Tsunami is a Japanese word with the English translation, "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 misnomer; 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" means an earthquake-related generation mechanism, but a tsunami can also be caused by a nonseismic event, such as a landslide or meteorite impact. A tsunami is often misnamed a tidal wave, but in fact a tsunami is not just one wave but usually a series of seven or eight, that have nothing to do with the tide. In the open ocean, tsunamis are only about one meter high, but as they approach shallower waters and the shore, they grow to heights as high as eighty-five meters. Every couple of years a tsunami is recorded along the Australian coast and, like the earthquakes, most are so small they are recorded on tide gauges but not observed by people. Since 1883 several of them along the northwest coast have had the potential for damage and as the Australian population expands, the risk of actual damage increases. The causes of previous or potential tsunamis in Australia are a tsunami is not a single wave but a series of waves which may last hours or, as in the case of the tsunami in Sydney Harbour resulting from the 1960 Chilean earthquake, several days. A tsunami can be generated by ANY disturbance that displaces a large water mass from its equilibrium position. Submarine landslides, 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. Gravitational forces then cause the tsunami. Similarly, a violent marine volcanic eruption can create an impulsive force that displaces the water column and generates a tsunami. Above water (subarial) landslides and space born objects can disturb the water from above the surface. Unlike ocean-wide tsunamis caused by some earthquakes, tsunamis generated by non-seismic mechanisms usually go away quickly and rarely affect coastlines far from the source area.
Earthquakes generate tsunamis when the sea floor abruptly deforms and displaces the overlying water from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. The main factor which determines the initial size of a tsunami is the amount of vertical sea floor deformation. This is controlled by the earthquake's magnitude, depth, fault characteristics, and coincident slumping of sediments or secondary faulting. Other features which influence the size of a tsunami along the coast are the shoreline and bathymeteric configuration, the velocity of the sea floor deformation, the water depth near the earthquake source, and the efficiency at which energy is transferred from the earth's crust to the water column. The most common causes of tsunamis are volcanoes, earthquakes and earthslides, which are mostly undersea. Volcanoes that have been erupting continuously for a long time have empty magma chambers. The roof then collapses forming a crater sometimes up to one kilometer in diameter. Water gushes into this crater in a very short amount of time, causing a tsunami. Earthquake caused tsunamis occur when parts of the Earth's crust on either side of a fault move past each other. For a tsunami to occur however, there must be some kind of vertical movement along the fault. This vertical movement must be capable of displacing huge amounts of water, thus causing waves. Tsunamis can also be caused by land sliding in to the sea with such great force that it creates a wave. Similar to the effect of throwing a pebble into a puddle of water. Most tsunamis occur in the Pacific ocean as it is located on a plate mostly made of water. The Pacific ocean is also surrounded by the Ring of Fire, a highly active volcano and earthquake zone. The Ring of Fire circles the ocean from Alaska down to the west coasts of North and South America and up along the east coast of Asia, taking in parts of China, Japan and Russia. Tsunamis can only occur in coastal regions but islands are the main targets. In the open ocean tsunamis are almost unpredictable. However a tsunami warning now operates in the Pacific. This system monitors sea movements and can be used to map the path of tsunamis and estimate the rate of approach. The authorities then notify the public and evacuations are undertaken.