Quake may have increased pressure on fault line

This GNS map shows the location of the 6.4 magnitude quake in red, with some of the first 'felt reports' it received from around the region. Photo / GNS Science

Scientists say a magnitude 6 earthquake centred near Arthur's Pass yesterday may have increased pressure on parts of the Alpine Fault, and lessened it in other parts.

The early morning quake, centred in the headwaters of the Wilberforce River, was followed by a series of aftershocks, including a magnitude 4.8 just after 5pm.

It also possibly triggered a landslide in the headwaters of the Arahura River. The tremors have stopped since sunset yesterday.

Geonet was today sending staff into the area to install more seismeters. A team will also fly over and check for landslides.

Seismologist John Ristau said underground stress build-up was released in an earthquake but rather than disappear, that stress was transferred. In turn, this could relieve, or increase, stress on nearby fault lines.

Mr Ristau said seismologists had been looking at whether the Wilberforce quake had transferred pressure to the main Alpine Fault.
"There's been a small increase in parts of the Alpine Fault, and a small decrease in other parts."

Geonet said it believed there were three scenarios over the next 30 days. The least likely - "extremely unlikely" - is the Wilberforce sequence of tremors will trigger a larger magnitude quake, a magnitude 7 or greater, on another fault, possibly the Alpine Fault.

The Alpine Fault is already known to have a high probability of rupture over the next 30 years, "however, it is unlikely that the occurrence of the Wilberforce earthquake has greatly increased this hazard".

The most likely scenario is that aftershocks will continue to decrease in frequency, with no future large earthquakes.

"It is very early in the aftershock sequence and we will know more as this sequence develops."

The middle scenario, which Geonet rated as "very unlikely", is that similar earthquakes may occur within the general region of the main shock.

The same area was hit by a 6.2 quake at Lake Coleridge in 1946, and aftershocks persisted until the end of 1949. In June, a 6.7 earthquake hit just 10km from Arthur's Pass township. That sequence started two years earlier with a 5.8 at the Wilberforce River. In May 1995 there was a 6.1, and in November 1995 the 6.3 centred at Cass.

West Coast regional manager civil defence Chris Raine said people could expect aftershocks, which may be frightening. "It could be a sharp, wrenching motion," Mr Raine said. He encouraged people to stock up with at least three days of supplies. "They could be on their own for three days if an earthquake is strong enough to disrupt power in the communities." People should also have an evacuation kit containing their medications, passport and insurance documents.

By this morning the Earthquake Commission had received 25 claims related to yesterday's quake, only four of them from the West Coast.