Quake moved mountains - about 18m

An enormous earthquake near Wellington which struck 150 years ago this Sunday established New Zealand's reputation as the "Shaky Isles". Now scientists have found that it was even bigger than we thought.

The rupture on the Wairarapa Fault lifted the eastern edge of the Rimutaka and Tararua Ranges by up to 6m and was said to have pushed the ranges about 12m north of previously adjoining points on the east.

It was thought to have measured magnitude 8.1 or 8.2 on the Richter scale, making it New Zealand's biggest earthquake since Europeans began recording them. It generated a tsunami that swept over what is now Wellington Airport and into shops along Lambton Quay.

Now geologists Tim Little and David Rodgers have found evidence that the ranges west of the Wairarapa town of Featherston actually moved as much as 18.5m north of the adjoining countryside - by far the biggest horizontal movement along a vertical fault line ever recorded.

Dr Little said the magnitude was at least 8.3, the same as the 1906 San Francisco earthquake and about six times bigger than the disastrous Napier quake in 1931 that measured 7.8 and killed 256 people. (Magnitude 8 has 31 times as much energy as magnitude 7 on the scale.)

But there were only 3000 people in Wellington in 1855. The earthquake's historian, Rodney Grapes, quotes reports at the time that between two and six Maori died when their whare collapsed in the southern Wairarapa, and two other people "died in a fissure in the Manawatu".

Far more people will be at risk when the next big quake strikes, as there are now more than 420,000 people in the Wellington region.

Last month's Indian Ocean earthquake was even bigger, at magnitude 9 on the Richter scale. But it was a different kind of rupture - a fault on a slightly-sloped, almost-horizontal plane where new land pushed up into the sea, in contrast to the near-vertical Wairarapa fault where the land moved northwards on one side and southwards on the other.

Ten smaller earthquakes, mostly of the Indian Ocean kind, hit the Cook Strait area on Tuesday, raising fears that another big one could be imminent.

Tuesday's biggest registered 5.3 on the Richter scale, but Government geologist Martin Reyners said yesterday that the swarm of quakes had now tailed off.

Dr Little, of Victoria University, said the 1855 Wairarapa quake caused north-south movement along a 150km fault line from Cook Strait to past Mt Bruce, north of Masterton.

Like a piece of rubber fixed at both ends, the movement was greatest in the centre of the fault line around Featherston where Dr Little and Dr Rodgers, from Idaho State University, looked for traces in landforms and stream courses.

"We saw very large displacements," he said.

"They may not be representative of what happened, but the biggest we saw was around 18.5m and just about everywhere else we saw at least 14m or 15m, sometimes 16m or 17m.

"In some places the crack which forms is a very simple single break and all the slip occurs in a single crack. In other areas there is a whole area of cracks that share the displacement."

The biggest vertical uplift was at Cape Turakirae on the south coast between Wellington and the Wairarapa, where the beach was raised 6m, forming the latest of four raised beaches generated by a series of quakes on the Wairarapa fault over the past 6000 years.

The vertical uplift west of Featherston ranged between 0.5m and 3.8m.

Dr Little said repeated earthquakes along the fault had created the Rimutaka and Tararua Ranges over millions of years, driven by the force of the great Pacific plate on the Earth's crust diving underneath the Australian plate.

1855 Quake

* Struck at 9.17pm on January 23, 1855.
* Magnitude 8.3 - NZ's biggest quake in European times.
* Killed 4 to 8 people.
* Caused the biggest horizontal movement along a vertical fault line ever recorded.