Twin quakes ‘unusually large’

Two earthquakes that struck near Levin on Monday and yesterday were unusually large for the area, GNS Science says.

A magnitude-5.8 quake 30 kilometres northwest of Levin on Monday morning was followed by a 5.2 shake in the same location yesterday. There were 19 ‘‘weak’’ or ‘‘light’’ aftershocks in between.

GNS seismic duty officer Jonathan Hanson said yesterday’s earthquake came from the same plate pressures that caused Monday’s strong quake.

The depth and area of the quake was in a location where the Pacific Plate was bending ‘‘relatively sharply’’, Hanson said.

The large aftershock was a result Monday’s shaking.

‘‘The events yesterday and today are larger than we generally observe. They are unusually large events.’’

The most notable thing about both earthquakes was the number of ‘‘felt it’’ reports, he said.

Monday’s earthquake was felt by almost of 40,000 people, while Tuesday’s was felt by 20,000. ‘‘Events of this magnitude will be quite widely felt, especially around highly populated areas like Wellington and the Manawatu¯ Coast.

‘‘We’re really happy to see people getting involved in that, it’s like citizen science.’’

It was almost guaranteed there would be more aftershocks in the coming weeks, probably based in the same place, Hanson said. ‘‘There’s still a chance of another, larger one . . . but I don’t think this significantly alters the chance of another earthquake in New Zealand.’’

Wellington geophysicist Professor Tim Stern said yesterday’s quake was a typical aftershock. ‘‘Generally, the strength of aftershocks will decay with time and they will get smaller.’’

The ‘‘bending’’, shaking motion was caused by the Pacific Plate being subducted by the Australian Plate near New Zealand.

‘‘It’s being caused by rocks being pulled apart,’’ Stern said.

‘‘We can tell that by the radiation pattern of the quake, as it’s recorded at stations all around New Zealand.’’