A 5.8-magnitude earthquake southwest of the Southland town Tuatapere last Thursday and two other sizeable quakes reflected one of the most seismically active parts of New Zealand, scientists said.
The GeoNet internet site said the initial quake, centred 190km southwest of Tuatapere at a depth of 12km at 7.48pm, was felt by more than 500 people across the lower South Island.
The earthquake was followed by a magnitude-5.5 quake centred about 190km from Tuatapere at 10.01pm on Thursday, and a magnitude-5 quake, at a similar location, at 1.46am on Friday, as well as by several smaller aftershocks, GeoNet officials said.
University of Otago geologist Prof Richard Norris said the southwestern corner of the South Island was one of the most seismically active parts of the country.
The latest earthquakes appeared linked to the Puysegur Trench, a 6300m-deep cleft in the floor of the south Tasman Sea south of New Zealand, which marks the boundary between two tectonic plates.
GeoNet officials noted the three quakes off Tuatapere were just south of the aftershock zone of the 7.8-magnitude "Dusky Sound" earthquake in July 2009, which was located 100km northwest of Tuatapere.
The 2009 quake was one of the biggest earthquakes in the world that year and the biggest in New Zealand since the 1931 Hawkes Bay earthquake, which killed more than 250 people.
Prof Norris said last Thursday's quake off Tuatapere had released a great deal more energy than an earthquake of about 4.9 magnitude, linked to nearby fault systems, which hit Dunedin in 1974.
That Dunedin quake toppled some chimneys and caused some other damage.
Had the Thursday quake been centred in the St Clair area, it would have caused much more serious damage, he said.
Despite the large magnitude of the Dusky Sound earthquake in 2009, it caused relatively little damage, even in the somewhat remote areas affected.
Researchers throughout the world, including Otago University geologists, were trying to understand why quakes of similar magnitude caused greatly different amounts of damage, he said.
