What phenomena generate a Tsunami? Return to index page
Tsunami can be created when sea floor deforms and displaces water from its equilibrium position. The tectonic earthquakes which are linked to a deformation of the earth crustal can generate this water movement. Thus in a subduction zone, where a denser oceanic lithosphere is sinking beneath a continental one (those phenomenen exist for example around the margins of the Pacific Ocean), earthquakes can appear and form waves due to the motion of the water mass. Subduction earthquakes are particularly effective in generating tsunamis.
The mecanism of a earthquake is shown below. The yellow line shows schematically that after the last earthquake the two litospheric plates are stuck. But the oceanic plate which is denser keeps on sliping under the continental plate. When an earthquake occurs, the energy travels outward in all directions from the source. Because the momentum of the waves is so great, a tsunami can cover enomous distances withlittle loss of energy. To set an exemple the 1960 earthquake of the coast of Chile generated a tsunami that had enough force to kill 150 people in Japan after a journey of 22 hours and 10.000 miles.
The mechanism is shown more in details in the first column above on the left. The second column relates how a tsunami is generated. After the last earthquake, the two plates are stuck together, but the oceanic plate (green) keeps on sinking (arrows). The overriding plate gets dragged down with the oceanic plate. As the front of the oceanic plate is flexed down, the coastline rises. Eventually, after a hundred years or so, the accumulated strain (deformation) becomes greater than the rock can bear. The fault ruptures, and the strain is relieved in an earthquake. During the earthquake, the coastline drops, while the nose of the overriding plate (in the deepest water) pushes out across the Pacific. This change in the shape of the sea bottom changes the shape of the sea surface, as shown in the last figure.
Thus, in these conditions a Tsunami is generated:
Wave propgation of a Tsunami.
A Tsunami may be generated by changing the shape of the ocean surface over a large area. A landslide can also do that just as an earthquake can. Instead of an excess lump of water at the surface, a landslide sucks down the water surface, producing a trough.
Gravity does not like that either:
it tries to make the surface horizontal. Water rushes into the void, the
trough rises from a swell, and again oscillations can go on for a very
Each oscillation produces
waves which radiate outwards. It can be several minutes to tens of minutes
from one wave to the next. As with the earthquake, the first sea level
change on land is a fall in water level ( a "leading depression wave"),
while out on the ocean the first change in sea level is a rise ("a
leading elevation wave").
Figures 1 to 7 explain how a tsunami may be created if an asteroid or comet were to land in the sea or oceans . As the bodyimpacts it displaces huge amounts of water which are thrown up in giant tidal waves. During impact the comet/asteroid changes shape, initially becoming flatter, until so much energy is produced that it vaporises. Conversely, marine landslides and cosmic-body impacts disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls.
As you can see in the following image computer can simulate such an impact:
A violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami. In fact the most effective tsunami in recorded history followed the eruption of Indonesia's Krakatoa volcano in 1883; an estimated 36000 people died as a result of the eruption, the majority of them from the tsunami.
You can see in the following image the travel time of a tsunami generated by the Honolulu's volcano