How a tsunami works
Tsunamis form through a complex, multi-stage process that emanates from the massive energy release of a submarine earthquake, underwater or coastal landslide, or volcanic eruption.
The first stage in this formation begins when the tectonic upthrust caused by the quake or impulse event causes massive amounts of ocean water to be displaced almost instantaneously. This action kick-starts a simple series of progressive and oscillatory waves that travel out from the event’s epicentre in ever-widening circles throughout the deep ocean. Due to severe levels of energy propagated from the impulse, the waves build in speed very quickly, reaching up to an incredible 500mph. However, due to the depth of water, the speed of the waves is not visible as they expand to have incredibly long wavelengths that can stretch between 60-120 miles. Because of this, the wave amplitudes (the wave height) are also very small as the wave is extremely spread out, only typically measuring 30-60 centimetres. These long periods between wave crests – coupled with their very low amplitude – also mean that they are particularly difficult to detect when out at sea.
Once generated, the tsunami’s waves then continue to build in speed and force before finally approaching a landmass. Here the depth of the ocean slowly begins to reduce as the land begins to slope up towards the coastline. This sloping of the seabed acts as a braking mechanism for the high-velocity tsunami waves, reducing their speed through colossal friction between the water and the rising earth. This dramatic reduction in speed – which typically takes the velocity of the tsunami to 1/10th of its original speed – also has the effect of reducing the length of its waves, bunching them up and increasing their amplitude significantly. Indeed, at this point coastal waters can be forced to raise as much as 30 metres above normal sea level in little over ten minutes.
Following this rise in sea level above the continental shelf (a shallow submarine terrace of continental crust that forms at the edge of a continental landmass) the oscillatory motions carried by the tsunami are transferred into its waters, being compressed in the process. These oscillations under the pressure of the approaching water are then forced forwards towards the coast, causing a series of low level but incredibly fast run-ups of sea water, capable of propelling and dragging cars, trees, buildings and people over great distances. In fact, these run-ups are often responsible for a large proportion of the tsunami’s damage, not the giant following waves. Regardless, however, following the run-ups the tsunami’s high-amplitude waves continue to slow and bunch into fewer and fewer megawaves before breaking at heights between five and ten metres over the immediate coastline, causing great damage and finally releasing its stored energy.
Cause and effect
Tsunamis initiate when an earthquake causes the seabed to rupture (bottom centre), which leads to a rapid decrease in sea surface height directly above it.
As the tsunami reaches the shore the shallow, long and exceedingly fast waves pile up, reducing the wavelength and increasing their height dramatically.
Due to the severe hazards that tsunamis pose, research into their causes and tracking of their formation has increased through the 20th and 21st Centuries. Currently, the world’s oceans are monitored by various tsunami detection and prevention centres, such as the NOAA (National Oceanic and Atmospheric Administration) run Pacific Tsunami Warning Center (PTWC) based in Honolulu, Hawaii.
Set up back in 1949, the PTWC utilises a series of tsunami monitoring systems that delivers seismic and oceanographic data to it on a daily basis, with information transferred to it and other stations by satellite connection. This is one of two American-run centres that monitor the Pacific Ocean and it is responsible for detecting and predicting the size and target of any approaching tsunamis.
Tsunami prevention has also seen advances as construction techniques and materials have developed over the past century. Now areas that are prone to tsunamis, such as Japan’s west coast, are fitted with large-scale sea walls, artificial deep-sea barriers, emergency raised evacuation platforms and integrated electronic warning signs and klaxons in coastal resorts and ports.
Areas that have been affected by tsunamis in the past are also fitted with physical warning signs and have specific evacuation routes that best allow for large numbers of people to quickly move inland. Unfortunately, however, despite many advances being made to ensure prone areas are protected and warned in advance, due to the transcontinental nature of generated tsunamis, remote or under-developed areas are still affected regularly, the consequences of which have been recently shown in the disastrous 2004 tsunami in the Indian Ocean that claimed over 200,000 lives.