Tuesday’s Quake Unlikely to Affect the Alpine Fault

Tuesday’s Quake Unlikely to Affect the Alpine Fault, Scientists Say

Scientists believe this week’s magnitude 6.0 quake west of Arthur's Pass has not 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 extremely minor and has not changed the likelihood of a rupture.

“The chances of the Alpine Fault rupturing are the same as they were a week ago. Tuesday’s quake is unlikely to have increased the chances of a rupture,” 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.

In the mid-90s there were three earthquakes at Arthur's Pass over magnitude 6 and they had no measurable impact on the Alpine Fault.

Dr Hamling 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 would normally be accompanied by an increase in the number of small earthquakes on the fault.

The level of 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.”

Seismologists had calculated that the rupture plane from Tuesday’s quake was near vertical, and measured about 8km long by 8km deep. The rupture was likely to have come to within 1km of the surface.

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.

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