THE earthquake of magnitude 8.7 that struck off the coast of Sumatra yesterday fulfilled many of the worst fears of scientists, who predicted just ten days ago that faults in the region were primed to deliver another big quake.

The latest earthquake appears to have followed precisely the pattern set out by researchers at the University of Ulster, who forecast that the quake that caused the Boxing Day tsunami had destabilised two neighbouring fault lines.

The epicentre of yesterday’s event, 125 miles northwest of the town of Sibolga, is at the edge of an area of particularly high stress on the Sunda Trench fault.

This line of seismic weakness extends directly to the south of the Sumatra-Andaman fault, which ruptured with such devastating consequences in December and was placed under greatly increased pressure by the previous event.

Yesterday’s earthquake is also close to the magnitude range predicted by the Ulster team, headed by John McCloskey, who calculated that an event along the Sunda Trench would measure between 8 and 8.5 on the Richter scale.

“It is just at the edge of where the worst stress was,” Dr McCloskey said. “Unfortunately it looks as if it is all entirely consistent with our study. We were concerned about two potential events, and this appears to be one of them.”

The latest earthquake may also have added to stress on the Sumatra fault which runs underneath the island close to Banda Aceh, which was also destabilised by the Boxing Day quake. “It probably will have increased the pressure on a fault we have already marked as being stressed, making a quake there more likely,” Dr McCloskey said. “We’ll be doing calculations overnight.”

Scientists were relieved, but initially perplexed by the apparent absence of another tsunami, despite the earthquake’s offshore epicentre.

Early indications were that this was because the epicentre was deeper than the Boxing Day event, at about 19 miles (30km) underground and because of its lower intensity.

The quake yesterday was a large event, initially estimated at magnitude 8.2, later raised to 8.7, which is likely to make it the most powerful earthquake that will be recorded anywhere in the world this year.

Nevertheless, it was considerably smaller than the magnitude 9.3 earthquake that struck in December. While the difference between 8.7 and 9.3 does not sound great, the Richter scale used to measure earthquakes is logarithmic, with each point signifying a 30-fold increase energy release.

Yesterday’s quake was thus about 15 times smaller than the Boxing Day event, Dr McCloskey said. At such a depth, this may not have been sufficient to cause any slippage on the sea floor — and it is such disturbances that cause tsunamis.

While the Boxing Day quake was only slightly shallower, its greater size would have been enough to crack the sea floor, even had it occurred at a much greater depth. Dr McCloskey said the chances of a tsunami being generated receded greatly when none was reported in Sumatra after the earthquake.

“If there was a tsunami, it would have been over as far as Sumatra was concerned within 15 minutes. It probably hasn’t happened and that is a great relief. Even at 8.7, which is major, it would not necessarily have produced one. Devastating tsunamis are, thankfully, rare.

“It would have been small enough that the rupture could have been confined to fairly deep within the Earth’s crust. A depth of 30km and this magnitude means there could have been nothing.” Last night, it appeared that in fact there may have been a small tsunami that did little or no damage.

The Sunda Trench fault, like the Sumatra-Andaman fault of which it is an extension, is of a variety known as a subduction fault, in which one tectonic plate slides beneath another. In subduction zones, earthquakes often occur in clusters: once one part of a fault has ruptured, stress is placed on neighbouring faults, which seek to “catch up” with the movement.

The Ulster team calculated that pressure on the Sunda Trench was raised by up to five bars after the December earthquake and that the Sumatra fault that passes under the Indonesian island was under increased pressure of up to nine bars.

These figures compare with an increase in pressure of four bars ahead of the magnitude 7.4 Izmit earthquake that struck Turkey in 1999. The epicentre of yesterday’s quake lies in the area of greatest pressure on the Sunda Trench predicted by Dr McCloskey’s team. The rupture is likely to have extended about 190 miles (300km) southwards along the Sunda Trench, seismologists said.

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