A build-up of stress on faults in Sumatra following the Indonesian earthquake is likely to trigger another large quake and perhaps a tsunami.
That is the claim made in Nature by a team from the University of Ulster, UK.
The slip that caused last year’s devastating quake placed increased stress on the Sumatran fault and on the adjacent undersea Sunda Trench.
A new rupture could trigger a magnitude 7-7.5 quake on land and a magnitude 8-8.5 quake beneath the sea, they say.
The 2004 earthquake occurred when the deep, flat Indian plate slipped under the Burma plate.
Major earthquakes tend to cluster in these subduction zones where two or more plates of the Earth’s crust grind and overlap.
When a quake takes place, the displacement causes the surrounding crust to become distorted. This places stress on other fault lines and structures in the area.
Researchers from the University of Ulster at Coleraine used information about displacement following December’s magnitude 9.0 quake to calculate the stresses it placed on the surrounding region.
They concentrated on the Sumatran fault, a so-called “strike-slip” fault which cuts through the island of Sumatra, and the Sunda trench, a continuation of the underwater subduction zone that ruptured to cause the tsunami last year.
“We found that both of them had been significantly loaded, in stress terms, by the 26 December quake,” said lead author John McCloskey.
Their results show a stress increase of up to five bars in the 50km stretch of the Sunda trench located next to the rupture zone. They show an increase of up to nine bars for about 300km on the Sumatra fault near the city of Banda Aceh.
Based on these findings, the scientists predict a possible magnitude 7-7.5 earthquake on the Sumatran fault and a magnitude 8-8.5 quake at the Sunda trench.
“The huge changes in stress mean that I am comfortable talking about a significant increase in the risk of another quake. But that is as far as I am prepared to go,” Professor McCloskey told the BBC News website.
The researchers stop short of predicting when another large quake will strike the region. But similar events elsewhere in the world have occurred within a few years of each other, or even a few months.
In Japan, at least five major quakes in the Nankaido segment of the Nankai subduction zone have been accompanied by similar events on the linked Tonankai/Tokai segment within five years – and three of the subsequent quakes ruptured in the same years as their precursors.
The magnitude 7.4 Izmit earthquake in Turkey in 1999 triggered the magnitude 7.1 Duzce earthquake three months later. Worryingly, the Anatolian fault – where these quakes occurred – has a very similar structure to the Sumatran fault.
Some researchers believe large earthquakes occur at the Sunda trench on a cycle of 200 years, which is determined by stress loading at the subduction zone. The last big event occurred about 150 years ago, but Professor McCloskey said the recent quake could have accelerated this cycle.
“The amount of extra stress could be equivalent to 50 or 60 years of loading. But I personally am not convinced by this theory,” he said.
A large earthquake at the undersea Sunda trench had the potential to cause another tsunami, the University of Ulster researcher added.
But Professor Nick Ambraseys of Imperial College London, UK, expressed concern about the Nature report:
“False alarms and inaccurate timing could create more problems than already exist,” he said.
“There is nothing in their article that enables, with any degree of certainty, the prediction of the immediacy of the next earthquake.”
The authors of the research paper say this makes a tsunami early warning system for the Indian Ocean all the more urgent
Image: HM Royal Navy