International scientists are intrigued at the complexity of the suspected twin quakes that rattled New Zealand overnight.

Professor of Geosciences Kevin Furlong, of Pennsylvania State University in the US, said what GeoNet now considers two separate earthquakes, striking at Culverden just after midnight with a magnitude of 7.5, showed evidence of "complex rupture history".

The area where the quakes appeared, near the boundary of the Pacific and Australian tectonic plates, contained numerous faults with a wide range of orientations, Furlong noted.

"The main rupture and aftershocks may exploit this complicated tectonic zone resulting in a range of earthquake mechanisms for the aftershocks and in comparison to the main earthquake."

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Dr Mark Quigley, formerly based at Canterbury University and now at the University of Melbourne, also commented on the quakes' dynamic characteristics.

"Preliminary data suggest that this is a structurally complicated earthquake, involving a mixture of reverse faults that thrust the northern South Island up over the Pacific Plate, and strike-slip faults, which slid northern crustal blocks sideways and towards the northeast relative to their southern counterparts," Quigley said.

"The majority of the energy release appears to be sourced from rupture of a large fault or fault system that could be up to 200km long and that strikes northeast and dips to the northwest, beneath the northern part of the South Island."

Due to ongoing convergence between the Pacific and Australian tectonic plates, faults in this orientation accommodated a mixture of reverse and strike-slip faulting.

Because of the large size of the fault, and the initial interpretation that the rupture started at the southwest end of the fault and propagated to the northeast, the seismic energy was released in a period of up to two minutes, he said.

"In Christchurch, the shaking felt quite different to the fatal 2011 Christchurch earthquake, which had a tremendous amount of high frequency energy and very strong ground accelerations; this most recent earthquake would have been dominated by lower frequency shaking manifested as a 'rolling-type' ground motion felt for up to one to two minutes."

Like Christchurch, this quake had also triggered liquefaction in coastal areas and in susceptible sediments, and landsliding along steep, susceptible cliffs in the northern South Island.

For an earthquake of magnitude 7.5-7.8, several large aftershocks were expected, and for each magnitude 6 aftershock 10 more magnitude 5 aftershocks are expected in the coming days and weeks.

"This region has been one of the most seismically active in New Zealand over the last few years, including magnitude 6.5 and 6.5 earthquakes that occurred as part of the Cook Strait earthquake sequence in 2013; it is likely that these sequences are related given their close spatial and temporal association."

Dr Paul Somerville, a geoscientist at Macquarie University, said that, although the earthquake appeared not to have occurred at the plate interface, known as the Hikurangi Subduction Zone, the direction of slip on the faults was similar to what was expected from earthquakes on the subduction zone, which underlies Wellington.

"It is important to understand the implications of this earthquake for the potential triggering of earthquakes on the Hikurangi Subduction Zone and other faults in the region around Wellington."

James Goff, director of the Australia-Pacific Tsunami Research Centre and Natural Hazards Research Laboratory at the University of New South Wales, said, yet again, New Zealand's earthquakes were proving to be anything but easy to understand.

"In addition, it is always difficult to know exactly what type of tsunami warning to issue as a result of such an event; a relatively 'small' earthquake for tsunami generation and near, but not at, the coast."

Large aftershocks were continuing, with the latest, a magnitude 6.8 quake, generating yet another tsunami information statement from the warning centres.

"While the earthquake itself may not generate a tsunami, it is quite possible that large local tsunamis can be generated by earthquake ground-shaking causing submarine landslides offshore," Goff said.

"The Kaikoura Canyon, close to Kaikoura's coast is just such a location and it has the potential to produce a devastating tsunami for an area already badly affected by the earlier earthquake."