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Bay quake highlights NZ's vulnerability - 02/02/2001

The 70th anniversary of the 1931 magnitude 7.8 Hawke’s Bay earthquake is a reminder of New Zealand’s vulnerability to earthquakes.

Fault diagram

The Hawke’s Bay earthquake struck on the morning of February 3, devastating Napier and Hastings and resulting in the deaths of 256 people – 161 in Napier, 93 in Hastings, and two in Wairoa. Hundreds of aftershocks, some almost as big as the first shock, continued to jolt the region during the following weeks.

Maximum uplift on land was 2.7m near the mouth of the Aropaoanui River, north of Whirinaki. At Ahuriri Lagoon, about 3500ha was thrust up by about 1.8m. At Napier, the uplift was less than 1m. There was also subsidence at several locations.

On a global scale, the Hawke’s Bay earthquake was impressively large. At magnitude 7.8, it was very similar in size to the recent devastating earthquake in the western Indian state of Gujarat. Research in recent years shows that the 1931 earthquake released only a small portion of the accumulated strain across the boundary of the Pacific and Australian plates.

 

Researchers have also concluded that the 1931 earthquake initiated on the Napier Fault just offshore from Whirinaki and within seconds had travelled north and south rupturing about 120km of the fault underground. The west side of the fault went up and the east side went down. The most severe vertical movement occurred offshore in Hawke Bay.

The earthquake was the sudden release of hundreds of years of accumulated strain on a blind thrust fault that, until February 3, 1931, was unknown. Blind thrust faults are earthquake-generating faults that do not extend up to the earth’s surface. 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. It is the explosive release of this energy that produces ground-shaking of such high intensity.

Scientists consider them to be a particularly lethal type of geological fault because there is little or no evidence of them at the surface and, even when detected, they are difficult to study.

Blind thrust faults tend to predominate on the North Island’s east coast from southern Wairarapa to East Cape. Some have been identified, but it is likely many remain undiscovered.

Seismologist Hugh Cowan, of the Institute of Geological & Nuclear Sciences Limited (GNS), warns against any tendency to think that just a few areas, such as Hawke’s Bay, Wellington and the South Island’s west coast, are prone to big earthquakes.

" Historical evidence and scientific research show convincingly that the risk to New Zealanders from geological hazards is significantly higher than the last 60 years would indicate," says Dr Cowan.

If an earthquake on the scale of the 1931 Hawke’s Bay jolt, and in the Wellington region (1848, 1855), or the volcanic eruptions at Tarawera (1886) and Taupo (1800 years ago) occurred today, they would cause devastating losses.

" There are many New Zealand communities located within 10km of an active fault. Scientists have identified more than 200 active faults, so it’s fair to say that many parts of New Zealand could experience an earthquake with effects similar those experienced in Hawke’s Bay in 1931.

" While it is not possible to say exactly where and when the next one will strike, monitoring with GPS technology can quantify the rate at which strain is building up in the earth’s crust to a depth of 30km. High strain tends to be a precursor to an earthquake."

Dr Cowan says in 1931 there was little appreciation of New Zealand’s location on an active plate boundary and the implications this has for the size of frequency of earthquakes.

" New Zealand’s earthquake activity is almost comparable to that of California. 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."


John Callan, Communications Co-ordinator, GNS