A shock waiting to happen since Boxing Day

Several hours after yesterday's tremors sent shivers of panic around south-east Asia, there was an unusual sight recorded on the remote Cocos Islands.

Several hours after yesterday's tremors sent shivers of panic around south-east Asia, there was an unusual sight recorded on the remote Cocos Islands.

The archipelago of 27 Indian Ocean islands, situated 1,000km (620 miles) south-west of Indonesia's Java island, was at the receiving end of a tsunami measuring 25cms.

Its arrival on the islands occurred as geologists across the globe debated the possibility of yesterday's Sumatran earthquake unleashing tidal waves of destruction across south-east Asia. Having reached a magnitude of 8.7 on the Richter scale - compared with 9.0 on Boxing Day last year - the risk of a second tsunami was clearly high.

But as the three-hour danger period lapsed without incident in the aftermath of the earthquake, governments slowly withdrew their warnings and the panic began to subside. Instead, speculation grew that the release of energy from the earthquake had been directed south - the opposite direction to the Boxing Day quake.

As a result, the risk of small-scale tsunamis remained last night in southern locations such as Madagascar, Mauritius and even western Australia.

Robert Cessaro, an official at the Pacific Tsunani Warning Center in Hawaii, said: "Because we think the event probably ruptured to the south, the beam of energy, rather than propagating towards the central Indian Ocean, may very well have propagated towards the south, towards Mauritius and Rodrigues."

There are roughly six major tsunamis each century, with about 90 per cent occurring in the Pacific Ocean, all of which can only be triggered by earthquakes over a magnitude of 6.75 on the Richter scale.

In the case of sub-marine earthquakes, the sea floor is lifted then dropped, pushing the entire water column up and down. The movement - sometimes up to 40 feet in a split second - displaces billions of tons of water. The immense energy created is transferred to the horizental movement of a wave. As the depth of the sea becomes shallower, the tidal force slows down but grows rapidly in height, piling the waves on top of themselves, resulting in peaks of 100 feet of more.

The earthquake was the biggest aftershock recorded since Boxing Day. However, experts were confident that if a tsunami had been unleashed, it would be much smaller than than December's.

David Booth, senior seismologist of the British Geological Survey, said: "Because the earthquake is of such a shallow depth and is offshore, it would be on a much smaller scale than the Boxing Day disaster."

In spite of its vulnerability to earthquakes and tsunamis, the Indian Ocean has yet to install a tsunami warning system which will provide little comfort for its millions of inhabitants.

A warning system will be installed by December 2006. However, despite the sophisticated arsenal of pressure sensors, seismographs and satellites involved in the technology, there is more work required on land. The implementation of the system is dependent on the ability to communicate the warning to communities in remote areas across the Indian Ocean.

STRONGEST QUAKES IN PAST 100 YEARS

CHILE, 22 May 1960: Measured 9.5; killed 5,000, made two million homeless

ALASKA, 28 March 1964: Measured 9.2; killed 125

ALASKA, 9 March 1957: Measured 9.1; no one killed

INDONESIA, 26 Dec 2004: Measured 9.0; killed nearly 300,000

RUSSIA, 4 Nov, 1952: Measured 9.0; no one killed

RUSSIA, 31 Jan 1906: Measured 8.8; killed 1,000

INDONESIA, 28 March 2005: Measured 8.7

ALASKA, 4 Feb 1965: Measured 8.7, no one killed

TIBET/INDIA 15 Aug 1950: Measured 8.6, killed at least 1,500 people

Source: Reuters/US Geological Survey