article: May 3 2005 - West Coasters Woken by Biggest Shallow Quake This Year

Today’s magnitude 6 earthquake near Haast is the largest shallow earthquake to occur in New Zealand this year.

The earthquake struck at 3:35 am and was located 10 km deep under mountains about 10 km south of Haast on the West Coast. It was followed five minutes later by another earthquake of magnitude 5.7 in the same area and at a similar depth.

The main shock was felt strongly in Haast and as far away as Westport, Invercargill, and Christchurch. Property owners at Haast reported minor damage.

A group of GNS seismologists has travelled to the Haast area to deploy up to five portable seismographs near the epicentre to record aftershocks.

Aftershocks provide information on the extent of the "fault break" in the Earth’s crust and enable seismologists to analyse any changes in stress on nearby faults.

Today’s earthquake was centred close to the Alpine fault, which runs 450km between Milford Sound and Marlborough. It is one of the largest active faults in New Zealand and is visible from space.

Four years ago after a similar sized earthquake near Jackson Bay, south of today’s quakes, GNS seismologists recorded 2000 aftershocks on their portable instruments over a three week period. Most were small – between magnitude 1 and 3. They were able to use about 400 for further analysis.

"On average, New Zealand can expect a shallow earthquake of magnitude 6 or greater about once a year," said duty seismologist Mark Chadwick.

Dr Chadwick posted details of today's quakes on the GeoNet website within 30 minutes of them occurring. Within that time, about 20 people logged reports on the GeoNet website describing how they felt the earthquake.