The Pacific Tsunami Warning System (PTWS), with its headquarters in Hawaii, detected the Sumatran earthquake that triggered the Boxing Day tsunami. But the PTWS was not geared up to tell Indian Ocean communities of the danger. Even had it done so many communities would not have known how to respond.

More than three-quarters of the world’s tsunamis occur in the Pacific Ocean, which is rich in tsunami-generating plate boundaries. Rapid-response tsunami detection and warning systems are in place in Japan and on the western coast of North America, where tsunami-generating plate boundaries lie close to heavily populated shorelines. Here a tsunami can make landfall within 15 minutes of an underwater earthquake.

This also happened at Banda Aceh in the Indian Ocean after the Sumatran earthquake, the difference being that the earthquake did not prompt a population evacuation that could have saved tens of thousands of lives. In the wake of the Boxing Day tsunami there was international outcry to set up a warning system in the Indian Ocean like the one in the Pacific. The Indian Ocean version will begin operating in 2006 and be fully functional by 2008.

The Pacific’s warning system is currently alerted when seismographs detect a strong shallow earthquake beneath the seabed. Sea-level monitoring with tide gauges and seafloor pressure sensors confirms a tsunami. Communication satellites then relay tsunami warnings to local governments, emergency services and the media, who alert local populations.

All the technology only makes sense if communities in at-risk regions know how to respond. People must escape inland to higher ground along prearranged evacuation routes. Some communities also need to recognise the telltale signs of a locally-generated tsunami. Following earth tremors, signs include the roar and spray of an approaching tsunami often preceded by an unexpected withdrawal of water from beaches.