Analysis of the Weber (Dannevirke) earthquakes of 2 February and 13 May 1990, and their aftershocks

Technical Abstract

A series of four large earthquakes with epicentres very close to one another occurred in 1990-1992 in the Weber region of the southeast coast of the North Island, New Zealand. The region is one of oblique plate convergence and subduction, the plate interface being at about 20km depth.

The first event, February 19 1990, ML 6.1, occurred within the upper part of the subducting Pacific plate on a steeply NW dipping normal fault, as defined by relocated aftershocks. The mechanism of this event reflects down-dip tension within the subducting plate due to “slab pull” forces, as is common in New Zealand.

The second event, May 13 1990, MW 6.4, occurred on an imbricate thrust fault in the overlaying Australian plate, dipping shallowly NW, with a component of dextral strike-slip motion. The slip vector is close to the direction of plate convergence.

Two subsequent large events, both of magnitude 5.5, occurred on August 15 1990 and March 2 1992, and extended the aftershock zone of the deeper first main shock to the NE, matching the extent of the shallower second main shock.

Mechanisms of aftershocks of the two larger events are quite variable but as a whole are similar to their relevant mainshock. The time history of the aftershocks of the first (deeper) event shows a period of quiescence beginning 35 days before the second (shallow) event; the mean magnitude of those events that did occur was high. This cannot be explained by station faults, weather, or changes in analysis methods.