Saturday, October 3, 2009

FT: Warning of bigger tremor to come

Source (click to view): Kevin Brown and Earth Observatory of Singapore, NTU

The earthquake that struck Sumatra three days ago is likely to be followed by an even larger tremor that could cause a tsunami as devastating as the one that hit Indonesia in 2004, according to a leading seismologist.

Professor Kerry Sieh, director of the Earth Observatory at Singapore’s Nanyang University, says earthquakes measured at 8.4 and 7.8 which hit southern Sumatra in September 2007 marked the beginning of a cycle of tremors that starts roughly every 200 years.

“This is 100 times smaller than the big one,” Prof Sieh said by telephone from Padang, where he was monitoring the aftershocks from Wednesday’s earthquake, estimated by the US Geological Survey at 7.6 on the scale used by modern seismologists.

“My timing has not changed as a result of this quake. This is not the big one, but we don’t know when that will happen – it could be in 30 seconds or 30 years,” he said.

Prof Sieh said there had been 30 or more earthquakes of magnitude six or greater in Sumatra or the adjacent seas in the last 10 years, including the devastating quake measured at 9.1 on Boxing Day 2004.

These tremors had released more pent up energy from below the earth’s surface than anywhere else in the world over the decade, marking the start of a “failure sequence,” comparable to watching an aircraft falling apart in mid-air, that would inevitably culminate eventually in a much bigger tremor, he said.

Records going back to 1300 suggested that there had been three such failure sequences in the seas around Sumatra, the last of which began 210 years ago. However, it was impossible to forecast how long the series of quakes would last before calm returned.

Prof Sieh said there was no tsunami on Wednesday because tidal waves accompanied earthquakes only when a subsea fault was so close to the sea floor that it was suddenly deformed and the displaced water rushed on to land.

The Padang earthquake did not trigger a tsunami because it originated about 80 kilometres below the sea floor. By comparison, the ruptures that caused the Sumatran earthquakes in 2004, 2005 and 2007 were within 30 kilometres of the sea floor.

Prof Sieh said in a technical explanation of Wednesday’s quake posted on the Earth Observatory’s website that most earthquakes off Sumatra occurred as a result of collisions between undersea tectonic plates – large slabs of the earth’s crust that move slowly under earth’s surface...

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