The phenomenon we call tsunami is a series of large waves of extremely long wavelength and period usually generated by a violent, impulsive undersea disturbance or activity near the coast or in the ocean.
When a sudden displacement of a large volume of water occurs, or if the sea floor is suddenly raised or dropped by an earthquake, big tsunami waves can be formed by forces of gravity. The waves travel out of the area of origin and can be extremely dangerous and damaging when they reach the shore.
The word tsunami (pronounced tsoo-nah’-mee) is composed of the Japanese words “tsu” (which means harbor) and “nami” (which means “wave”). Often the term, “seismic or tidal sea wave” is used to describe the same phenomenon, however the terms are misleading, because tsunami waves can be generated by other, non seismic disturbances such as volcanic eruptions or underwater landslides, and have physical characteristics different of tidal waves.
The tsunami waves are completely unrelated to the astronomical tides – which are caused by the extraterrestrial, gravitational influences of the moon, sun, and the planets. Thus, the Japanese word “tsunami”, meaning “harbor wave” is the correct, official and all-inclusive term. It has been internationally adopted because it covers all forms of impulsive wave generation.
How do earthquakes generate tsunamis?
By far, the most destructive tsunamis are generated from large, shallow earthquakes with an epicenter or fault line near or on the ocean floor.
These usually occur in regions of the earth characterized by tectonic subduction along tectonic plate boundaries. The high seismicity of such regions is caused by the collision of tectonic plates.
When these plates move past each other, they cause large earthquakes, which tilt, offset, or displace large areas of the ocean floor from a few kilometers to as much as a 1,000 km or more.
The sudden vertical displacements over such large areas, disturb the ocean’s surface, displace water, and generate destructive tsunami waves. The waves can travel great distances from the source region, spreading destruction along their path.
For example, the Great 1960 Chilean tsunami was generated by a magnitude 9.5 earthquake that had a rupture zone of over 1,000 km. Its waves were destructive not only in Chile, but also as far away as Hawaii, Japan and elsewhere in the Pacific. It should be noted that not all earthquakes generate tsunamis.
Usually, it takes an earthquake with a Richter magnitude exceeding 7.5 to produce a destructive tsunami.
How do submarine landslides, rock falls and underwater slumps generate tsunamis?
Less frequently, tsunami waves can be generated from displacements of water resulting from rock falls, icefalls and sudden submarine landslides or slumps. Such events may be caused impulsively from the instability and sudden failure of submarine slopes, which are sometimes triggered by the ground motions of a strong earthquake. For example in the 1980’s, earth moving and construction work of an airport runway along the coast of Southern France, triggered an underwater landslide, which generated destructive tsunami waves in the harbor of Thebes.
Major earthquakes are suspected to cause many underwater landslides, which may contribute significantly to tsunami generation. For example, many scientists believe that the 1998 tsunami , which killed thousands of people and destroyed coastal villages along the northern coast of Papua-New Guinea, was generated by a large underwater slump of sediments, triggered by an earthquake.
In general, the energy of tsunami waves generated from landslides or rock falls is rapidly dissipated as they travel away from the source and across the ocean, or within an enclosed or semi-enclosed body of water – such as a lake or a fjord. However, it should be noted, that the largest tsunami wave ever observed anywhere in the world was caused by a rock fall in Lituya Bay, Alaska on July 9, 1958.
Triggered by an earthquake along the Fairweather fault, an approximately 40 million cubic meter rock fall at the head of the bay generated a wave, which reached the incredible height of 520-meter wave ( 1,720 feet) on the opposite side of the inlet.
A initial huge solitary wave of about 180 meters (600 feet) raced at about 160 kilometers per hour (100 mph) within the bay debarking trees along its path. However, the tsunami’s energy and height diminished rapidly away from the source area and, once in the open ocean, it was hardly recorded by tide gauge stations.
How do volcanic eruptions generate tsunamis?
Although relatively infrequent, violent volcanic eruptions represent also 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.
What are the factors of destruction from tsunamis?
There are three factors of destructions from tsunamis: inundation, wave impact on structures, and erosion. Strong, tsunami-induced currents lead to the erosion of foundations and the collapse of bridges and seawalls. Flotation and drag forces move houses and overturn railroad cars. Considerable damage is caused by the resultant floating debris, including boats and cars that become dangerous projectiles that may crash into buildings, break power lines, and may start fires.
Fires from damaged ships in ports or from ruptured coastal oil storage tanks and refinery facilities, can cause damage greater than that inflicted directly by the tsunami.
Of increasing concern is the potential effect of tsunami draw down, when receding waters uncover cooling water intakes of nuclear power plants.
How does tsunami energy travel across the ocean and how far can tsunamis waves reach?
Once a tsunami has been generated, its energy is distributed throughout the water column, regardless of the oceans depth. A tsunami is made up of a series of very long waves. The waves will travel outward on the surface of the ocean in all directions away from the source area, much like the ripples caused by throwing a rock into a pond.
The wavelength of the tsunami waves and their period will depend on the generating mechanism and the dimensions of the source event. If the tsunami is generated from a large earthquake over a large area, its initial wavelength and period will be greater.
If the tsunami is caused by a local landslide, both its initial wavelength and period will be shorter. The period of the tsunami waves may range from 5 to 90 minutes. The wave crests of a tsunami can be a thousand km long, and from a few to a hundred kilometers or more apart as they travel across the ocean.
On the open ocean, the wavelength of a tsunami may be as much as two hundred kilometers, many times greater than the ocean depth, which is on the order of a few kilometers. In the deep ocean, the height of the tsunami from trough to crest may be only a few centimeters to a meter or more – again depending on the generating source.
Tsunami waves in the deep ocean can travel at high speeds for long periods of time for distances of thousands of kilometers and lose very little energy in the process. The deeper the water, the greater the speed of tsunami waves will be.
Why are locally generated tsunamis so dangerous?
A locally generated tsunami may reach a nearby shore in less than ten minutes. There is not sufficient time for the Pacific Tsunami Warning Center or for local authorities to issue a warning. For people living near the coast, the shaking of the ground is a warning that a tsunami may be imminent.
For tsunamis from more distant sources, however, accurate warnings of when a tsunami might arrive are possible because tsunamis travel at a known speed.
What are the types of tsunami?
There are three types of tsunami which are based on the distance from the source and the tsunami travel time:
Local Tsunami originates from a nearby source for which its destructive effects are confined to coasts within 100 km or less than 1 hour tsunami travel time from its source. These can often be the most dangerous because there is often little warning between the triggering event and the arrival of the tsunami.
Regional Tsunami is capable of creating destruction in a particular geographic region, generally within 1,000 km or 1-3 hours tsunami travel time from its source. Regional tsunamis occasionally have very limited and localized effects outside the region
Distant Tsunami (also called an ocean-wide, distant, tele or far-field tsunami) is a tsunami that originates from a far away source, which is generally more than 1,000 km away from the area of interest or more than 3 hours tsunami travel time from its source. A distant tsunami is capable of causing widespread destruction, not only in the immediate region of its generation, but across an entire ocean. All ocean-wide tsunamis have been generated by major earthquakes in the subduction regions.