Graham Leonard: Tsunami triggers lurk off coast

A tsunami hit came as a result after a powerful earthquake in Natori, Miyagi prefecture, Japan. Photo / AP

Opinion

The Japan earthquake and tsunami is relevant to New Zealand because we are also exposed to large tsunami triggered by near-shore subduction zone earthquakes.

We have a section of subduction zone off the east of the North Island that is similar to that off the Tohoku coast in Japan.

These are tectonic plate boundary faults found around most of the rim of the Pacific plate, often called the Pacific ring of fire.

When the next large subduction zone earthquake occurs we can expect similar strong ground shaking across significant parts of the North Island, depending on which segment of the zone ruptures.

The closest segments are offshore from Wellington to East Cape, but the zone continues all the way to Tonga.

An earthquake on a segment beside New Zealand could possibly cause a tsunami of similar size to that seen in Japan, with the coast facing the fault receiving run-up heights that might in some places exceed 30m on steep coastal hills, and run-in distances over coastal plains of five or more kilometres.

New Zealand's science capacity and capability around understanding subduction zone earthquakes and modelling tsunami is similar to that of Japan, with our scientists often collaborating with the Japanese.

Japan has invested heavily in monitoring technology including earthquake early warning, where in earthquakes on faults 100 or more kilometres away, people can be warned of shaking seconds to tens of seconds ahead of time.

This happened in Tokyo after the 2011 earthquake began.

It's important to understand that this is warning of ground shaking, not of a tsunami threat.

The Japanese have also invested heavily in trying to predict tsunami within minutes of large near-shore earthquakes.

This is very difficult and they do have false alarms. It also requires a heavy financial commitment to monitoring equipment for earthquakes and scientists sitting waiting to interpret data within seconds 24 hours a day.

The likely rate of false alarms, even if such monitoring was funded here, continues to raise concern.

In New Zealand, we rely on the Pacific tsunami warning system for tsunami from sources across the Pacific where the earthquake is too far away to be felt here.

GeoNet provides additional advice on those warnings.

Luckily, the best warning of the largest and nearest tsunami sources is the earthquake itself - longer than a minute or hard to stand up, not necessarily both.

Graham Leonard, volcanologist and hazard scientist, GNS Science.