Double jolts made tsunami fierce

The September tsunami killed nearly 200 people. Photo / Brett Phibbs

The devastating tsunami that struck Tonga and Samoa last year, killing nearly 200 people, was the result of a rare geological event - two almost simultaneous earthquakes.

Global earthquake readings and GPS recordings from Samoa initially indicated the September 29 tsunami was the result of a single "normal-faulting" 8.0-magnitude quake.

However, tsunami modellers could not reconcile the apparent "thrust", or compression movement, within the Earth's crust with the usual extensional motion produced by a normal-faulting quake.

Six weeks after the event, GPS measurements from a small Tongan island showed there were two large quakes within minutes of each other.

Simultaneous earthquakes are unusual and would have increased the size of the tsunami and its destructiveness.

The finding appears as this week's cover story in the science publication Nature.

Lead author John Beavan, a geophysicist with NZ's GNS Science, said yesterday that the conflicting data had been "very perplexing".

"In the end, it was pure detective work that uncovered the two earthquakes," he said.

"The files arrived from Tonga in early November 2009. When I processed the data and looked at the GPS results I was astonished to see that the [Tongan] island of Niuatoputapu had moved nearly 400mm to the east."

This was a much bigger displacement than expected, and in a completely different direction.

Researchers found the quakes occurred under the ocean floor, about 70km apart, within a couple of minutes of each other. While it was not clear which quake occurred first, the thrust quake was magnitude 8.0, while the other measured 7.9.

Dr Beavan said it was possible scientists had not noticed large, "double-whammy" earthquakes before because seismic waves become entangled, making it almost impossible to distinguish them.

It was possible, but unlikely, that a similar event would occur under New Zealand, he said.

"This is a rare phenomenon, but it is possible wherever there is a subduction-type plate boundary."

The quakes and subsequent tsunami killed 150 people in Samoa, 32 in neighbouring American Samoa and nine in Niuatoputapu.

FREAK OF NATURE
* The tsunami that struck Samoa and Tonga was the result of two almost simultaneous earthquakes.
* They occurred under the ocean floor about 70km apart.
* One quake was magnitude 8.0, the other measured 7.9.
* This is a rare phenomenon and would have increased the size of the tsunami.


- NZPA