What causes Tsunamis

Tsunamis are usually caused by earthquakes occurring below or near the ocean. Volcanic eruptions, submarine landslides, and coastal rock falls can also generate a tsunami, as can a large asteroid impacting the ocean. Tsunamis originate from a vertical movement of the sea floor with the consequent displacement of water mass.

From the area where the disturbance has occurred and the tsunami has been formed, the waves travel outward in all directions and can propagate across entire ocean basins. As a tsunami wave approaches the shore, it builds in height.  The topography of the coastline and the ocean floor will influence the size of the wave. That is why a small tsunami at one beach can be experienced as a giant wave a few miles away. Remember that a tsunami may be series of waves and successive waves may be larger than the previous one(s).   

 

EARTHQUAKES:

The earth's crust is comprised of a number of plates and the region where two plates come in contact is called a plate boundary. 

map of plates

Earthquakes can be generated by movements along fault zones associated with plate boundaries.  The way in which one plate moves relative to another determines the type of boundary.

  • spreading, where two plates move away from each other;
  • subduction, where two plates move towards each other and one slides beneath the other;
  • transform where two plates slide horizontally past each other.

Fault zones

Lithospheric plates, which cover the entire surface of the earth, and contain both the continents, and the seafloor, move relative to each other at rates of up to ten cm/year. Most strong earthquakes occur in subduction zones where an ocean plate slides under a continental plate or another younger ocean plate.

Regional Context 

All earthquakes do not cause tsunamis. There are generally four conditions necessary for an earthquake to cause a tsunami:

  1. The earthquake must occur beneath the ocean or cause material to slide into the ocean.
  2. The earthquake must be strong, at least magnitude 6.5 on the Richter Scale
  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) over a large area (up to a hundred thousand square meters).

 Sea Surface - Sea Floor

The amount of vertical and horizontal motion of the seafloor and horizontal motion of the sea floor, the area over which it occurs, the simultaneous occurrence of slumping of underwater sediments due to the shaking, and the efficiency with which energy is transferred from earth’s crust to the ocean water are all part of tsunami generation mechanism.  Waves are formed as the large amount of water displaced tries to regain its equilibrium with the help of gravity. 

Tsunami Stages

 

Year – Location

Magnitude

Effects

1690 – Near Antigua

7.5-7.8

Earthquake triggered landslide in Nevis, tsunami waves generated and observed in Nevis and Antigua

1692 – Jamaica

7.5

Sunken land mass in Port Royal set up waves, observed in Kingston Harbour

1842 – Hispaniola

8.3

Tsunami waves of up to 2 m reported on north coast of Hispaniola. 200-300 fatalities

1867 – Virgin Islands

7.5

Earthquake and associated landslide generated tsunami waves 4.5-6m which inundated a number of islands damaging houses

1907 – Jamaica

7.5

North coast earthquake triggered tsunami with recorded waves of 2.5 m

1918 – Puerto Rico

7.5

Earthquake triggered 6.1 m waves, extensive damage to western Puerto Rican coast claiming 29 lives

1946 – Dominican Republic

8.1

Earthquake devastated Dominican Republic, generating a 2.5 m tsunami that impacted NE coast of Hispaniola resulting in at least 1800 victims

2010 – Haiti

7.0

Earthquake SW of Port au Prince generated local tsunami, 4 persons killed

Select Earthquake-induced Tsunamis in the Caribbean and Adjacent Regions

Source: The University of the West Indies, Seismic Research Centre

LANDSLIDES:

Both underwater and on-land landslides into the ocean can result in tsunami formation. In underwater landslides, the earth is moved to lower levels of the ocean basin due to gravity or a tectonic trigger, and this earth movement pushes water in front of it which can cause a tsunami if enough earth is moved at once.  On-land landslides (huge chunks of glaciers, volcanoes, rockfalls, land or other debris which slide into the sea) can cause tsunamis if the land along the coastline is forced into the ocean, creating a displacement of water.  Landslide generated tsunamis are generally local tsunamis since the source of is near the shore.  The warning time may therefore be only a few minutes. 

Landslides

The largest tsunami reported in the Caribbean was possibly the landslide-generated tsunami which impacted the northern coast of Guadeloupe.  The event was triggered by an earthquake in 1867. 

 

VOLCANIC ERUPTIONS: 

Although relatively infrequent, violent volcanic eruptions represent impulsive disturbances, which can displace a great volume of water and generate extremely destructive tsunami waves in the immediate source area. According to this mechanism, waves may be generated by the sudden displacement of water caused by a volcanic explosion, by a volcanos slope failure, or more likely by a phreatomagmatic explosion and collapse/engulfment of the volcanic magmatic chambers.

Volcanic Eruptions
Source: GEOSCIENCE AUSTRALIA

  

 Volcanic eruptions can generate tsunamis

  

Year – Location

Effects

1902 –  Martinique                

Pyroclastic flow from Mt. Pelee entered  sea, generating a local tsunami about 4.5 m in height, approximately 100 fatalities

1939 – Kick ‘em Jenny Grenada        

 

Eruption of submarine volcano generated numerous tsunami waves 2m in height in Northern Grenada, the Grenadines and Barbados

1997 – Soufriere Hills, Montserrat

Debris avalanche into ocean from Soufriere Hills volcanic eruption generated tsunami waves that inundated up to 80m inland, waves also reported in Guadeloupe

Select Tsunamis in the Caribbean and Adjacent Regions induced by Volcanic Activity

Source: Seismic Research Centre, University of the West Indies

 

METEORITES:

Tsunamis caused by extraterrestrial collision (i.e. asteroids, meteors) are an extremely rare occurrence. Although no meteor/asteroid induced tsunami have been recorded in recent history, scientists realize that if these celestial bodies should strike the ocean, a large volume of water would undoubtedly be displaced to cause a tsunami. Scientists have calculated that if a moderately large asteroid, 5-6 km in diameter, should strike the middle of the large ocean basin such as the Atlantic Ocean, it 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. An asteroid 5-6 kilometers in diameter impacting  between the Hawaiian Islands and the West Coast of North America, would produce a tsunami which would wash out the coastal cities on the West coasts of Canada, U.S. and Mexico and would cover most of the inhabited coastal areas of the Hawaiian islands.

Explosions: Underwater explosions of marine vessels such as submarines, as well of explosions of on the surface vessels can cause large displacements of water which can result in tsunamis.

 

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