Scientists solve riddle of Gisborne tsunami

Undersea volcanoes off the coast of New Zealand have the potential to unleash 10-metre-high tsunamis on shorelines, seismologists say.

East of Gisborne, where the Pacific crust subducts - moves underneath - the Australian plate, extinct volcanic mountains are creating sticking points, catching against the rock they are being forced under.

These built up seismic energy, which was finally released as an earthquake, GNS Science seismologist William Power said.

Many such volcanoes are sitting along the Hikurangi fault, off the coast of Gisborne, waiting to be crunched under the Australian plate over hundreds of thousands of years.

"There will probably be multiple earthquakes associated with those sea mounts as they subduct."

The danger of those particular quakes was they did not feel like very large ones - sometimes no bigger than magnitude 5.6 - yet could produce waves up to 10m high.

Before this research, seismologists were puzzled by two tsunamis that struck the Gisborne coast in 1947. In March, 10m waves followed a magnitude 5.9 shake, and then in May a 6m wave again hit the coast after a magnitude 5.6 quake.

"You don't necessarily feel a lot of shaking, but you still get a large tsunami."

Power and a team of experts, including Rebecca Bell at Imperial College London and GNS scientists Caroline Holden and Xiaoming Wang, set out to solve the mystery, using old seismographs plus the latest knowledge on undersea "tsunami earthquakes".

Tsunami earthquakes were one of three distinct types of ground shaking, the others being the classic earthquake that was fast and powerful, and the "slow-slip" quake, where movement happened unfelt by people over weeks or months, Holden said.

Tsunami quakes were in between: fast enough to be felt slightly, but much of their seismic energy was unnoticed, generating unexpectedly large waves.

Piecing together data from newspapers and other records, the team found both 1947 quakes were centred around undersea volcanoes off Gisborne, discovered recently in geological surveys.

Also confirming their suspicions were residents' descriptions of the quakes as not violent, but as if the ground was rolling under their feet for several minutes - making some feel seasick.

The potentially life-saving find means coastal residents are now being warned to head for higher ground for any earthquake lasting longer than a minute.

The sparse population of the Gisborne coast in 1947 meant no lives were lost, but a similar tsunami could be devastating today, the research said.

The Dominion Post