Bay of Plenty Quake Swarm

Published: Mon Jul 19 2004 8:00 AM

Scientists monitoring the swarm of earthquakes in the Bay of Plenty say they appear to be tailing off.

More than 100 shallow earthquakes have occurred north-east of Rotorua since the swarm started on Sunday afternoon. The largest, a magnitude 5.4 quake centred 20 km northwest of Kawerau, was felt widely in the Bay of Plenty. At least another five jolts were big enough to have been felt throughout Bay of Plenty. People living closer to the centre of the swarm would probably have felt dozens of tremors.

"If the pattern follows historical trends, the earthquakes will become less frequent and smaller from now on", said seismologist Ken Gledhill of Geological and Nuclear Sciences Ltd (GNS). "As the swarm tails off, there's always a small possibility of a larger quake going against the general trend."

Swarms of small shallow earthquakes were reasonably common in the Bay of Plenty, Dr Gledhill said. "However, this swarm is slightly unusual in that the magnitudes of the largest earthquakes are bigger than normal. We see this type of swarm perhaps once every decade in the Bay of Plenty."

The earthquake epicentres are clustered north-west of Kawerau and are related to long term tectonic stretching of the region between Rotorua and East Cape. At this stage, there are no indications that the earthquakes are related to volcanic activity.

Scientists from GNS's Wairakei office are intending to deploy two portable seismic instruments near the centre of the swarm later today. And within the next few days, GNS scientists will document the landslips and other possible ground movements.

Data recorded by the battery-powered seismic instruments will give scientists a more accurate fix on the depth and location of any further quakes in the swarm and a better idea of the relationship between the earthquakes and known active faults in the region.