Feb 27 2007 - Edgecumbe Earthquake, Twenty Years On

Bay of Plenty network map 1987.

On March 2 1987, at 1:42 in the afternoon, the Bay of Plenty was rocked by a magnitude 6.1 earthquake which caused widespread damage. Although centred close to Matata, the destruction that occurred in and around Edgecumbe has forever associated that day with the town.

Hardest hit were the towns of Edgecumbe, Te Teko, Kawerau, Matata and Thornton, where chimneys toppled and poorly constructed houses suffered serious damage. In Edgecumbe the intensity of the shaking cracked asphalt in roads and footpaths, damaged river embankments, toppled a locomotive and tore large power transformers, weighing up to 20 tonnes each, from their mountings. The town's railway lines were also extensively damaged, bending and buckling under the huge force of the quake. In the Rangitaiki Plains the quake ruptured the ground to create a spectacular 7 km rift over one metre wide, part of a longer 13 km tear in the earth's crust.

The quake was felt over most of the North Island, including Hamilton, Taupo, Napier and Gisborne. It was preceded by a number of foreshocks in Matata and Thornton, and followed by a series of over 200 aftershocks. One of the largest foreshocks was of magnitude 4.9, and occurred only 7 minutes before the main shock. Luckily, a number of weak and poorly constructed buildings were evacuated in response to this foreshock and were consequently empty when they collapsed in the main earthquake. There were in fact no fatalities, but over 20 people required medical attention.

In 1987, GNS Science's predecessor organisation, the Department of Scientific and Industrial Research (DSIR), operated the seismic monitoring equipment. The map on the right shows the installations operating on the day of the Edgecumbe earthquake; the earthquake's epicentre is indicated by the star.

The stations at WIZ, TAZ and UTU (small red circles) radioed their information to Rotorua, where the seismic waves were drawn onto pen and paper drum recorders. This was the only place to see the events unfold as they happened. Instruments at the Matahina Dam east of Kawerau (green square)recorded their data onto 35 mm film, and these were collected several days after the earthquake. They represented some of the best recorded strong shaking in New Zealand at that time.

In the subsequent 20 years New Zealand has significantly developed its monitoring capability. The first improvements were to collect seismic signals digitally. By late 1990, New Zealand had established a network of seismographs that stored their data on magnetic tapes. This included a countrywide network of stations spaced at about 100 km apart, as well as regional networks to monitor volcanic activity around Tongariro National Park, Lake Taupo and the Rotorua Lakes, and to study the earth processes affecting the Wellington region. However, apart from these regional networks, data were only received by the Seismological Observatory on a weekly basis, as magnetic tapes were changed and the data posted to Wellington.

Bay of Plenty network map 2007.

This is in contrast with the present day situation shown at right. The main features are an expanded Bay of Plenty network (small red circles), with the newest station becoming operational at Ohinepanea (OPRZ) last week. Note too that strong motion recorders (green squares) are located at many more sites.

Besides this expansion, the major advancement from 1987 is that all data from these recording stations are received at the data management centres in Wellington and Taupo within seconds of it occurring.

Whilst we still cannot predict the occurrence of an earthquake such as Edgecumbe, if the same thing was to happen today we could do much more. Within five to ten minutes the areas to experience greatest shaking would be known. Within another ten minutes the epicentre and magnitude of the earthquake would be determined. Over the subsequent hours the GeoNet web site would collect responses from the affected community, and these would be shown on the recently released shaking maps. The quicker we know these things, the quicker those tasked with reacting to disaster can formulate and carry out their plans.