Following the great Sumatran Earthquake on December 26 with its attendant tsunami there have been strident calls for a tsunami warning system for the Indian Ocean to match that which has been in place for the Pacific since the mid-1960s.
In the Pacific Ocean the impact of tsunamis comes not just from local earthquakes, but from major earthquakes elsewhere around the basin. A formal warning system was established in Hawaii after devastation from a 1946 earthquake in the Aleutian chain, and this was extended to the whole Pacific following the impact of tsunamis from the 1960 event off the coast of Chile. The system works because the seismic waves produced by earthquakes travel through the Earth with high velocity and reach to even 12,000 km away from the source in less than 15 minutes, whereas the tsunami waves travel at speeds comparable to jet aircraft and take hours to cross a major ocean basin. The Pacific centre monitors the signals from seismic stations across the globe and can issue a tsunami watch notice within 30-40 minutes of an earthquake that might be capable of generating a tsunami. When tide stations near the earthquake confirm the generation of a tsunami, the watch notice becomes a warning accompanied by predicted arrival ti
Most tsunamis are produced by earthquake activity beneath the sea, either by direct deformation of the seabed or by submarine slumping of the seafloor induced by an earthquake. Other circumstances come from explosive volcanism at sea as in the destruction of the island of Krakatoa in Indonesia in 1883, or, potentially, from the impact of a large meteorite on the ocean.
There is unfortunately little that can be done to provide warnings in the immediate neighbourhood of a tsunami generating earthquake, since the interval between the ground shaking and the arrival of the ocean waves is so short. Such was the effect on the north coast of Papua New Guinea in 2001 and on the Aceh coast in this recent great earthquake.
To understand what we can hope to achieve with a warning system we need to understand the nature of the class of earthquakes that are likely to produce damaging tsunami effects. The earthquake needs to be big and occur in the marine environment. The source process has to produce vertical deformation of the sea-floor so that large volumes of water are displaced to set in motion the waves that cross the oceans. These circumstances are met for shallow events megathrusts on the subduction zones where oceanic material is descending into the interior of the Earth. The recent event off Aceh is of this nature, with a combination of both horizontal and vertical motion. Detailed studies by different groups around the world indicate that there was a strong initial disturbance followed about a minute later by much larger fault displacement starting about 200 km further north extending for a further 300-400 km along the plate boundary. Weaker slow faulting completed the process for a total fault length exceeding 1000
The initial faulting created the tsunamis that hit the Aceh coast. The strong waves radiated toward Thailand that have cost the lives of so many visitors were induced by the second phase of the earthquake. An event further south along the Sumatran coast would have produced a much smaller effect on the Thai coast. The dominant orientation of the thrust fault is slightly east of north and this lead to strong radiation of tsunami energy to the east to reach the coasts of the Maldives, SriLanka and India. The weaker faulting in the north meant that little tsunami energy was transmitted towards Bangladesh and Burma, where casualties have been low.
There have been strong tsunamis in the northern Sumatran region before, notably in 1861 and 1833 at locations slightly further south than 2004. Stress has to build up over a long period before it is released in a great earthquake and so such events do not occur frequently. Seismological studies can indicate the potential for large earthquakes, but even in those regions such as Japan and California with the most comprehensive instrumentation the goal of earthquake forecasting remains elusive.
Outside the Pacific Rim that has many different subduction zones, the frequency of potential tsunamis is quite low. This is why warning systems for, e.g., the Indian Ocean, have in the past been a low priority. The long time span between events also imposes a difficult burden if a warning system is established: constant vigilance but highly infrequent activity.
The international community has made an extraordinary investment in global monitoring capability, particularly in seismology, through the 1996 Comprehensive Nuclear-Test-Ban Treaty with the object of detecting any clandestine testing of nuclear devices. Data is transmitted by satellite to an International Data Centre in Vienna where automated estimates of event location are made, to be later refined by careful analysis. The system is not yet fully operational because the treaty has yet to enter into force. Yet we have here the capability for a global system for tsunami warnings to relevant Governments particularly with additional technical investment in ocean tide gauges. Australia played a major role in securing the signing of the 1996 treaty, let us continue the process of beating swords into ploughshares by turning the already existing investment to greater humanitarian benefit.
Brian L.N. Kennett is Professor of Seismology at the Research School of Earth Sciences, Australian National University and Past-President of the International Association of Seismology and Physics of the Earths Interior.