New Zealand sits right on the cusp of a plate margin between the Australian and Pacific plates. Both plates are moving in similar directions but at different speeds, and there there is a convergent relative velocity of 23mm/year between the plates, due to convection currents circulating towards each other.
Below: Diagram showing relative plate velocities, with proportional arrows
The result of this motion is that the continental Pacific crust is subducted below the continental Australian crust; as the Pacific plate descends into the Beniof zone it becomes locked with the underside of the Australian plate. Friction prevents slippage until the force becomes too much and the plates slide and move past each other. This particular event is known as a "slow release" earthquake as the plates slip past at a rate very little faster than those typically measured by GPS instruments in Wellington and Kapiti.

It is estimated that the earthquake is a 7 on the Richter scale, but is clearly far less destructive (I on Mercalli scale "Generally not felt by people unless in favorable conditions". Typical fast release M7 earthquakes would rank roughly X, described as "Many well-built structures destroyed, collapsed, or moderately to severely damaged"). The reason for the unusual devastation for an earthquake of such energy is that rather than the plates slipping over a period of days, in New Zealand the earthquake has been ongoing since January 2013 and "will most likely continue for several months".

The focus of the earthquake is only 40km below sea-level and acts in the vicinity of Wellington, a medium-sized city of 395,600 inhabitants.
Above: National and Local scale maps showing measurement of relative plate motion using GPS stations

Below: Cross section diagram of subduction
From the above diagram it is possible to conclude that the "North Island" of New Zealand, on which the activity has been registered, was formed by precisely the same tectonic margin over several thousands of years. The Island was formed by the upward crumpling of the sediments originally contained in a geosyncline below sea level. This is proved by the geology of New Zealand - predominantly Greywacke with many fossils of sea creatures. As a side note, melted sections of the subducted Pacific plate that have descended to the Beniof zone and below into the asthenosphere are melted by conductive heat from the mantle as well as friction with the Australian plate and pressure. The magma presently rises in its less dense state and results in a number of composite volcanoes around the area, explosive due to their high silica content and viscous magma.
Above: Well-preserved fossil of a fish found in Wellington, New Zealand

Tectonic Plate velocity data:
Earthquake information, Subduction diagram:
Location diagrams:
Geological information:
New Zealand fish fossil:
Mercalli Scale:

Thomas Ruddell, 19/07/2013

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    Josh Long studied geology at the University of East Timor, consequently completing a PhD in geology at the prestigious University of Panama.

    Tom Ruddell studied Geophysics with French at the University of Easter Island where he later did his PhD. Now editor of the National Geologic magazine.

    Adam Willis once picked up a rock and remarked "what the blazes is this?". An expert in the highly technical areas of Radish people and the pronunciation of German names, he has won several awards to commemorate his discoveries.


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