1855 quake cuts repeat risk

A Wairarapa earthquake 157 years ago has lowered the risk of a major earthquake striking the region.

The 1855 quake, which remains the largest recorded quake since European settlement, has been cited as the reason for a reduced risk of the faultline rupturing again to anything like the same extent.

This was confirmed yesterday by Russ Van Dissen, an earthquake geologist for GNS Science, who said the bumper shake of 1855 meant the odds of a big quake caused through the Wairarapa fault rupturing had been cut from 5 per cent to "1 or 2 per cent" in the next century.

Mr Van Dissen said scientists, helped by university students, had done a lot of research on the faultline in recent years. The faultline extended along the base of the Rimutaka Hill and Tararua Ranges and, although other faultlines criss-cross Wairarapa, it was by far the most dominant.

He said the reduced risk does not mean there is no risk at all, and the results of the research should be treated as allowing extra time to be prepared.

"It's a numbers game and the 1855 quake greatly reduced the odds of the faultline rupturing like that again but it takes generations for communities to prepare."

Other Wairarapa faultlines included the Martinborough fault, the Carterton fault and others further north.

The 1855 earthquake was caused by a faultline rupture that extended over 150km triggering horizontal displacement of up to eight metres.

It was estimated to have been over 8 on the Richter Scale making it significantly larger than other devastating earthquakes such as the Murchison earthquake in 1929, the Napier earthquake in 1931 and the Inangahua quake in 1968.

Mr Van Dissen said the 1855 Wairarapa quake had been the first to establish a relationship between earthquakes and faults.