Central New Zealand Earthquakes

Last night’s 5th magnitude earthquake in the Tasman Sea west of Paraparaumu was the latest in a short series of deep to very deep earthquakes to strike central New Zealand in the past ten days.

The earthquake struck close to where a magnitude 5.0 quake at a similar depth early in the morning of Monday May 15th caused items to fall from shelves in the Wellington and Marlborough regions.

Since September 5th, seven earthquakes have been felt in the Nelson, Marlborough and Wellington regions. These regions surround a transition zone where the colliding Pacific and Australian plates change from sliding under to sliding past each other.

As the Pacific Plate moves south-westward relative to the Australian Plate at the rate of about 40 millimetres per year this transition zone in central New Zealand, where the plates are locked together, accumulates stress. Rock is compressed and, as it reaches breaking point, it cracks and earthquakes occur. The stress is then either released or transferred to adjoining areas.

The very deep earthquakes under Nelson are occurring in the subducting slab of the Pacific Plate which is descending into the Earth’s mantle where it is melted and recycled. Recent deep quakes occurred at 90 km (mag 4.2) on the 13th and 190 km (mag 5.2) on the 11th.

Two shallower quakes in Cook Strait at 50 km (mag 5.0) on the 15th and 40 km (mag 3.4) on the 6th struck in the subducted Pacific Plate close to the interface with the overlying Australian Plate.

The shallow swarm of earthquakes that was felt in Remuera, Marlborough, 40 km south of Seddon on the 15th indicate stress changes in the region where the two tectonic plates begin to slide past each other.

Whilst 7 earthquakes have been felt in the area since September 5th, Geonet’s databases show that more than 14 earthquake events have occurred at various depths.

These sequences of events, whilst connected, illustrate the different forces playing out in this fascinating tectonic region.

They also remind residents that it is an active earthquake region and strong ground shaking from a movement on nearby major faults could occur at any time. Civil Defence and EQC are not banging on via their television advertisements about “being prepared” just for entertainment value.

[Compiled from data made available by GNS Science, Geonet and their contributing agencies.]

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