WELLINGTON (AFP) – The devastating tsunami which killed nearly 200 people in Samoa and Tonga last September was the result of a rare double earthquake and not one as previously believed, scientists said Wednesday.
Simultaneous earthquakes, with one hiding the other, are unusual “and almost certainly increased the size of the tsunami and its destructiveness on some Tongan islands,” New Zealand’s GNS Science said in a statement.
Global earthquake readings initially indicated a single large “normal faulting” quake of magnitude 8.0 had occurred, producing an extensional motion while the tsunami waves indicated a “thrust” event with compressional movement.
The scientists said they were unable to reconcile the conflicting data until six weeks after the event when measurements from a small Tongan island showed there must have been two large earthquakes.
Their findings appear this week as the cover story in the prestigious science publication “Nature”.
“This is a rare phenomenon,” lead author John Beavan, a geophysicist at GNS Science, said Wednesday.
“In the end, it was pure detective work that uncovered the two earthquakes. 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 island of Niuatoputapu had moved nearly 400 mm to the east.”
This was a bigger displacement than expected, and in a completely different direction.
“We worked hard for a couple of months to ensure there was no other explanation for the GPS and tsunami observations, which were telling us that two nearly simultaneous earthquakes had occurred.”
The scientists determined that the earthquakes occurred under the ocean floor about 70 kilometres apart with one at magnitude 8.0 and the other of magnitude 7.9.
Parts of Samoa and Tonga were devastated by the September 29 earthquakes when the tsunami smashed into the Pacific islands.
As a result nearly 150 people were killed in Samoa while another 32 were confirmed dead in neighbouring American Samoa and nine on Tonga’s northern island of Niuatoputapu.
Earthquakes are common on the Pacific “Ring of Fire” where continental plates in the Earth’s crust meet.