Tsunamis

Tsunamis (sometimes called seismic sea waves) are large sea waves that are created by underwater earthquakes, volcanic eruptions, or even nonseismic events such as landslides and meteorite impacts. They are not tidal waves, which are waves created by the gravitational influence of the sun and moon. The word tsunami is Japanese for “harbor wave.”

Tsunamis are not easily seen on the open water, since they have extremely long wavelengths on the order of tens of kilometers. The speed of the wave is directly related to the depth of the water; therefore, as water depth decreases, the tsunami moves slower. As the waves propagate toward the coast, the speed will decrease, but the amplitude or the height of the waves can achieve extraordinary levels. Tsunamis lose energy as they approach the coast, but they still have incredible amounts of energy as they often cause beach erosion and undermine trees and other types of coastal vegetation.

The fast-moving water is capable of flooding several hundreds of meters inland, well above normal flood levels, and destroying buildings and other structures. Tsunamis can extend to heights well above sea level, sometimes in extreme cases as high as 100 ft. (30 m).

Causes of Tsunamis

Volcanic activity and earthquakes are the prime causes of tsunamis. When the sea floor starts to buckle, the overlying water will begin to displace. As the sea floor rises and sinks, the displaced water will form waves due to the effects of gravity. Most of the major earthquakes occur at plate boundaries. There are three different types of plate boundaries. A divergent boundary takes place where two plates move away from each other. At this type of boundary, volcanoes will form, out of which molten material will flow. Weaker, shallow-focus earthquakes can also occur along these boundaries. 

Divergent boundaries are very common in the mid-ocean, such as the Mid-Atlantic Ridge, the East Pacific Ridge, the Mid-Indian Ridge, and the Southeast Indian Ridge. Convergent boundaries occur where two plates moving in opposite directions collide. One plate is denser and will subduct underneath the other. These subduction zones are a very common location for earthquake activity. There are three different types of convergent boundaries: oceanic–continental, oceanic–oceanic, and continental–continental convergence.

At an oceanic–continental convergent boundary, the oceanic crust is denser and will subduct underneath the continental crust. Volcanoes will form along the continental boundary, while deep trenches will form off the coast. Shallowfocus earthquakes often form along these subduction zones, such as along the west coast of South America. At an oceanic–oceanic convergent boundary, two ocean plates collide, forming a volcanic island arc on the ocean floor. Examples of this type of boundary include the Aleutian Islands, the Mariana Islands, and Japan. At a continental–continental convergent boundary, two continental plates collide, typically forming huge mountain ranges, such as the Himalayas or the Alps. Under this type of convergence, volcanic activity is rare, but earthquake activity is very common. The final type of boundary is called a transform boundary, where two plates slide past each other. Transform boundaries occur along vertical fractures called faults, which are noted for great magnitudes of earthquake activity. Most faults are found near mid-oceanic ridges, but can also extend through continents, as evidenced by the San Andreas Fault in California. Tsunamis can be formed by anything that displaces a large volume of water from its equilibrium state. When earthquakes or volcanoes generate tsunamis, water is displaced due to the uplift, or subsidence, of the sea floor and water column.

Sometimes, submarine landslides, which are common with large earthquakes and volcanic collapses, can displace great volumes of water. However, these types of water disturbances occur from above, rather than from below. Tsunamis derived from these types of mechanisms usually do not last long and have minimal impacts on coastlines. The most recent deadly tsunami struck Tohoku, Japan, on March 11, 2011 (now known as the Great East Japan Earthquake), after a 9.0 magnitude earthquake (on the Richter Scale, devised to estimate the amount of energy released in an earthquake) struck off the east coast of Japan. 

Over 15,000 deaths were reported and several thousand were injured or missing. Waves were reported to be as high as 124 ft. (38 m) and went approximately 6 mi. (10 km) inland. Nearly 250 mi. (400 km) from the epicenter, near Tokyo, waves were approximately 4 ft. (1.3 m) high. The waves from this tsunami propagated across the Pacific and could be felt in countries as far away as Chile (over 10,500 mi., or 17,000 km) with waves about 6.5 ft. (2 m) high. The Early Earthquake Warning system from the Japan Meteorological Agency issued the warning shortly after the major earthquake and saved many lives.

Thousands were left homeless, and other infrastructural damage included the cooling failure of two major nuclear power plants. Another deadly tsunami was the Asian tsunami that occurred after the 2004 Indian Ocean earthquake on December 26. The epicenter (the location at the Earth’s surface directly above the focus of the earthquake) took place off the coast of Sumatra, Indonesia. The magnitude of this earthquake was estimated at between 9.1 and 9.3 on the Richter scale. This made it the fourth most powerful recorded earthquake since 1900. The earthquake lasted almost 10 min. and was the second most powerful earthquake ever recorded on a seismograph (an instrument that measures seismic waves from an earthquake). Waves were reported as high as 100 ft. (30 m) and it was the deadliest earthquake in recorded history, with approximately 230,000 lives lost, mostly in the countries of Indonesia, India, Sri Lanka, and Thailand.

Prior to this event, the 1782 Pacific Ocean tsunami was the deadliest in recorded history, with an estimated 40,000 casualties in the South China Sea. Other powerful tsunamis include the 1883 tsunami after the eruption of Krakatoa, a volcanic island in Indonesia, and the 1908 tsunami that occurred in the Mediterranean Sea near Messina, Italy.

How Does Global Warming Contribute?

There has been speculation about the possible effects of global warming after the 2004 tsunami, with proponents suggesting that the increase in average temperature allows the atmosphere and the oceans to gather energy, which may cause more earthquake activity. Critics, however, claim that if this were the case, there would be more of a correlation between El Niño and tsunamis, since El Niño warms the ocean over an active region with many plate boundaries. Therefore, links to global warming and tsunamis are unsubstantiated because it is difficult to associate what is happening at the surface of the ocean with the depth at which the focus of the earthquakes takes place. However, increased sea levels due to global warming may make coastal areas more susceptible to tsunamis. Over the last 100 years, sea levels have risen about 7 in. (18 cm) per year. However, considering the magnitude of such devastating tsunamis as the 2011 Japanese earthquake and tsunami, which had waves over 124 ft. (38 m), this would only constitute an approximate 0.5 percent increase.