On 26 December 2004, the coast of Sri Lanka was hurled into devastation beyond imagination and into a maelstrom of confusion. According to the U.S. National Geophysical Data Centre (NGDC) that catalogues global tsunami events, within the last 250 years there have been in excess of 60 tsunamis in the Indian Ocean region. Tsunamis are not that rare.

The source of these tsunamis is related to the tectonic activity of the region. Of these 60 events, eight tsunamis affected Sri Lanka excluding the December 26, 2004 tsunami. There were two tsunamis triggered off the coast of Pakistan in 1819 and 1945. In the Bay of Bengal there have been at least 4 tsunamis that affected the eastern coast of Sri Lanka; in 1762, 1847, 1881 and 1946. The last three were related to earthquakes near the Nicobar Islands, India. It is also probable that in 1882, a small tsunami was triggered off the northeast coast of Sri Lanka and subsequently a tsunami was registered in the northeastern town of Trincomalee. However, prior to 2004, the most significant tsunami to affect Sri Lanka within the last 250 years was a result of the eruption of the Krakatau volcano in Indonesia on August 27, 1883.

As most tsunamis in the Indian Ocean are earthquake induced, it is likely that the next destructive tsunami to strike Sri Lanka would also originate off the coast of Sumatra. The potential for future destructive tsunamis from this region is still great. The December 26, 2004 Aceh earthquake did not relieve all the stress along the faults in the Sundra trench. This fault system is still highly stressed. Furthermore, the large displacement caused by the earthquake has heightened the stress of neighboring faults. These tsunami-triggering earthquakes could vary significantly in size and destructive force.

The role of a tsunami monitoring centre is to notify and alert local warning centers to prompt civil defence actions against an oncoming tsunami. A monitoring centre must therefore rely on information from geophysical instruments such as seismic sensors, ocean-bottom pressure gauges and tide gauges. From global and regional arrays and seismic and oceanographic stations, a tsunami monitoring centre can accurately determine the location and magnitude of a tsunami-producing event and evaluate the probability of a tsunami.

Although tsunami warning systems would do little to reduce the vulnerability to the near-source area, they would have a significant impact in far-from-source areas such as Sri Lanka. A good EWS should include tools such as monitoring, scenario development and forecasting. A EWS should also be mindful of integration into the larger social system. Because reducing vulnerability is essentially a social process, EWS should be used only in conjunction with other disaster preparedness steps. More scientific investigations into the patterns of tsunami hazards in the Indian Ocean region could prove extremely valuable in mitigating future threats from this up until now under-estimated natural hazard.

Rashmin Gunasekera has a PhD in Earthquake Seismology from University of Durham, UK and an MPhil in GIS and Remote Sensing from the University of Cambridge, UK. He can be contacted at rashmin_gunasekera@hotmail.com