Tsunamis & Coastal Hazards

A tsunami is a series of ocean waves with very long wavelengths (typically hundreds of kilometres) caused by large-scale disturbances of the ocean,

These disturbances can either be from below (e.g. underwater earthquakes with large vertical displacements, submarine landslides) or from above (e.g. meteorite impacts).

Tsunami is a Japanese word with the English translation: “harbour wave”. In the past, tsunamis have been referred to as “tidal waves” or “seismic sea waves”. The term “tidal wave” is misleading; even though a tsunami’s impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. (Tides result from the gravitational influences of the moon, sun, and planets.) The term “seismic sea wave” is also misleading. “Seismic” implies an earthquake-related generation mechanism, but a tsunami can also be caused by a non-seismic event, such as a landslide or meteorite impact.

Tsunamis are also often confused with storm surges, even though they are quite different phenomena. A storm surge is a rapid rise in coastal sea-level caused by a significant meteorological event – these are often associated with tropical cyclones.

The adequacy of early warning that can be provided in the face of an impending tsunami hazard will depend, in part, on where the tsunami originates from. If it is a local tsunami early warning might be limited simply by virtue of the short time period between generation of the waves and when they make landfall. In these situations therefore, it is important for coastal users to recognise the signs that a tsunami is about to occur. These include a rapid fall in the sea level, causing the sea floor to be exposed, and an audible roar like an oncoming train. When these signs are observed, coastal users should immediately head for higher ground.

With distant tsunamis the lengthier time before coastline impact can allow for more adequate warning. This provides the opportunity to put into effect evacuation plans and response protocols. In this regard it is important to understand the terminology used in Tsunami messages. Inherent in these specific terminologies are associated mechanisms, roles and responsibilities for organizations and individuals to undertake in order to mitigate against losses in the face of tsunami threat. Tsunami messages range from:

  • Tsunami Watch– this is the second highest level of alert. They are issued without confirmation of an impending tsunami to warn populations within one to three hours of potential impact. Watches are updated hourly and may include a warning for other locations
  • Tsunami Advisory– it is issued to coastal populations that are not within the warning areas to advise that a warning has been issued
  • Tsunami Information Bulletin/Statement– generated as a result of seismic action, it advises of major earthquakes and gives general information about tsunami threats
  • Tsunami Warning– this highest level of warning. It is issued where there is imminent threat of a tsunami from a large under sea earthquake or following confirmation of a large tsunami. Warnings are issued hourly or as conditions require

For our country, the Meteorological Services of Trinidad and Tobago, acting on advice from the Pacific Tsunami Warning Centre, is in charge of issuing tsunami messages. When these messages are received, the Office of Disaster Preparedness and Management (ODPM) coordinates relevant stakeholder agencies to produce an appropriate response to the tsunami hazard.