Cluster of quakes 'normal' for Hanmer Springs
The South Island has rocked and wobbled to more than 20 earthquakes in the past month, but scientists say such a sudden increase in activity is normal.
Since September 30, 22 earthquakes and aftershocks have shaken communities across the South Island.
Thirteen tremors registered magnitudes of between three and five on the Richter scale – five were moderate to strong events between five and six on the Richter scale and three were large quakes, measuring from magnitude six to seven.
The biggest, of 7.3 on the Richter scale on September 30, was centred about 470km south-west of Invercargill in the Southern Ocean and was the largest earthquake in the New Zealand region for nearly 74 years.
Since that quake, southern parts of the South Island have been rattled most frequently by tremors, with a 6.8 magnitude aftershock to the September 30, 7.3 shake in about the same place.
Then, on October 16, 6.7 and 6.2 earthquakes were registered 50km to 60km west of Milford Sound. In all, 13 quakes were located off Milford Sound from October 16 and 25.
Central and northern parts of the South Island were shaken quite strongly on October 4 by an earthquake of magnitude 5.6 centred between Hanmer Springs and St Arnaud.
An aftershock of 4.7 was recorded in much the same location three days later.
Last week, the Reefton area and the north of the South Island were rocked sharply by a 5.1 magnitude tremor.
However, geologists say there is little connection between the quakes, or evidence they may be precursors of something larger.
Otago University geology department Professor Richard Norris said seismically it had been a more active month than usual, but there was no indication that would continue.
"Earthquakes don't necessarily space themselves out evenly," he said. "It's hard to put this down to just one thing. My feeling is there probably isn't any connection between some of them, but given they are on the same section of (crustal) plate boundary, you can't completely rule that out."
The higher incidence of earthquakes along the Fiordland plate boundary and around the Marlborough section of the Alpine Fault highlighted the relative quietness of the middle section of the fault.
"So there's a lot of those magnitude around Fiordland – 6s and 7s – and the same up in Marlborough, but we don't really get much of those in the middle. That reflects that the middle breaks in rather larger earthquakes."
A magnitude eight quake released 30 times the energy of a seven and had the potential to cause a surface rupture hundreds of kilometres long.
GNS Science duty seismologist Brian Ferris said an analysis of earthquake data in recent years showed this year was "about average" so far.
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