The tsunami that struck the Pacific Ocean islands of Samoa in September 2009 had Caribbean visitors and residents wondering, can it happen here?
The earthquake that caused September’s tsunami was centered about 120 miles south of Samoa. The first tsunami wave roared ashore on American Samoa about 15 minutes after the quake, reaching up to a mile (1.5 kilometers) inland. According to Associated Press reports, the Pacific Tsunami Warning Center in Hawaii issued an alert, but the waves arrived so quickly that residents had only about ten minutes to respond. Along with coastal residents, visiting cruisers were affected by the four 15- to 20-foot (four- to six-metre) waves that pummeled the shoreline. There were some 180 fatalities reported, including one cruiser, Dan Olszewski of Mainly, a Floridabased Freedom 39 schooner.
According to the University of the West Indies Seismic Research Centre (SRC), in the past 500 years there have been ten earthquake-generated tsunamis in the Caribbean Basin, with four of these tsunamis causing fatalities — an estimated 350 people in total.
There are three conditions necessary for an earthquake-generated tsunami: the earthquake must occur beneath the ocean and be at least of magnitude 6.5; the earthquake must occur at a relatively shallow depth (less than 30 miles/50 kilometres below the Earth’s surface); and the earthquake must be “dip-slip” in nature (i.e. involving vertical rather than lateral ground motion, which displaces a large volume of water).
Potentially, there are two groups of earthquakes that might generate tsunamis in
the Caribbean. These are: 1) Earthquakes occurring within the region, which might generate “local” tsunamis (i.e. only nearby islands are affected). In the past 500 years there have been approximately 50 potentially tsunamigenic local earthquakes but, as noted above, only ten
to 20 percent of these earthquakes actually generated tsunamis that caused significant inundation.
2) Distant earthquakes occurring outside of the region may generate tele-tsunamis.
For example, in November 1755, a major earthquake near Portugal resulted in a teletsunami which crossed the Atlantic and was noticed throughout the Eastern Caribbean from Barbados to Antigua and as far west as Cuba. The amplitude of the tsunami in all Caribbean islands was about two to three metres (six-and-a-half to ten feet) and waves continued to arrive for many hours. No damage or casualties were reported.
The Cumbre Vieja Volcano is located on the island of La Palma in the Canary Islands, off the west coast of Africa. There is a school of thought that asserts that if the western flank of that volcano were to collapse and drop 150 to 500 cubic kilometres of rock into the sea, massive tsunamis could be generated which could devastate islands in the Caribbean. While this might be possible, it is in fact a worst-case scenario and scientists at the SRC believe that its likelihood is remote. Also, a tsunami generated in that region would take four to six hours to reach the Caribbean, so
there would be ample time for a warning.
Tsunamis caused by large volcanic eruptions at or below sea level also pose a potential threat to the Eastern Caribbean. The submarine volcano Kick ’em Jenny located just north of Grenada erupts on average every 11 years. At least two of those eruptions, in 1939 and 1965, generated small tsunamis that were witnessed on the north coast of Grenada. Detailed studies of Kick ’em Jenny conducted between 2002 and 2004 have shown that the volcano does not currently pose an immediate tsunami threat, but it is possible that future eruptions could change this situation.
The historical record suggests that potentially destructive tsunamis occur at an average rate of one or two per century in the Caribbean. The hazard is not the same throughout the islands. The northeastern Caribbean region near Puerto Rico and Hispaniola is more susceptible to tsunamis. The average rate of occurrence in this area has approached one every 50 years in the last 200 years. In other sub-regions, such as the southern Caribbean, there are no historical records of destructive tsunami impacts.
It is important to note that all oceans can experience tsunamis but there are more destructive tsunamis in the Pacific Ocean because of the many major earthquakes along the margins of the Pacific Ocean and also because “dip-slip” earthquakes are more common there. As a result of the immediacy of the tsunami hazard to countries in the Pacific, there is currently a tsunami early warning system in that region.
Two components are essential for any tsunami warning system. The first is a network of instruments that allows scientists to rapidly determine when and where potentially tsunamigenic earthquakes occur and determine whether a tsunami has actually been generated and, if so, its size. For this, an efficient seismograph network and an efficient tide gauge network, both with real-time response capability, are needed.
The second component of the system is an efficient public information and education system that allows agencies to issue tsunami warnings and ensures that the public knows how to respond to these warnings. This second component is particularly important since it makes little sense to issue a warning unless people know how to respond.
In the Caribbean and most regions of the world, the first component of this system is mostly in place. Within the past decade seismograph networks and communication systems have improved such that it is now possible to detect and locate earthquakes down to about magnitude 4.5 within a few minutes anywhere in the world. This is particularly true for the Eastern Caribbean, which has one of the densest seismograph networks in the world.
Since June 2005, under the coordination of the Intergovernmental Oceanographic Commission of UNESCO, nations bordering the Caribbean Sea have been working to establish a Tsunami Warning Network. In September 2006, the SRC began upgrading its seismic monitoring network as part of this regional effort. In May 2007, the SRC purchased five broadband Libra VSAT (Very Small Aperture Terminal) telemetry.
This purchase followed the installation of nine VSATlinked seismic stations in the Caribbean by the United States Geological Survey to monitor tsunamis. These stations are an extension of the Global Seismic Network and will be operated in partnership with host institutions in the region. Three of these stations were deployed in the Eastern Caribbean, on Grenada, Barbados and Barbuda. The SRC’s new VSAT stations are located on Tobago, St. Vincent, St. Kitts, St. Lucia
and Dominica. Until a regional tsunami warning centre has been established, the Pacific Tsunami Warning Center provides cover for the region. Under this arrangement, if an earthquake occurs that can or has triggered a tsunami that may affect the Caribbean, the Pacific Tsunami Warning Center will send a warning to specific government agencies in the Caribbean, including the SRC.
One of the most effective methods of mitigating the potential impact of tsunamis is educating vulnerable island communities to identify the physical changes that signal an approaching tsunami and informing them of how to respond. The first sign of an approaching tsunami is usually a significant retreat of the sea and/or very strong ground shaking. As a result of the sea retreating, the trailing waves pile on top of the waves in front of them, thereby significantly increasing the height of the wave before hitting the shore. If you live in a lowlying area, make yourself familiar with the quickest way to get to high ground. Make sure all family members know the evacuation plan. If you are close to the sea and the water retreats by an abnormal amount,
move to high ground at once. Boats should make every attempt to reach deeper water as fast as possible. Do not stay to see what happens.
For more information visit www.uwiseismic.com.
