NZ gets off lightly with tremors

The tsunamis triggered by the massive Indonesian earthquake had a tiny impact on New Zealand.

Seismologist Dr Warwick Smith of the Institute of Geological and Nuclear Sciences said he had been told a tidal gauge off the coast of Fiordland had recorded a tiny effect from the tsunamis, the ocean rising between 10cm and 20cm.

The Asian quake happened when two giant tectonic plates which rub together, the Australian plate and the Eurasian plate, ruptured along a 1000km intersection, or faultline, below the ocean.

One plate pushed up and over the other with unimaginable force.

Dr Smith said the Asian earthquake was "absolutely huge".

"What we're talking about is a length of the plate boundary that's about 1000km long that has ruptured, lifted up under the water, and displaced all that ocean," he said.

All along the rupture, the seafloor was shunted vertically by about 10 metres.

The surface of the Earth is composed of seven large plates and a dozen or more smaller ones. Most earthquakes happen where the giant plates intersect, called faultlines. All the plates move a few centimetres a year.

New Zealand's position on the boundary of the Australian and Pacific plates is the reason for the large number of earthquakes in this country. Underneath New Zealand, the two huge plates are grinding together in three ways.

To the east of the North Island the Pacific plate is being forced under the Australian plate. Under the South Island the two plates push past each other sideways, and to the south of New Zealand the Australian plate is being forced under the Pacific plate.

The boundary of the Australian and Pacific plates is included in what's known as the "Ring of Fire", along which earthquakes are likely to occur and on which New Zealand sits.

It stretches past Chile and California up to Alaska, down to Japan and the coast of Southeast Asia and to the north of Australia to New Zealand.

The earthquake, which sent huge waves rolling around the southeast Asian coast measured 9 on the Richter scale, the fifth-largest since 1900.

Dr Smith said despite more than three decades of intensive scientific research, it might never be possible to predict earthquakes of the magnitude of this latest one.

"In terms of ever being able to say batten down the hatches at half-past two on Tuesday, no, I don't think we'll ever be able to do that," he said.

"A lot of seismologists feel it's just so random and it really is inherently too unpredictable to be able to specify a date and place."

New Zealand has two major faultlines, the Wellington fault and the Alpine fault in the South Island. The Wellington fault last moved around 300 years ago and research shows it moves on average once every 600 years.

The alpine fault moves about once every 300 years but had also not moved in historical time.

New Zealand's biggest earthquake was in 1855 near Wellington. At 8.2 on the Richter scale, it "sent a few colonists packing back to Mother England", he said.

But New Zealand had got off "lightly" in earthquake terms over the past 50 years, he said.

But whether that meant the big one was looming was impossible to say.

"When you go over the previous 100 years, we had rather more than we've had in the past 50 years in terms of what the normal pattern might be."

The Institute of Geological & Nuclear Sciences records about 14,000 earthquakes in and around New Zealand each year. Most are small, but between 100 and 150 are big enough to be felt.

The highest ever recorded tsunami was on July 9, 1958, in Alaska, which caused a wave around half a kilometre high.