article: Jan 18 2005 - Principal Earthquakes of New Zealand in 2004

With a total of just nineteen earthquakes of magnitude 5.0 or greater, and just one of these over magnitude 5.4, 2004 could be classed as a relatively light year for earthquakes in New Zealand.

Felt earthquakes in 2004. Green are likely to have been felt, yellow had the potential to cause contents damage, red had the potential to cause structural damage.

The most destructive event of the year in the North Island was a magnitude 5.4 event that occurred near Lake Rotoehu in the Bay of Plenty on the afternoon of July 18. This was the largest of a sequence of shallow events that lasted some weeks and was at the same time as severe flooding in the Bay of Plenty region. Several homes in the Rotorua Lakes area suffered severe damage, and the Earthquake Commission received about 120 earthquake-related claims, mainly from the Rotorua and Whakatane areas, with the estimated cost of about $800,000. The sequence was associated with the spreading of the Taupo Volcanic Zone due to plate subduction, rather than indicating any volcanic activity.

There were also a sequence of earthquakes centred close to Upper Hutt, which started early on April 3 and continued intermittently until May 10. The largest event of these had a magnitude of 4.6 and was widely felt in the lower North Island, causing minor damage in the Wellington region. At least a dozen others were reported felt.

On May 12, a shallow magnitude 4.9 earthquake centred close to Eketahuna was felt throughout the lower North Island, and a magnitude 4.3 aftershock the following day was also widely felt.

The two most widely felt earthquakes in the South Island in 2004 were fortunately centred some distance offshore. On November 23, a shallow magnitude 7.3 earthquake centred about 200 km south-west of Fiordland was felt widely throughout the South Island, and at several localities on the western side of the North Island as far north as Auckland. Widespread minor damage was reported in the lower South Island, and the Earthquake Commission received over 350 damage claims. This event occurred in the Puysegur Trench, a feature which marks the boundary between the Australian and Pacific tectonic plates.

The second, with a magnitude of 8.1 and so classed as a great earthquake, occurred on December 24 and was centred 430 km west of the Auckland Islands and over 700 km south-west of Stewart Island. This was at a depth of 10 km, and was felt widely in the South Island and at a few places in the North Island. At the time of occurrence, this was the largest earthquake that had occurred worldwide for over a year, subsequently surpassed by the devastating Indian Ocean event on Boxing Day.

On June 23, a shallow magnitude 5.1 event centred 20 km north of Arthur’s Pass was felt throughout the West Coast and Canterbury.

There were no significant events of depth greater than 50 km in New Zealand in 2004.