International Committee on issues of Global Changes of the Geological Environment, “GEOCHANGE”

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Double whammy caused tsunami

The 12-metre tsunami that killed 192 people on 29 September 2009 in Tonga and Samoa was the result of a couple of earthquakes that occurred within a few minutes of each other, two teams of scientists report in Nature today.

Because the earthquakes occurred so close together — and from epicentres only 60-115 kilometres apart — they masked each other, producing a confusing rumble that took months for geophysicists to tease into separate events.

news.2010.416.samoaOne earthquake, around magnitude 7.9, was produced when a piece of the Pacific plate snapped as it was bending downwards, en route to the Tonga subduction zone, where it dives beneath the Tonga platelet. This type of earthquake is caused by a 'normal' fault, in which two plates pull away from one another, causing one to slip upwards and the other downwards with respect to the fault plane.

The other quake — magnitude 8.0 — occurred in the subduction zone, and is known as a 'megathrust' event, similar to the larger earthquake that recently rocked Chile. In these earthquakes, compressional forces cause one chunk of plate to slide beneath another.

At first, scientists thought the event was a single, normal-fault earthquake. But when they started to look in detail, they found several oddities.

For example, a normal-fault earthquake should initially cause the seabed to drop, says John Beavan, a geophysicist at the Institute of Geological and Nuclear Science in Lower Hutt, New Zealand, and lead author of one of the studies. A megathrust earthquake, on the other hand, should primarily cause the seabed to rise. Both can produce tsunamis, but the waves should show a different pattern as they pass over the seabed buoys that track their progress across the open ocean — and the buoys indicated that it was more like a megathrust earthquake than a normal-fault earthquake, says Beavan.

260px_news.2010.416.graphicA second clue came from a Global Positioning System (GPS) monitoring station on the Tongan island of Niuatoputapu. "It was astonishing," says Beavan. "The island had gone in the wrong direction. We expected 80 millimetres to the west, but it actually moved 350 millimetres towards the east." The only explanation that the researchers could think of was a previously unrecognized megathrust earthquake at nearly the same time as the normal-fault event.

Meanwhile, several seismologists, led by Thorne Lay of the University of California, Santa Cruz, were reaching the same conclusion on the basis of a bizarre pattern of seismic reports, in which stations that should have seen minor shaking had instead seen strong shaking. Lay likens it to looking at the surface of a drum, and trying to determine whether it is being hit by one or two drumsticks. "If you have two," he says, "each sets off patterns that interfere and beat against each other. We can model that."
By Richard A. Lovett