Waves keep rolling into NZ after quake

Small tsunami waves could continue around New Zealand's coastline until late tonight after yesterday's magnitude-8.2 earthquake off the coast of northern Chile.

GNS Science senior geophysicist William Power said for most of the coastline the tsunami waves had a range of about 20 centimetres, going from about 10cm above normal to 10cm below.

Tide gauges indicated a larger impact on the Chatham Islands, where the range from top to bottom was almost a metre – from almost 50cm above normal to 50cm below.

"What's happening in the Chatham Islands is reasonably sizeable and big enough to be a genuine cause for caution for small boats or if anybody was swimming," Power said.

"For most of New Zealand it's observable on instruments but probably people aren't going to notice it at the beach."

The tsunami could induce stronger than normal currents and care was needed.

It had been reasonably clear yesterday, once the tsunami reached a deep ocean tsunami buoy about an hour after the quake, that it would not be highly serious for this country, Power said.

"You can usually tell quite a lot just from the magnitude of the earthquake but there are other factors that can affect how much tsunami is generated, other than just magnitude.

"That's why it's particularly useful to get the information from the deep water buoy to be sure about things."

Tsunami activity would probably stay at the current level through to maybe 10pm and then slowly die off. The peak had possibly been reached in the Chatham Islands by 10.30am.

In 1868, an estimated magnitude-9 earthquake off what is now southern Peru caused a tsunami that reached up to 10m high in the Chatham Islands and 3m-4m around the main islands.

"That would be a big deal if it happened again, that would have quite an impact," Power said.

With quakes off western South America, magnitude-8.5 was a kind of cut-off level at which the tsunami threat to New Zealand started to be taken very seriously.

Yesterday's quake was a rupture between two plates, along part of the area where the Nazca Plate is being thrust eastwards and subducting underneath the South American continent.

"Essentially the one underneath is thrust under the overlying plate," GNS Science seismologist Martin Reyners said.

"We have a similar situation along the east coast of the North Island where the Pacific Plate is actually sliding under the Australian Plate."

In Fiordland it was happening the other way round, with the Australian Plate sliding under the Pacific Plate.

Hazard models developed for New Zealand included events like the Chile quake, Reyners said.

"They're eventually going to happen along the east coast of the North Island, for example, but the recurrence interval is quite long, so we haven't had any in our historical record of this size."

Yesterday's quake was centred in an area referred to by seismologists as a seismic gap, meaning a part of an active fault known to produce significant earthquakes but which has not slipped in an unusually long time.