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Last Updated: Tuesday, 29 March, 2005, 12:38 GMT 13:38 UK
New quake makes only small waves

News of the magnitude 8.7 earthquake off the coast of Sumatra on Monday caused panic in coastal regions hit by the devastating Asian tsunami.

But unlike December's quake, this latest one did not lead to a wave with anywhere near the same destructive force.

Scientists say this may be because the new quake did not release enough energy to cause slippage in the sea floor - and the quake's focus was also deeper.

Measurements show a small tsunami did result but its effects were hardly noticeable.

It is likely that we did not have nearly as large a displacement as with the December 26 event
Greg Romano, Noaa
Tide gauges in the Maldives and Cocos islands in the Indian Ocean recorded waves of between 20cm and 50cm (8-20 inches) in height.

Energy values

The epicentre of Monday's quake was about 200km (125 miles) northwest of the town of Sibolga, at the edge of a particularly high stress area on the undersea Sunda trench fault.

Major earthquakes tend to cluster at these subduction zones, where two or more plates of the Earth's outer material grind and overlap.


The magnitude 9.3 quake on 26 December occurred further to the north - where the deep, flat Indian tectonic plate slipped under the Eurasian (Burma micro-) plate off the coast of Banda Aceh on Sumatra.

The disturbance this caused in the water column above generated waves that, by the time they reached land, were up to 10m (30ft) high.

The figures quoted by agencies for the size of the quakes are on the moment magnitude scale - and although the jump from 8.7 to 9.3 may not sound like much, the way the calculations are made means there is actually a big difference between the two.

In terms of the energy released, Monday's quake was probably 5-6 times smaller than the Boxing Day event.

"That's crucial, because the bigger the energy released, the greater the chance that the sea bed will move," said John McCloskey of the University of Ulster.

Deep underground

Just 10 days ago, Professor McCloskey's team forecast in the journal Nature that the Boxing Day quake had increased stress on two nearby fault lines - the Sumatran fault and on the undersea Sunda trench.

Greg Romano, of the US National Oceanic and Atmospheric Administration (Noaa), told the BBC: "A tsunami is generally generated when you have a large displacement of water in a vertical direction and it is likely that we did not have nearly as large a displacement as with the December 26 event."

An employee points at a graphic of the strong earthquake that rocked Sumatra island at the Meteorology and Geophysics office in Jakarta, Indonesia, Tuesday, March 29, 2005     Image: AP
The earthquake's magnitude may yet be revised up
Early indications suggest the quake occurred about 30km (19 miles) underground - deeper than the Boxing Day event.

"We don't know what depth it nucleated at but if the earthquake did nucleate slightly deeper then the effect on the surface is much less," Professor McCloskey told the BBC.

"Tsunamis of the type we observed on Boxing Day are thankfully very, very rare."

Monday's quake was close to the magnitude range predicted by the Ulster team, which calculated that it would measure between magnitude 8 and 8.5.

It could also have added to stress on the Sumatran fault, which cuts through the Indonesian island and was, according to Professor McCloskey's team, also destabilised by the December event.

Analysis of Monday's earthquake

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