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. (ft.com)

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.

“The Indian and Australian plates are sliding north-eastward at about 7cm per year under the Sunda plate, which includes Sumatra and Singapore,” he said. “The process of one slab sliding beneath another is called subduction, and the gently sloping upward thrust that occurs where they join is called the Sunda megathrust.

“Until the “subducting” oceanic plates drop to about 30km beneath Sumatra, the megathrust resists slippage and the plates becomes coupled, or locked together. As a result, strain accumulates over many decades and even more than a century, until the megathrust gives way.

“When that happens, the oceanic plate lurches suddenly many metres north-eastward beneath Sumatra, and a big earthquake occurs. That’s what happened in the earthquakes and related tsunamis of December 2004, March 2005 and Sept 2007.

“In contrast, however, initial analysis by the US Geological Survey suggests that the September 30 earthquake was caused by a sudden rupture within the descending Indian and Australian plates, about 80km beneath the earth’s surface.”

Prof Sieh said a follow-up earthquake on October 1, measured at 6.6, appeared not to be an aftershock. It occurred more than 200km away from the September 30 earthquake on the Sumatran fault, a “strike-slip” fault that runs the length of Sumatra, from near Krakatoa in the south to Banda Aceh in the north.

That fault produced many large earthquakes with magnitudes of between 7 and 7.5 between 1892 and 1953, but has been largely quiet for the past 50 years.

Prof Sieh said there was no way to avoid the impact of earthquakes, but deaths and damage could be reduced through better building codes and more effective emergency evacuation procedures he said.

He said this was more likely to save lives than the early warning systems favoured by many governments.

“We need to have a long-term strategy of training people to run away or evacuate vertically – go up – if they can’t run away,” he said.

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