INDONESIA: Measuring between 8.2 and 8.7 on the Richter scale, yesterday's earthquake off the northern coast of Sumatra was the most violent of at least seven to have struck the region in as many days.
The earthquake, which occurred just after 11pm local time and shook the ground for almost three minutes, was not unexpected. Earlier this month, scientists analysing the aftermath of the December 26th earthquake and resulting tsunami - which brought havoc to the region and claimed more than 300,000 lives - warned that another devastating quake was likely.
Although a huge earthquake can release stresses built up over hundreds of years, it rarely means that the region is safe afterwards. The slippage only heaps stress on other parts of the fault, leading to further aftershocks.
John McCloskey, of the University of Coleraine, who led the research, found that the seismic slip which triggered the St Stephen's Day tsunami piled dangerous levels of stress on to two vulnerable parts of the fault zone, significantly raising the chances of an earthquake of a magnitude greater than 7.5 on the Richter scale.
After the December 26th earthquake, stresses in one region, a 50km stretch of the undersea Sunda trench, were believed to have increased by 5 bars. The stress in a second region, a 296km fault running directly beneath the island of Sumatra, close to the city of Banda Aceh, was thought to have been pushed up by as much as 9 bars.
Scientists last night reported that one ocean sensor near the Cocos Islands off the south-west coast of Sumatra had detected a minor tsunami wave.
The Indian Ocean region is prone to earthquakes because of the slow but unstoppable movement of the giant Burma and Indian tectonic plates. The Indian Ocean is sliding under Indonesia at a rate of 7cm a year, but as the plates rub past one another, they jam, causing huge stresses to build up. When the stresses give, the plates judder with tremendous ferocity.
In the December 26th earthquake, which measured 9.5 on the Richter scale and was the biggest detected in 40 years, the slip of the tectonic plates forced the sea floor up by almost 20 metres and raised the level of the Sumatran coastline. Despite plans to deploy ocean sensors to detect tsunamis throughout the Indian Ocean, no system is yet in place. Instead, yesterday evening's earthquake was detected by the Pacific Tsunami Warning Centre, which immediately issued a bulletin. The announcement recording the earthquake advised that a tsunami was possible.
Because there are no sensor buoys in the Indian Ocean to detect tsunami waves, Pacific Tsunami Warning Centre scientists were unable to confirm shortly after the earthquake whether a tsunami was due.
Vasily Titov, a tsunami researcher at the Pacific Marine Environment Laboratory in Seattle, said a tsunami bulletin was usually issued if a sub-sea earthquake of more than 7.5 on the Richter scale was detected.
Earlier this month, countries in the Indian Ocean region and UN experts met in Paris to agree plans for a tsunami early warning system, but this will not be completed until the end of this year. Until the network is in place, the region must rely on alerts from the US and Japan.
The Indian government has pledged to invest about $25 million in a tsunami warning system, putting in place up to 12 seabed sensors to transmit information to the mainland. - (Guardian Service)