WASHINGTON, D.C. (BNO NEWS) — Not one major earthquake but two major earthquakes generated a destructive tsunami that left nearly 200 people killed in Tonga and Samoa on September 29, 2009, scientists revealed on Wednesday.

The 12-meter (13-yards) tsunami mainly affected Tonga and the Samoan Islands, leaving scores of people dead and many more injured. Researchers initially believed this was caused by a single 8.1-magnitude earthquake centered about 185 kilometers (115 miles) east-northeast of Hihifo on Tonga.

A team of scientists revealed in the ‘Nature’ journal on Wednesday, however, that the tsunami followed two major earthquakes which were located so close together that they masked each other.

One 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 earthquake, magnitude 8.0 on the Richter scale, occurred in the subduction zone and is known to geophysicists as a ‘megathrust’ event, similar to the 8.8-magnitude earthquake that struck off the coast of Chile. In these earthquakes, compressional forces cause one chunk of plate to slide beneath another.

Scientists noticed after the earthquakes how buoys indicted that the seabed had risen, which is usually seen as a result of a megathrust earthquake. A normal-fault earthquake should initially cause the seabed to drop, not rise. The pattern of tsunami waves recorded was also different from a tsunami generated as a result of a normal-fault earthquake.

A second clue came from a Global Positioning System (GPS) monitoring station on the Tongan island of Niuatoputapu. “It was astonishing,” said 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. “The island had gone in the wrong direction. We expected 80 millimeters to the west, but it actually moved 350 millimeters towards the east.”

Paul Earle of the United States Geological Survey (USGS) told BNO News that it is the first time that scientists have documented two major earthquakes that struck two different fault lines close to each other at approximately the same time. He said it probably happened before, but said researchers now have better instruments to analyze earthquakes.

Earle said the new findings most importantly have implications for tsunami warnings as such events as on September 29, 2009 are difficult to detect in real-time. “It is still a research topic,” he said.

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