Earthquake assessment in the Bay of Plenty

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NIWA scientists are discovering geological faults beneath the seabed in the volcanic coastal region of the Bay of Plenty.

A 25-day survey of the seabed in the coastal waters of the Bay of Plenty will begin on 10 November. It will be carried out by geologists and geophysicists from the National Institute of Water and Atmospheric Research (NIWA), in collaboration with British scientists from the Southampton Oceanographic Centre. The survey will be undertaken on NIWA’s research vessel Tangaroa, which will be carrying sophisticated and expensive equipment designed to image the geological faults on and beneath the seafloor. Seabed samples will also be collected to try to date the rocks that are being faulted, and to help identify which faults are still active and likely to rupture in the future. It is these active faults which can cause potentially damaging earthquakes.

The scientists hope that studying the submarine faults that appear to have been the most active in the recent geological past will give them a better understanding of how the Taupo Volcanic Zone extends offshore, and which of the geological faults have the potential to cause large earthquakes that may damage urban centres such as Tauranga and Whakatane. Four earthquakes with a magnitude greater than 5 occurred within 50 km of Whakatane during the 50 year period before the 6.2 Edgecumbe earthquake in 1987. Earthquake damage can be caused either directly by ground shaking or by earthquake-induced "tsunamis" or "tidal waves", such as that which destroyed the Alaskan city of Anchorage in 1964.

On land, geologists are sometimes unable to see the geological faults because they are buried beneath the ground or are covered by areas of dense bush. Furthermore, geophysical surveys designed to image faults under the ground in such areas are slow and very expensive. In contrast, the seabed is a good place to study active faults because there are few obstacles to the imaging techniques of a research vessel like Tangaroa. The seismic reflection equipment used on Tangaroa is a little like an echosounder that a fishing boat would use, but it is much more powerful, so not only is the seabed imaged, but so are the faulted rocks beneath it. Another piece of equipment, the sidescan sonar, sends sound out sideways, and echoes build up an image like an aerial photograph of the seabed and its faults and folds. The scientists hope to improve understanding of how earthquake faults grow.

The survey will repeatedly pass over the submarine faults that are thought to be the most active. These include the offshore parts of the Waiohau, Whakatane, Waimana, and Waikaremoana faults. Each pass will generate a picture, called a seismic reflection profile, which looks like a slice through the faulted rocks beneath the seabed. The profiles will enable the geologists to map the faults carefully and work out their history of fracturing. From the maps that show how big the faults are, and where they occur, it will be possible to estimate the size of an earthquake that could be generated when the fault suddenly ruptures.

The size of a possible earthquake, its ability to generate a tsunami, its distance from population centres, and the frequency at which large earthquakes might occur are important to engineers and town planners, who are responsible for ensuring that building design codes and coastal structures are adequate.

The offshore Bay of Plenty corresponds to a zone of active tectonics within the Taupo Volcanic Zone (TVZ). The TVZ is a zone of volcanism and active faulting associated with the plunging of the Pacific crustal plate beneath the eastern coast of the North Island. Volcanism at White Island and Motuhora Island appear to be the offshore extension of a line of onshore volcanoes extending from Mt Ruapehu to Mount Tarawera. Active faults are dominated by structures restricted to a 15-20 km wide zone on the east side of the TVZ. This zone is a region of intense faulting, with repeated movements in the last 50 000 years. The fault zone extends offshore into a 15 km wide seabed depression known as the Whakatane Graben, and the White Island Fault, which will be extensively surveyed.

Tangaroa leaves Wellington on 10 November and will arrive in the Bay of Plenty on the 11th. The vessel will call into Tauranga on 23 November for a crew change and to calibrate one of the instruments. Tangaroa is expected to be tied up between 10 a.m. and 3.00 p.m.

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