Seismicity and structure of North Island, New Zealand

The configuration of New Zealand's major seismic zone, which occurs with other geophysical features in the pattern that is typical of active continental margins and island arcs, is examined using computer-determined hypocenters for approximately 500 earthquakes since 1962. Earthquakes in the mantle occur in a tabular volume about 50 km thick, dipping 50° to 60° northwest under North Island from the landward side of the Hikurangi trench. Maximum depth of earthquake occurrence increases from about 200 km in the southwest to almost 400 km in the northeast. The more intense mantle activity takes place in the deeper part of the mantle zone and is partly overlain by the volcanic belt. Most crustal earthquakes lie between the trench and the northwestern limit of mantle earthquake activity. Active faults generally exhibit the same trend as the other features and lie within the region of crustal earthquakes. The structure of the North Island region is similar in some respects to that of the Tonga-Kermadec region to the northeast, but the two regions are separated by a gap in seismicity, a change in trend, and a discontinuity in the oceanic trench system.