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Tsunamis and Coastal Hazards
The coastal hazard of a tsunami (soo-NAH-mee) is a series of travelling ocean waves of extremely long wave length. These phenomena are most frequently caused by shallow focus earthquakes, underneath or near the ocean, that cause vertical movement of the seafloor (up to several metres) over a large area (up to a hundred thousand square kilometres). This is why tsunamis are also sometimes called seismic sea waves. The 2004 Boxing Day tsunami that affected several countries in the Indian Ocean and more recently, the 2011 tsunami that battered north east Japan, were both caused by massive earthquakes occurring along Pacific fault subduction zones. In addition, although far less frequent, tsunamis have also been known to be triggered by submarine volcanic eruptions, submarine landslides and large meteorite impacts in the ocean.
Characteristics of a Tsunami
Depending on the location of the tsunami trigger impact time on coastlines can range from less than an hour (a local tsunami) to over three hours (a teletsunami/distant tsunami). In the deep ocean the waves can travel at speeds of 500 to 1000 km per hour and may be only about one metre in height. Upon approaching the shore however, tsunami height can grow to tens of metres and speeds reduce to just a few tens of kilometres per hour. The associated shortening of wave length and concurrent increase in wave height concentrates the tsunami’s energy, giving this hazard its destructive potential. Local conditions such as near-shore bathymetry, shape of the coastline, state of the tide and degree of coastal development can further augment the extent of impact a tsunami may have.
Tsunami Warning and Response
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 Information Bulletin/Statement– generated as a result of seismic action, it advises of major earthquakes and gives general information about tsunami threats
· Tsunami Advisory– it is issued to coastal populations that are not within the warning areas to advise that a warning has been issued
· 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 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.