Series of Milford Sound quakes ‘not unusual’
A 5.2-magnitude earthquake and ongoing aftershocks felt across the lower South Island over the weekend are ‘‘not unusual’’, a seismologist says.
More than 700 people reported feeling the first in the series of quakes, centred about three kilometres deep and 30km north of Milford Sound, at 10.37pm on Saturday night.
GeoNet has reported 10 aftershocks ranging from magnitude 2.5 to 3.5 since the initial tremor, which was close to the southwest tip of the Alpine Fault.
A major quake on the Alpine Fault historically occurs roughly every 300 years – the last one was in 1717. As strong as magnitude 8, the next rupture is expected to dam rivers, cause landslides and cut power across the South Island.
While it is unusual for an earthquake in sparsely-populated Fiordland to be felt by so many people, GNS Science seismologist John Ristau said the strength and recurrence of the tremors was not necessarily concerning.
‘‘A 5.2 earthquake, if it’s shallow like this, will predict an aftershock sequence. Fiordland is traditionally one of, if not the most seismically active areas in New Zealand.
‘‘It doesn’t tell us whether there’s going earthquakes.’’ anything about to be [further]
Fiordland has a very complex tectonic system, Ristau said. The region is considered a subduction zone, where the Australian tectonic plate passes below the Pacific plate.
The plates can move horizontally and vertically, and there are many fractures in the area’s geology.
It is ‘‘highly unlikely’’ this weekend’s quakes occurred on the Alpine Fault, Ristau said.
‘‘It’s probably on some other fault close to the Alpine Fault but we wouldn’t be able to say precisely where it was.’’
Reports that people had felt Saturday’s quake as far north as Tauranga were likely inaccurate, Ristau suggested, as ‘‘some people feel a bump and see there’s been an earthquake’’ and report it as such.