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Two years of aftershocks

NZ Newswire logoNZ Newswire 2/09/2016

The East Cape area can expect up to two years of aftershocks following the magnitude 7.1 earthquake which sparked a 30cm tsunami, GNS Science says.

Friday morning's quake, centred 125km northeast of Te Araroa at a depth of 22km, struck in one of the more seismically active regions of New Zealand, where the Pacific Plate pushes beneath the Australian Plate.

Large earthquakes, greater than magnitude 6, occur regularly in this area, says GNS Science seismologist John Ristau.

"The aftershock sequence is likely to continue for an extended period of time," he told the Science Media Centre.

"The aftershocks for the 1995 M 7.2 earthquake continued for around two years, and since this earthquake is similar in size it will likely have a similar aftershock sequence."

Statistical seismologists are working on detailed aftershock forecasts.

The largest aftershocks were typically about 1 magnitude unit smaller than the mainshock, he said.

Hundreds of aftershocks, less than magnitude 4, would strike offshore and would not be widely felt.

Friday's quake did generate enough displacement of the seafloor to cause a small tsunami of around 25cm at East Cape.

Dr Ristau said there were two "slow-rupture" earthquakes off the coast of Gisborne in 1947 with magnitude about 7, which produced tsunamis with heights of several metres.

"With these earthquakes the speed at which the fault ruptures is much slower than is typical for an earthquake, and as a result it causes a large displacement in the seafloor but does not produce significant high-frequency energy.

"The high-frequency energy is what people normally feel, which means the earthquakes were only weakly felt and no one was aware that a large tsunami was approaching."

No one was killed by the tsunamis, which swamped the coast from Muriwai to Tolaga Bay, but if they had struck during summer beaches would have been crowded and there could have been deaths, according to Te Ara, the online encyclopedia of New Zealand.

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