This is the responsibility of two Tsunami Warning Centers; The West Coast and Alaska Tsunami Warning Center and the Pacific Tsunami Warning Center.
The West Coast/Alaska Center is the primary warning center for the western and eastern coasts of the United States mainland and Canada.
The Pacific Warning Center provides warnings for Pacific basin teletsunamis (tsunamis that can cause damage far away from their source) to almost every country around the Pacific rim and to most of the Pacific island states, the Indian Ocean basin and countries, and Caribbean countries.
Following the 2004 Indian Ocean tsunami, other warning centers have been established in Indonesia, Australia, and New Zealand. Efforts are currently underway to establish tsunami warning centers in the Caribbean and Mediterranean Seas.
Since most tsunamis originate near coastal regions the initial response following an earthquake is issued very quickly. This initial response is based on pre-set criteria such as the, location, magnitude and depth of quake. The initial response could be one of four products; a 'Tsunami' Watch, Warning, Advisory, or Information Bulletin/Statement.
- Tsunami Warning
- The highest level of tsunami alert. Warnings are issued due to the imminent threat of a tsunami from a large undersea earthquake or following confirmation that a potentially destructive tsunami is underway. They may initially be based only on seismic information as a means of providing the earliest possible alert.
Warnings tell appropriate actions be taken in response to the tsunami threat. Such actions could include the evacuation of low-lying coastal areas and the movement of boats and ships out of harbors to deep water. Warnings are updated at least hourly or as conditions warrant to continue, expand, restrict, or end the warning.
- Tsunami Watch
- The second highest level of tsunami alert. Watches are issued based on seismic information without confirmation that a destructive tsunami is underway. It is issued as a means of providing an advance alert to areas that could be impacted by destructive tsunami waves.
Watches are updated at least hourly to continue them, expand their coverage, upgrade them to a Warning, or end the alert. A Watch for a particular area may be included in the text of the message that disseminates a Warning for another area.
- Tsunami Advisory
- The third highest level of tsunami alert. Advisories are issued to coastal populations within areas not currently in either warning or watch status when a tsunami warning has been issued for another region of the same ocean.
An Advisory indicates that an area is either outside the current warning and watch regions or that the tsunami poses no danger to that area. The Tsunami Warning Center issuing the 'Advisory' will continue to monitor the event, issuing updates at least hourly. As conditions warrant, the Advisory will either be continued, upgraded to a watch or warning, or ended.
- Tsunami Information Bulletin/Statement
- This is issued to inform that an earthquake has occurred and to advise regarding its potential to generate a tsunami. In most cases, a Tsunami Information Bulletin indicates there is no threat of a destructive tsunami, and are used to prevent unnecessary evacuations as the earthquake may have been felt in coastal areas.
A Tsunami Information Bulletin may, in appropriate situations, caution about the possibility of a destructive local tsunami. A supplemental Tsunami Information Bulletin may be issued if important additional information is received such as a sea level reading showing a tsunami signal. A Tsunami Information Bulletin may also be upgraded to a watch or warning if appropriate.
Following the first message, whether it is a warning, watch, advisory or bulletin/statement, the tsunami threat is analyzed using observed sea level data, forecast models, historic data, and further seismic processing. Based on this analysis, supplemental messages are issued if needed. Observing sea level height data is aided by a special buoy system.