Northern Westland Earthquakes

Residents of northern Westland may have felt slightly more local earthquakes than normal during March. The five earthquakes occurred near Westport, Reefton and Murchison.

Westland and the Nelson province are the only parts of the South Island that lie on the Australian tectonic plate – the rest of the island is made up by the Pacific tectonic plate. This explains the wrinkled and crumpled look of the landscape of Nelson and Westland as the land is being crushed by the two plates colliding with each other.

Four of this month’s earthquakes have been shallow events in the upper levels of the Australian plate. Two quakes with magnitudes 4.7 and 3.2 struck 30 km south-east of Karamea (50 km north of Murchison) at a depth of 5 km on March 6th. A magnitude 3.9 event struck at a depth of 6 km at a location 20 km north of Reefton on the 28th. It was followed by a magnitude 3.0 earthquake at a depth of 5 km located 50 km north-west of Reefton (within 5 km of Westport) this morning, the 30th of March 2007.

A deeper quake of magnitude 4.7 struck at a depth of 80 km on March 23rd. This event was located 40 km south-east of Murchison and probably represents activity on the interface between the two plates.

Earthquakes in the area often occur in pairs, but interestingly some of the pairs consist of a deep and a shallow event. Some examples of this activity have occurred during the past two years.

During February 2007, a pair of events struck 3 days apart. An 8 km deep quake struck 10 km north-west of Murchison on the 20th, followed by a magnitude 4.4, 100 km deep event 45 km east of Westport on the 23rd.

During January 2007, another pair of quakes struck, but both were shallow. On the 20th a magnitude 3.0 quake struck 10 km south-west of Westport (50 km north-west of Reefton) at a depth of 6 km. Five days later a magnitude 3.2 event occurred 20 km north-east of Murchison at a depth of 5km.

Last year a series of 3 shallow quakes struck near Reefton during March. A pair of quakes with magnitudes 3.8 and 3.6 struck at a depth of 8 km on the 18th. One was located 20 km east of Reefton, the other 10 km south-east of Reefton. Six days later, a 5 km deep magnitude 3.6 quake struck 40 km north-west of Reefton (30 km south-west of Westport).

A solitary magnitude 4.0 quake struck 20 km north-west of Murchison at a depth of 5 km on July 18th.

August 2006 saw stronger activity, when a damaging magnitude 5.9 quake struck 30 km east of Murchison at a depth of 100 km on the 13th. It was followed by a shallow magnitude 3.5 quake located 10 km north-west of Murchison at a depth of 5 km on the 21st.

November was quieter with a single magnitude 3.5 quake located 20 km north-east of Murchison at a depth of 10 km on the 4th.

Activity in the area during 2005 occurred at a similar rate, with 8 events. A pair of quakes struck 30 km north of Reefton on May 11th 2005. Both were 8 km deep and had magnitudes of 3.5 and 3.0.

During July a single magnitude 4.2 quake struck 30 km north-west of Reefton at a depth of 11 km on the 27th.

A pair of quakes struck near Reefton on August 6th 2005. One was magnitude 3.1, 15 km deep located 30 km north of Reefton, the other was magnitude 4.8, 9 km deep located 20 km north-west of the town.

Activity moved nearer Murchison during September with a magnitude 3.8, 20 km deep quake located 20 km north-west of the town on the 19th. The following month a pair of quakes struck on the 7th and 13th. One was magnitude 3.7 located 10 km west of Murchison at a depth of 10 km whilst the other was magnitude 4.8, 20 km north-west of Reefton at a depth of 7 km.

With the Alpine Fault running to the west of the alpine boundary of Westland, it is easy to forget that the Westland and Nelson provinces host serious earthquakes of their own from time to time. The shallow Buller earthquake of 1929 struck between Westport and Murchison at a depth of about 10 or 20 km. The magnitude 7.8 earthquake killed 17 people and was felt throughout the country. The Inangahua earthquake of 1968 was also shallow, having a magnitude of 7.0 and causing 3 deaths.

The Inangahua quake remains New Zealand’s most recent onshore quake of 7th magnitude.

[Compiled from data provided by Hazardwatch and the Geonet project and its sponsors EQC, GNS Science and FRST.]

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