Tuesday's quake unlikely to greatly affect the Alpine Fault scientists say - 07/01/2015

Scientists believe this week’s magnitude 6.0 quake west of Arthur’s Pass has not greatly increased the chance of a rupture on the Alpine Fault, about 20km to the west of Tuesday’s epicentre.

In a preliminary study, they found the Wilberforce earthquake has increased the stress on parts of the Alpine Fault and reduced the amount of stress elsewhere on the fault.

Overall, any impact on the fault has been minor. The on-going chances of an Alpine Fault earthquake are potentially elevated, but still very similar to what they were before Tuesday’s quake.

“The chances of the Alpine Fault rupturing are only slightly higher than they were a week ago,” said GNS Science geodesist Ian Hamling.

Dr Hamling said in the overall scheme of things, Tuesday’s quake was not large and any changes it had caused in the stress regime in the earth’s crust would be modest. A larger quake would have changed the stress regime more.

The modelling had also shown that small sections of the Hope and Porters Pass Faults had also become slightly less stressed.

By way of comparison, Dr Hamling said in the mid-90s there were three earthquakes at Arthur’s Pass over magnitude 6 that did not trigger a large Alpine Fault earthquake.

He emphasised that his analysis was preliminary and could be regarded as a guide rather than a definitive study.

A significant change in the stress regime on the Alpine Fault could potentially be accompanied by an increase in the number of small earthquakes on or near the fault that might signal a greater chance of a large earthquake.

The level of currently observed quake activity on the Alpine Fault had not changed following this week’s earthquake.

“We monitor the Alpine Fault longterm for any signs of increased earthquake activity. There has been no change in activity this week.”

Dr Hamling said he was waiting on satellite data from overseas. When it became available, he would undertake a more detailed study of the stress changes.

Seismologists had calculated that the rupture plane from Tuesday’s quake was near vertical, and measured about 5km long. The rupture had not broken the surface, but was likely to have come with 1km of it.

Stress modelling had become a fairly routine activity after all moderate to large earthquakes in New Zealand. It gave an indication if faults near to the epicentre had become loaded with stress, or unloaded (destressed).

The Alpine Fault ruptures once every 330 years on average, although intervals between ruptures can vary between 140 and 450 years. It last ruptured 298 years ago producing an earthquake of about magnitude 8.

It has a 28 percent chance of rupturing in the next 50 years, which is high by global standards.

Scientists say there is no better time than the present to prepare for the next earthquake, regardless of which fault it occurs on. The more thorough the preparation, the lower the eventual impact will be.