Foreshocks of the 1990 Tennyson & Weber earthquakes - would they have been recognised?

Technical Abstract

The foreshocks of the ML = 5.8 Tennyson earthquake had very similar waveforms indicating that they occurred in the same location (within a few 100 metres) inside the final aftershock zone. Two other clusters of activity occurred in the 38 days before the main shock in one location 40km outside the aftershock zone. These separate clusters were apparently related to the following main shock, but the actual physical relationship is not clear.

This study shows that such clustering activity is unusual in this area, but more investigation is needed before we can be confident that it is always related to an impending main shock. Stress drops of three events before the Tennyson main shock were lower than for earlier events and aftershocks.

This is an unexpected result that can possibly be explained by pre-seismic creep occurring before the main shock. A major limitation in the stress drop analysis was the low 50 Hz sampling rate used by the national network seismograph stations. There were only four Weber foreshocks, and they did not have similar waveforms. Spatial clustering of background seismicity prior to the Weber main shock was observed, but it was well outside the aftershock zone. This is considered to be part of the normal background activity within the subducting oceanic crust along Hikurangi Margin.