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Understanding the causes of tsunamisThe phenomenon known as a tsunami – when it seems as if all the might of the sea is lined up for an assault on a landmass – and the destructive tragedy for human life and the environment associated with it, often has dominated the news in recent years.
It has put humanity’s collective ability to deal with disasters under considerable pressure.
But what exactly is this phenomenon and what causes it?
A tsunami (pronounced tsoo-nah-mee) is a wave train, or series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the water column.
Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies such as meteorites can generate tsunamis.
They can attack coastlines savagely, causing devastating property damage and loss of life.
Tsunami is a Japanese word with the English translation of “harbour wave”: Represented by two characters, the top character “tsu” means harbour, while the bottom character “nami” means “wave”.
In the past, tsunamis sometimes were 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” implies an earthquake-related generation mechanism, but a tsunami can be caused also by a non-seismic event such as a landslide or meteorite impact.
How tsunamis differ from other water waves
Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterised as shallow-water waves, with long periods and wavelengths.
The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, may have a period of about 10 seconds and a wavelength of 150 metres.
A tsunami, on the other hand, can have a wavelength in excess of 100km and a period in the order of one hour.
As a result of their long wavelengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wavelength becomes very small.
Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s/s) and the water depth. Let us see what this implies: In the Pacific Ocean, where the typical water depth is about 4 000m, a tsunami travels at about 200m/s, or over 700km/h. Because the rate at which a wave loses its energy is inversely related to its wavelength, tsunamis not only propagate at high speeds, they also can travel great, transoceanic distances with limited energy losses.
How do earthquakes generate tsunamis?
Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water.
Tectonic earthquakes are a particular kind which are associated with the Earth’s crustal deformation; when these 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. Plates interact along these boundaries, known as faults.
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.
Landslides, volcanic eruptions, and cosmic collisions
A tsunami can be generated by any disturbance that displaces a large water mass from its equilibrium position.
In the case of earthquake-generated tsunamis, the water column is disturbed by the uplift or subsidence of the sea floor.
Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, also can disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor.
Similarly, a violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami.
Conversely, supermarine 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.
Generally speaking, tsunamis generated from these mechanisms – unlike the Pacific-wide tsunamis caused by some earthquakes – dissipate quickly and rarely affect coastlines distant from the source area.
What happens to a tsunami as it approaches land?
As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it transforms. Since a tsunami travels at a speed that is related to the water depth, 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 metres or more in height near the coast.
When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore.
When a tsunami encounters land
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-propagating 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 potential, stripping beaches of sand that may have taken years to accumulate, and undermining trees and other coastal vegetation.
Capable of inundating, or flooding, hundreds of metres inland past the typical high-water level, 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 – often called a run-up height – of 10, 20, and even 30m.
Information from the website of the Department of Co-operative Governance and Traditional Affairs, for the South African National Disaster Management Centre: http://web.ndmc.gov.za.
