3. Why do tsunamis occur?

The impulses that create tsunamis can originate from landslides, volcanoes and impacts of objects from outer space (such as meteorites, asteroids, and comets), but mostly, submarine earthquakes.

Generation of Tsunamis

Once the event which initiates the tsunami occurs the potential energy that results from pushing water above mean sea level is then transferred to horizontal propagation of the tsunami wave (kinetic energy). The return of the sea level to its normal position generates a series of waves propagating in all directions from the initially deformed area (Figure 1).

Tsunami propagation

The speed at which the tsunami travels depends on the water depth. If the water depth decreases, tsunami speed decreases. In the mid-Pacific, where the water depths reach 4.5 kilometers, tsunami speeds can be more than 900 kilometers per hour. Refraction and diffraction of waves are important to the tsunami propagation.


Earthquakes generated tsunamis occur when the sea floor abruptly deforms and displaces the overlying water from its equilibrium position (Figure 1). However, not all earthquakes generate tsunamis. To generate a tsunami, the fault where the earthquake occurs must be underneath or near the ocean, and cause vertical movement of the sea floor (up to several meters) over a large area (up to a hundred thousand square kilometers). Shallow focus earthquakes (with a depth less than 70 km) along subduction zones (where an oceanic plate slides under a continental plate or another younger oceanic plate) are responsible for most destructive tsunamis.

Figure 1 Initiation of a tsunami due to an earthquake (Source: UNESCO)


Landslides (both submarine and subaerial), which often occur during a large earthquake, can also create a tsunami. During a submarine landslide, the equilibrium sea level is altered by sediment moving along the sea floor.


A violent marine volcanic eruption too can create an impulsive force that displaces the water column and generates a tsunami.


Fortunately it is very rare for a meteorite or an asteroid to reach the Earth. No asteroid or meteorite has been documented to have generated a tsunami in recorded history. However, since evidence of the fall of meteorites and asteroids on Earth exists, some might have landed in oceans and seas, since 80% of the planet is covered by water. Their fall into oceans has the potential to cause tsunamis of cataclysmic proportions.

One of the most destructive tsunamis in recent history was generated along Chile’s coast by an earthquake in May 22, 1960 (Figure 2). Every coastal town between latitudes 36S and 44S was destroyed or severely damaged by the action of the tsunami and the earthquake. In Chile, the double combination of earthquake and tsunami produced more than 2,000 deaths, 3,000 injured, two million homeless, and damage worth $550 million (US). The tsunami caused 61 deaths in Hawaii, 20 in the Philippines, 3 in Okinawa, and more than 100 in Japan. The height of the waves ranged from 13 meters at Pitcairn Islands, 12 meters at Hilo, Hawaii, and 7 meters at several places in Japan, to minor oscillations in other areas.

Figure 2 Area affected by the tsunami of May 22, 1960. (source: http://pubs.usgs.gov)

One of the most interesting landslide generated tsunamis happened in 1958 where about 81 million tones of ice and rock crashed into Lituya Bay, Alaska, Fifuren3). An earthquake had shaken the enormous mass loose. The landslide created a tsunami which sped across the bay. Waves splashed up to an astonishing height of 350 to 500 metres – the highest waves ever recorded. They scrubbed the mountain slope clean of all trees and shrubs. Miraculously, only two fishermen were killed.

Figure 3 Lituya Bay, Alaska (http://www.usc.edu/dept/tsunamis)

In 1883, a series of volcanic eruptions at Krakatau in Indonesia created a powerful tsunami. As it rushed towards the islands of Java and Sumatra, it sank more than 5,000 boats and washed away many small islands. Waves as high as a 12-story buildings wiped out nearly 300 villages and killed more than 36,000 people. Scientist believe that the seismic waves traveled two or three times around the Earth.

Figure 4 Krakatau island evolution during volcanic eruptions (“I invite you to know the earth”, education material prepared by Chilean government and UNESCO-IOC, http://www.ioc-tsunami.org

In 1997, scientists discovered evidence of an asteroid with a four kilometre (2.5 mile) diameter that landed offshore of Chile approximately two million years ago, producing a huge tsunami that swept over portions of South America and Antarctica. Scientists have concluded that the impact of a moderately large asteroid, five to six kilometres (three to four miles) in diameter, falling between the Hawaiian islands and the west coast of North America would produce a tsunami that would attack cities on the west coasts of Canada, the United States and Mexico and would cover most of the inhabited coastal areas of the Hawaiian islands. (“Tsunami Teacher”, Educational material prepared by UNESCO-IOC www.unesco.org)

It has not happened, but conceivably, a tsunami could also be generated by very large nuclear explosions.