Fault responsible for 10 major earthquakes

New Zealand’s arguably most hazardous fault system has produced 10 major earthquakes in

7500 years, a figure scientists say should help them forecast the magnitude of powerful quakes able to generate highly damaging tsunami.

A new study of the Hikurangi subduction zone – which runs about 50-100km offshore from Gisborne to Marlborough – has found the last large quake it generated, of about magnitude 8, was between 470 and 520 years ago and on its southern section.

The next most recent major quake, between 815 and 870 years before now, was much stronger and may have been a ‘‘full-margin rupture’’ of at least 350 kilometres, with a possible magnitude of more than 8.5.

Two others may also have been at least as powerful – with evidence of a 333km-long rupture from a quake sometime between

2070 and 2300 years ago, and a

419km-long movement in a quake around 3780 to 3930 years before present.

Scientists involved in the study say there is too little data to more accurately calculate the magnitude of these quakes.

The subduction zone marks the boundary of two tectonic plates, where the Pacific plate is being dragged below the Australian plate.

The research, led by earthquake geologist Dr Kate Clark of GNS Science and published in Marine Geology this week, says four of the 10 large quakes triggered tsunami large enough to cause damage along the coast.

Clark told Stuff the most active part of the zone appeared to be its southern margin, from just south of Cape Turnagain on the Wairarapa coast underneath Wellington and Cook Strait to about Cape Campbell in Marlborough.

She was not sure if it was good or bad news that the most recent magnitude 8 quake in the zone was only about 500 years ago.

‘‘That’s because it’s only one earthquake – we still don’t have a good handle on the recurrence of those big quakes.’’

This paper was the first time all evidence, such as coastal uplift or subsidence and tsunami traces, for the large subduction zone quakes had been assessed. Information collected from 22 sites studied during the past 25 years had been collated, along with that from several new sites.

While it would remain difficult to forecast when such quakes might occur, the information provided a better understanding of their magnitude and how to plan for them, Clark said.