Earthquake sirens sound in Napier again

Large crowds attended today's 75th anniversary service commemorating the 1931 earthquake with Prime Minister Helen Clark speaking at the Napier event.

About 300 people who survived the disaster were expected to be among the gatherings in both Napier and Hastings.

People were taking up their seats on the grassed area in front of the Sound Shell from 9.15 and by 9.45 there were already about 300 people assembled. Many were dressed in the '30s styles.

The service got under way at 10.30am after a concert by the Napier Tech Band, and at the conclusion of speeches from Mayor Barbara Arnott and the Prime Minister, the Veronica Bell, which had been set up on the Sound Shell stage, was rung to mark the moment, at 10.46am, the quake struck.

That was followed by the peeling of church bells and the wail of sirens.

The catastrophic Napier earthquake was one of five big shakes to hit New Zealand over six years in the late 1920s and 1930s.

All the quakes were larger than magnitude seven, and at magnitude 7.8, the February 3 1931 quake was the largest in the sequence.

It was New Zealand's second-largest quake since European settlement.

The quake, centred near Napier, struck at 10.47am. The energy involved was the equivalent of 100 million tonnes of TNT being detonated.

New Zealand's largest quake was the magnitude 8.2 quake in the Wairarapa on January 23, 1855, which significantly reshaped the young town of Wellington.

Recent research has indicated it was closer to an estimated magnitude of 8.3, about the same size as the disastrous 1906 San Francisco quake and six times more ground-shaking than the Napier quake, which killed 256 people.

But seismologist Warwick Smith, of GNS Science, said that though the string of quakes from the late 1920s may have seemed like a cluster of random events, modern research has shown that big quakes can sometimes trigger quakes on other faults, a phenomenon known as "stress triggering".

Since the Napier quake, New Zealanders have had a relatively easy ride in terms of killer quakes, but Dr Smith said the real risk from geological hazards is significantly higher than the last 60 years would indicate.

If an earthquake on the scale of the 1931 Hawke's Bay jolt occurred anywhere near a population centre today, it would cause devastating losses.

And there were so many New Zealand communities located within 10km of an active fault that people in many parts of New Zealand could experience an earthquake similar in size to the 1931 event.

Denser populations of modern communities mean potential losses would be greater.

Dr Smith said the 75th anniversary of the Hawke's Bay quake was a good opportunity to reflect on our vulnerability to earthquakes.

New Zealand's earthquake activity was comparable to that of California, he said.

"It's inevitable when two massive slabs of the earth's crust are converging at a rate of 40mm-a-year, as they are under the North Island, that the accumulated strain will release periodically causing a large earthquake.

"A major earthquake, no matter where it was located, would affect the whole of New Zealand society and economy."

Historical records and geological studies show that New Zealand can expect an average of one magnitude 6 quake each year, a magnitude 7 quake every decade, and a magnitude 8 quake every century.

In the case of the Hawke's Bay earthquake, scientists now know that it occurred on a buried, or blind thrust, fault.

These are earthquake-generating faults that do not extend up to the earth's surface and they occur in most of the tectonic plate boundary zones of the world.

As well as being hidden, blind thrust faults also pack a big punch when they rupture. Because the two sides of a thrust fault are being compressed like a vice, it takes a lot of energy to rip them apart.

The explosive release of this energy produces high intensity ground-shaking, and scientists consider them to be a particularly lethal type of geological fault because they show little or no evidence at the surface and, even when detected, are difficult to study.

These blind thrust faults predominate on the North Island's east coast between Wairarapa and East Cape.

Some had been identified, but it was likely many remained undiscovered, Dr Smith said.

Though geologists have identified just over 300 onshore active faults in New Zealand in recent years, fewer than half have been studied in detail.

GNS Science geologists are studying another fault in central Hawke's Bay, just north of Waipawa, to give the Hawke's Bay Regional Council a better understanding of the hazard involved.

The geologists are digging a trench 3m to 4m deep across the fault and mapping the displacements on either side of the fault, and organic material from each rupture -- preserved in the soil -- is being dated to map out the pattern of earthquake activity on the fault.

The scientists expect to estimate of the size of the earthquakes the fault is capable of generating, and work out the hazard it poses.

- HAWKE'S BAY TODAY, NZPA