Tsunami warning systems exist in the Pacific Ocean on a basin-wide scale, in many of the countries around the Pacific Ocean, and in certain other tsunami-prone areas. In particular, efforts are currently in progress to set up a counterpart to the international systems in the Pacific that will cover the many small nations of the Caribbean (see http://woodshole.er.usgs.gov/puertoricoworkshop for a progress report). However, by far the most developed system is that in the Pacific and this will be discussed here to illustrate the structures, strengths and limitations of such systems.

The current international warning system in the Pacific, the International Tsunami Warning System, is an outgrowth of the Pacific Tsunami Warning Centre (PTWC), set up in Honolulu, Hawaii, as a branch of the United States Weather Service after the destruction caused by the 1946 Alaska tsunami in Hawaii. Further information on the structure of this organisation can be found on the website of the International Tsunami Information Centre at http://www.shoa.cl/oceano/itic/frontpage.html. Present membership of the group of nations participating in ITIC and ITWS activities is Australia, Canada, Chile, China, Colombia, Cook Islands, Costa Rica, Democratic People's Republic of Korea, Ecuador, Fiji, France, Guatemala, Indonesia, Japan, Mexico, New Zealand, Nicaragua, Peru, Philippines, Republic of Korea, Singapore, Thailand, Russian Federation, U.S.A., and Western Samoa.

The basic methodology of the system is designed to deal with earthquake-generated tsunamis. It takes advantage of the fact that earthquake waves travel substantially faster within the earth (at velocities ranging from 2000 m/s to as much as 8000 m/s, compared to maximum deep water tsunami wave velocities of around 200 to 300 m/s ---- see Implications of the equations for tsunami motion and energy). Thus at distances of more than several hundred kilometres from the fault rupture, the tsunami waves generated by a large earthquake will arrive more than an hour after the seismic waves. Therefore, the seismic records of large Pacific region earthquakes can be processed to define location and magnitude in time to provide an indication that a tsunami may have been generated long before the actual waves would arrive in most regions of the Pacific Rim. Present policy is to for the PTWC to institute a tsunami watch all around the Pacific Rim when an earthquake with magnitude greater than 7.5 and an epicentre beneath the ocean or close to the coast is detected, and a tsunami warning in the regions closest to the earthquake. The principal watch activity is to closely monitor tide gauges, especially those closer to the earthquake source, and to issue predictions based on past experience and tsunami travel time models, for the times of arrival of the tsunamis in different places. If these record the sea level changes indicative of a tsunami, the watch is converted to a warning for progressively larger regions as the tsunami propagates, and civil protection measures put into operation; if not, or if the tsunami dissipates, then the watch and any warnings put into place are cancelled. The emergency measures principally consist of evacuations of coastal areas known to be at risk from tsunamis and, in the United States at least, clearing of harbours of shipping. In Japan, coastal defences are also activated. Emergency management organisations are also put on alert. The whole warning and evacuation procedure, if activated, causes major disruption in coastal areas for a day or more. As an example of the costs involved, the evacuation of coastal areas in Hawaii following a 1986 earthquake in the Aleutians, which did not in fact produce a major tsunami, cost about $US 30 million, almost entirely due to interruption of business.

Although it is extremely useful in the reduction of casualties from distant, transoceanic tsunamis, this system has a number of problems at present:

• The false alarm rate is high
  
• The tide gauge records are difficult to interpret
  
• The system does not help mitigation in areas closer to the source
  
• The system does not predict the time of arrival of the last waves in a series
  
• The system does not function efficiently when tsunamis are generated by non-earthquake mechanisms