Calculating the threat of tsunami
Box 3 | The warning system in the Pacific
There are two basic ways to guard against the effects of a tsunami. The first could be labelled forward planning. It involves using seismological and other geological data to predict where tsunami-generating earthquakes and landslides are most likely to occur. In those areas, information on the shape and depth of the sea floor is used to model the effects of tsunami on adjacent coasts.
When high-risk areas are identified, planning procedures could be put in place to ensure that residential developments and major constructions such as power stations are restricted to higher ground, and sea walls built to deflect some of the energy of a tsunami. One such wall, standing about 15 metres high and made of reinforced concrete, was built on Japan's Okushiri Island after it was struck by a tsunami that killed 120 people in 1993.
The second method involves monitoring and warning systems such as the one already in place around the Pacific. The Pacific Tsunami Warning System consists of a series of seismic monitoring stations and a network of gauges that measure sea levels. When a seismic disturbance is detected, its location and magnitude are computed. In some susceptible regions, warnings are issued if the magnitude is above a certain threshold. Then the gauging stations are monitored for abnormal changes in sea level. If a tsunami is detected, computer-based mathematical models are used to calculate its speed and direction – taking into account diffraction, refraction and reflection effects, as well as peculiarities in the shape of the sea bed. Coastlines lying in the predicted path of the tsunami are warned of the approaching wave train.
This system, coupled with proposed innovations such as deep ocean sensors that can pick up changes in water pressure as a tsunami passes, represents a significant advance in tsunami warning. But it is not so effective against tsunami caused by local or regional events. In the recent tragedy in Papua New Guinea, about half an hour elapsed between the occurrence of the earthquake and its associated tsunami, insufficient time for a warning system to have been of any use.
Australia is a participating member of the Pacific Tsunami Warning System, and scientists are considering the development of a tsunami prediction system for vulnerable coastlines around the continent.
Boxes
Box 1. What caused the Indian Ocean tsunami on 26 December 2004?
Box 2. The disappearance of an ancient civilisation
Related sites
The tsunami warning system in the Pacific (Intergovernmental Oceanographic Commission, UNESCO)
Tsunami forecasting and modelling (Pacific Marine Environmental
Laboratory, National Oceanic and Atmospheric Administration, USA)
Posted May 1999.