January 22, 2005
The Dominion Post
HAYDON DEWES and KIM RUSCOE
A day out from the anniversary of New Zealand's biggest earthquake, scientists are analysing a series of jolts that have rocked the lower North Island.
Violent tremors shook goods from shelves and jammed hundreds of lifts yesterday as the Wellington region was hit by its biggest earthquake in 30 years.
The quake, centred five kilometres north of Upper Hutt and measuring 5.5 on the Richter scale, struck at 7.56am. It was quickly followed by three short aftershocks, each of about 3.7 magnitude. Another tremor was recorded at 8.27pm.
They followed a flurry of seismic activity in the lower North Island on Tuesday, when 10 earthquakes were recorded in as many hours. The biggest measured 5.3 and eight were centred at the same location, 40 kilometres southeast of Martinborough in Wairarapa.
Martin Reyners, a seismologist at the Institute of Geological and Nuclear Sciences, said scientists were collating data and would use computer models to elicit the possible outcomes. "We're working out what all this means. We're still wondering why so many are happening in such a short period," he said.
There was the possibility of a large earthquake on the boundary of the Australian and Pacific plates, which meet 25 kilometres below Wellington. "The plates are locked together at the moment so eventually they will have to separate," Dr Reyners said. Exactly when, no one was willing to speculate.
Recent quakes in Wairarapa, Wellington and Porirua had happened on the edges of the locked section and had added a small degree of stress. Dr Reyners said that, if the plate boundary - or interface - failed, it could produce a quake similar to New Zealand's biggest known, the 1855 Wairarapa quake.
Earthquakes on the interface were rare, happening hundreds of years apart. The Boxing Day earthquake off Sumatra happened on an interface. "We haven't had one of these earthquakes between the plates since the historical settlement of Wellington."
New Zealand has had, on average, a quake of magnitude 6 to 7 every decade and a magnitude 8 quake every century.
Tomorrow is the 150th anniversary of the Wairarapa quake, which measured 8.2. It released more than 10,000 times the energy of yesterday's 5.5 earthquake, felt as far north as Wanganui and Woodville and as far south as Kaikoura.
The shaking yesterday set off fire and car alarms, strewed supermarket aisles with smashed jars and sent people running. Firefighters checked dams and water supplies in the Hutt Valley for damage or leaks, while Civil Defence warned people to check on emergency supplies.
The Earthquake Commission confirmed it had received 84 "relatively minor" damage claims totalling $262,968 by yesterday afternoon. It was expecting several hundred more.
The region's last big earthquake - 5.9 in magnitude with almost the same epicentre as yesterday's - struck on January 4, 1975.
http://www.stuff.co.nz/stuff/0,2106,3163450a10,00.html