is also evidence that the degree of coupling of the plate interface at shallow depth may control the state of stress in the dipping seismic zone at the Hikurangi Margin.