A tsunami is a wave train, or series of waves, generated in a body of water. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis. These waves can savagely attack coastlines, causing devastating property damage and loss of life.
In English, the word tsunami means “harbor wave.” Represented by two characters, the top character, “tsu,” means harbor, while the bottom character, “nami,” means “wave.” In the past, tsunamis were sometimes referred to as “tidal waves” by the general public, and as “seismic sea waves” by the scientific community. The term “tidal wave” is a deceiving term; although a tsunami’s impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term “seismic sea wave” is also misleading. “Seismic” implies an earthquake-related generation mechanism, but a tsunami can also be caused by a non-seismic event, such as a landslide or meteorite impact.
When the sea floor abruptly deforms and vertically displaces the overlying water, Tsunamis are formed. When tectonic earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created. Large vertical movements of the earth’s crust can occur at plate boundaries. Around the margins of the Pacific Ocean, for example, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis.
As a tsunami is forming, it leaves the deep water of the open ocean and travels into the shallower water near the coast, it transforms. It travels at a speed that is related to the water depth. So, as the water depth decreases, the tsunami slows. The tsunami’s energy flux, which is dependent on both its wave speed and wave height, remains nearly constant. Consequently, as the tsunami’s speed diminishes as it travels into shallower water, its height grows. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be several meters or more in height near the coast. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide or a series of breaking waves.
Yet, as a tsunami approaches shore, it begins to slow and grow in height. Just like other water waves, tsunamis begin to lose energy as they rush onshore – part of the wave energy is reflected offshore, while the shoreward-moving wave energy is dissipated through bottom friction and turbulence. Despite these losses, tsunamis still reach the coast with tremendous amounts of energy. Tsunamis have great erosional effects, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. The fast-moving water associated with the inundating tsunami can crush homes and other coastal structures. Tsunamis may reach a maximum vertical height onshore above sea level of 10, 20, and even 30 meters. Leaving mass destruction behind them Tsunamis are a very real and very dangerous natural occurrence.