Some hazards, like tropical cyclones and flooding are all too frequent across the Lesser Antilles – particularly after the 2017 Hurricane Season. Other hazards, such as earthquakes, are not uncommon but in the Eastern Caribbean but it is a hazard that is dangerously under-prepared for. The Eastern Caribbean is the most seismically active area along the Caribbean Plate with numerous volcanoes – both above ground and submarine. These volcanoes have the potential to cause landslides that generate tsunamis in addition to earthquake-generated tsunamis along this plate boundaries.

Each year, over 1200 earthquakes are recorded in the Eastern Caribbean. These earthquakes aren’t necessarily a precursor for a larger event. On average, the Eastern Caribbean has seen a pattern of quakes within M7.0 to M7.9 every 20 to 30 years. That pattern has stayed true and was last seen in an event north of Martinique in 2007. Historical patterns indicate earthquakes at and above the magnitude of 8.0 on the Richter Scale have occurred every century or so in the region, and the probability of another event at that level is high since the last >M8.0 earthquake occurred in 1843. These larger earthquakes, depending on the magnitude, depth and location of the epicenter may generate tsunamis.

To reduce panic and increase the awareness of tsunamis, events that can generate tsunamis, what messages, advisories and warnings will likely be issued if T&T ever is under a tsunami threat and most importantly – how to prepare and react, we will have a four part series on Tsunamis in the Eastern Caribbean.

1) What Causes a Tsunami 2) Tsunami Characteristics 3) The History of Eastern Caribbean Tsunamis 4) Tsunami Advisories & Safety

A tsunami is one of the most powerful and destructive natural forces. It is a series of extremely long waves (multiple waves tens-to-hundreds of miles between crests) caused by a large and sudden displacement of the ocean. Tsunamis radiate outward in all directions from the point of origin and can move across entire ocean basins. When they reach the coast, they can cause dangerous coastal flooding and powerful currents that can last for several hours or days.

Tsunamis are not tidal waves since it is not related to tides, which result from the gravitational attraction of the Sun and Moon on Earth’s oceans.

To begin, not all earthquakes can generate a tsunami. The University of the West Indies Seismic Research Center (UWI SRC) outlines the following criteria for tsunamigenesis: (1) The earthquake must occur beneath the ocean or cause material to slide in the ocean. (2) The earthquake must be strong, at least magnitude 6.5. (3) The earthquake must rupture the Earth’s surface and it must occur at shallow depth – less than 70km below the surface of the Earth. (4) The earthquake must cause vertical movement of the sea floor (up to several metres).

Note that the Pacific Tsunami Warning Center, the current warning agency for the Caribbean in the event of a tsunami, has a lower magnitude threshold (M6.0+) and a greater depth threshold (>100 Kilometers) for determining if an earthquake has the potential to generate a tsunami.

A tsunami is caused by a large and sudden displacement of the ocean (or sometimes – a large water body such as a lake). Large earthquakes below or near the ocean floor are the most common cause, but landslides, volcanic activity, certain types of weather, and near earth objects (e.g., asteroids, comets) can also cause tsunamis. Most of the tsunamis (88%) in the Global Historical Tsunami Database were generated by earthquakes or landslides caused by earthquakes.

Thrust/Reverse Tsunamigenic Earthquakes Most of the earthquakes that generate tsunamis (tsunamigenic earthquakes) are usually on thrust or reverse faults. In the Eastern Caribbean, this large thrust fault would be the subduction zone where the North and South America Plates subduct beneath the Caribbean plate along the Lesser Antilles Trench at approximately 20 millimeters per year– east of the islands.

It is still important to note that while it is uncommon, 10-15% of historical damaging tsunamis have been generated by strike-slip earthquakes which occur along transform faults. These tsunamis are likely generated by associated landslides, movement of a sloping ocean floor, or the presence of seamounts, which are underwater mountains (that can act like paddles and push the water horizontally). Tsunamis generated by strike-slip earthquakes normally affect regions near the source only. Specifically for Trinidad and Tobago, most of the larger regional faults across our region are transform faults – specifically the North Range Fault Zone, the El Pilar Fault System and the Southern Range Fault System.

The last large (>M6.0) earthquake that occurred along the El Pilar/Northern Range Fault System occurred at 10:10PM October 11th, 2013 at a Magnitude 6.4 and at a depth of 60 Kilometers actually did have a localized tsunami threat no more than 100 Kilometers from the epicenter. Note these values are from the UWI SRC, the USGS has this earthquake at a M6.0 and depth of 63 KM.

A Tsunami Information Statement was issued by the Pacific Tsunami Warning Center with the following information: “A destructive widespread tsunami threat does not exist based on historical earthquake and tsunami data.

With a relatively low earthquake magnitude when determining its tsunami potential – it was clear that although a tsunami was possible, it was unlikely. This is typically the case for most earthquakes along the transform faults northwest of Trinidad.

Earthquakes at divergent plate boundaries are not likely to generate tsunamis as earthquakes at divergent boundaries tend to remain below M6.0. The strike-slip areas from fracture zones resulting from the divergent plate boundaries do have the potential to generate localized tsunamis.

Tsunamis generated by volcanoes, both above and below water, are infrequent, but several types of volcanic activity can displace enough water to generate destructive tsunamis. These include:

Like other nonseismic tsunamis, such as those generated by landslides, volcanic tsunamis usually lose energy quickly and rarely affect distant coastlines – i.e. this is a relatively localized event.

From the UWI SRC: “While it is probable that large explosions or landslides at Kick’em Jenny could generate tsunamis, the threat from tsunamis has been largely over exaggerated. Not all eruptions at Kick’em Jenny will generate tsunamis and not all tsunamis will be large.

The best-studied example of underwater dome growth in the West Indies occurred at the Soufriere volcano in St. Vincent in 1971. On that occasion a dome began to grow at the bottom of the 175 meter deep crater lake in early October 1971 and reached the surface in late September. By analogy with this episode we would expect a period of one to two months of steady eruption before Kick’em Jenny grows into the violently explosive range of depths.”

Relating to tsunami generation, “landslide” is a general term that incorporates a number of types of ground movement, including rock falls, slope failures, debris flows, slumps and for outside of the Caribbean – ice falls/avalanches, and glacial calving (the breaking off of large pieces of ice from a glacier).

Tsunamis can be generated when a landslide enters the water and displaces it from above (subaerial) or when water is displaced ahead of and behind an underwater (submarine) landslide. Tsunami generation depends on the amount of landslide material that displaces the water, the speed it is moving, and the depth it moves to. Landslide-generated tsunamis may be larger than seismic tsunamis near their source and can impact coastlines within minutes with little to no warning, but they usually lose energy quickly and rarely affect distant coastlines.

Most landslides that generate tsunamis are caused by earthquakes, but other forces (like gravity, wind, and increased precipitation) can cause overly steep and otherwise unstable slopes to suddenly fail. Earthquakes that are not large enough to directly generate a tsunami may be large enough to cause a landslide that in turn can generate a tsunami. A landslide-generated tsunami may occur independently or along with a tsunami directly generated by an earthquake, which can complicate the warning process and compound the losses.

While these events are unheard of in the Caribbean to date, they are possible with strong, fast moving thunderstorms with a localized area of very low pressure area to affect the water column below the system. They are caused by air pressure disturbances often associated with fast moving weather systems, such as squall lines. These disturbances can generate waves in the ocean that travel at the same speed as the overhead weather system. Development of a meteotsunami depends on several factors such as the intensity, direction, and speed of the disturbance as it travels over a water body with a depth that enhances wave magnification.

Like an earthquake-generated tsunami, a meteotsunami affects the entire water column and can become dangerous when it hits shallow water, which causes it to slow down and increase in height and intensity. Even greater magnification can occur in semi-enclosed water bodies like harbors, inlets, and bays.

Most meteotsunamis are too small to notice, but large meteotsunamis can have devastating coastal impacts (although not to the extreme of the 2004 Indian Ocean and 2011 Japan tsunamis). Damaging waves, flooding, and strong currents can last from several hours to a day and can cause significant damage, injuries, and deaths.

A meteotsunami should not be confused with storm surge associated with tropical storms and other large coastal storms. Storm surge is a wind-driven effect that occurs when strong winds push water onshore, causing water levels to steadily rise over the course of several hours. Recent research has shown that meteotsunamis are more common than previously thought in areas along the Mediterranean Sea and Great Lakes of the U.S. It suggests that some past events may have been mistaken for other types of coastal floods, such as storm surges or seiches, which also tend to be wind-driven.

It is indeed very rare for a meteorite or an asteroid to reach the earth. Although no documented tsunami has ever been generated by an asteroid impact, the effects of such an event would be disastrous.

Most meteorites burn as they reach the earth’s atmosphere. However, large meteorites have hit the earth’s surface in the distant past. This is indicated by large craters, which have been found in different parts of the earth. Also, it is possible that an asteroid may have fallen on the earth in prehistoric times – the last one some 65 million years ago during the Cretaceous period. Since evidence of the fall of meteorites and asteroids on earth exists, we must conclude that they have fallen also in the oceans and seas of the earth, particularly since four fifths of our planet is covered by water.

The fall of meteorites or asteroids in the earth’s oceans has the potential of generating tsunamis of cataclysmic proportions. Scientists studying this possibility have concluded that the impact of moderately large asteroid, 5-6 km in diameter, in the middle of the large ocean basin such as the Atlantic Ocean, would produce a tsunami that would travel all the way to the Appalachian Mountains in the upper two-thirds of the United States. On both sides of the Atlantic, coastal cities would be washed out by such a tsunami.

It should be reiterated that the possibility of these events occurring are very, very low.

Conceivably tsunami waves can also be generated from very large nuclear explosions. However, no tsunami of any significance has ever resulted from the testing of nuclear weapons in the past. Furthermore, such testing is presently prohibited by international treaty.

From the International Tsunami Information Center: “The Discovery Channel has replayed a program alleging potential destruction of coastal areas of the Atlantic by tsunami waves which might be generated in the near future by a volcanic collapse in the Canary Islands. Other reports have involved a smaller but similar catastrophe from Kilauea volcano on the island of Hawai`i. They like to call these occurences “mega tsunamis”. We would like to halt the scaremongering from these unfounded reports. We wish to provide the media with factual information so that the public can be properly informed about actual hazards of tsunamis and their mitigation.

– These considerations have been published in journals and discussed at conferences sponsored by the Tsunami Society.”