One of today’s lead stories in The Gleaner advised us not to worry about Kick ’em Jenny, the underwater volcano located eight kilometres north of Grenada, the eruption of which would lead to seismic activity that would affect several parts of the Caribbean. This advice from Dr Parris Lyew-Ayee comes after the seismic unit at the University of the West Indies (UWI), St Augustine campus in Trinidad, raised the alert level from yellow to orange. This means there is a highly elevated level of seismic and eruption may begin with less than 24 hours notice.
Dr Lyew-Ayee also pointed out that Jamaica has experienced two tsunamis before. Did you know about them? They were in 1692 and 1907, following the two biggest earthquakes to ever shake the island.
In the first documented major earthquake in the Caribbean, Port Royal was devastated on June 7, 1692. According to this paper by Dr Margaret D Wiggins-Grandison, head of the Earthquake Unit at the University of the West Indies – Mona,
“The sinking of a part of Port Royal into the sea in 1692 caused a wave of 1.8 metres (6 feet) to cross the [Kingston] Harbour with a withdrawal of 274 metres. At Yallahs a withdrawal of 1.6 km is mentioned though not substantiated by the Tomblin & Robson earthquake catalogue or by any consequent damage reports. In 1907, waves of 1.8 – 2.4 metres were reported along the north coast between Portland and St Ann, accompanied by withdrawal of the sea by 70-90 metres. In Kingston Harbour, waves of 2.5 metres were observed. Both events followed strong to major local earthquakes which displaced land at Port Royal and probably along the north coast suddenly and vertically into the sea. There are no reports that anyone was killed by the tsunamis. The death toll from the earthquakes was 2000 out of a population of 8,000 at Port Royal (1692) and 1,000 in Kingston (1907).”
Other accounts have attributed some of the deaths to the tsunamis.
Fast facts about tsunamis
- Tsunamis are a series of waves generated in the sea by sudden vertical displacements of a column of seawater. They come in all sizes from less than 1cm to tens of metres.
- An earthquake must have a magnitude of 6.5 or higher to trigger a tsunami. In addition, the earthquake must displace the sea floor vertically, either up or down.
- There are four types of tsunamis:
- Teletsunamis are tsunamis originating more than 1000 km from the affected area. Since they originate at a considerable distance there is time for tsunami warning systems to detect the existence of a tsunami and to warn the population at risk. Only two historical teletsunamis are known to have affected the Caribbean – both occurred off the coast of Portugal.
- Landslide tsunamis are generated by landslides are usually but not always triggered by earthquakes. They can have devastating effects locally, but the effects are limited to a small area.
- Volcanic tsunamis can occur in a number of ways including explosions, caldera collapse, and landsliding. These have been observed in the Caribbean from eruptions of Mt Soufriere and Nevis.
- Tectonic tsunamis are produced when one portion of the sea floor moves vertically with respect to an adjacent portion.
- Tsunamis cause damage in a number of ways: the force of the wave can destroy buildings, piers, bridges and other structures. Even relatively small waves can cause strong currents. Damage can also be caused by battering by water carried debris such as logs, boats, automobiles, etc. The retreating waves can scour the support for bridges, piers, breakwaters, etc. and cause failures. Chemical spills and fires caused by ruptured storage tanks are also common. Waves can also travel long distances up rivers as bores.
- The 1947 quake in El Cibao, Dominican Republic, is believed to be the largest recorded earthquake to have occurred in the Caribbean. It reportedly measured magnitude 8.1 and generated a tsunami that caused 75 deaths and rendered 20,000 homeless, with aftershocks extending through 1947 and 1948.
Tsunami facts are taken from A Brief History of Tsunamis in the Caribbean Sea by James F Lander, Lowell S Whiteside and Patricia A Lockridge of the National Geophysical Data Center in Boulder, Colorado.