Gravity, seismicity, and tectonics of the North Island, New Zealand

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Abstract
The distribution of earthquakes and gravity anomalies in New Zealand are compared. Shocks classified as 12 km or less are more widespread than those classified as 33 km, but all crustal shocks are symmetrically disposed about the axis of the negative isostatic gravity anomalies. A minimum of seismicity occurs between the base of the crust and about 80 km depth which conflicts with the author's previous suggestion that low densities between these depths are associated with seismicity. The Benioff zone dips at 63° and high densities associated with this zone would be consistent with the positive gravity anomaly. In plan, the main belt of shallow earthquakes follows the negative gravity anomaly axis in changing direction from SW to SSE through Cook Strait. A relatively aseismic region in the upper crust following a parallel strike from south Taranaki through Tasman Bay to Kaikoura separates the main zone from a further active belt running from New Plymouth through north-west Nelson to Canterbury. All these directions cross the main geological structures almost at right angles in the Cook Strait-Marlborough area. There is some evidence that the deeper earthquakes tend to change their strike in a similar manner