A tsunami is a giant wave hitting the coast. It is usually caused by an earthquake or underwater landslide (also often caused by an earthquake). At sea the wave is hardly noticeable, but when it is slowed down by a shallow area, such as near the coastline, it becomes steeper and can rise to a height of tens of meters, with an enormous destructive force.
If the epicenter of an earthquake is under the ocean or a large lake, the earthquake will move huge amounts of water. Even a small earthquake can also cause an underwater landslide, with similar effects. The tsunami risk depends on in what direction the water is moved. In the worst case, such as when the edge of a tectonic moves up- or downwards along the plate boundary, the water will move more or less in a coordinated direction, causing a huge wave.
Tsunamis can strike the coast minutes or hours after an earthquake, depending on how far away from the coast the epicenter was. Tsunamis may travel as fast as a commercial jet and are often invisible before it is too late, as they grow in height only as they hit the shallower waters near the coast. Moreover, tsunamis can travel very far – for instance the 2004 Indian Ocean tsunami was caused by an earthquake north of Sumatra but also wreaked havoc on the east coast of Africa several hours later.
There has long been a tsunami warning center for the Pacific Ocean and after the 2004 tsunami one was installed for the Indian Ocean as well. Although they are not perfect and false alarms do occur, their warnings should be heeded, as the worst that can happen in case of a false alarm is a ruined day, while a tsunami can easily kill you and your company.
Tsunamis cause damage by two mechanisms: the smashing force of a wall of water travelling at high speed, and the destructive power of a large volume of water draining off the land and carrying a large amount of debris with it, even with waves that do not appear to be large.
While everyday wind waves have a wavelength (from crest to crest) of about 100 metres (330 ft) and a height of roughly 2 metres (6.6 ft), a tsunami in the deep ocean has a much larger wavelength of up to 200 kilometres (120 mi). Such a wave travels at well over 800 kilometres per hour (500 mph), but owing to the enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has an amplitude of only about 1 metre (3.3 ft). This makes tsunamis difficult to detect over deep water, where ships are unable to feel their passage.
The velocity of a tsunami can be calculated by obtaining the square root of the depth of the water in metres multiplied by the acceleration due to gravity (approximated to 10 m/s2). For example, if the Pacific Ocean is considered to have a depth of 5000 metres, the velocity of a tsunami would be the square root of √(5000 × 10) = √50000 = ~224 metres per second (735 feet per second), which equates to a speed of ~806 kilometres per hour or about 500 miles per hour. This is the formula used for calculating the velocity of shallow-water waves. Even the deep ocean is shallow in this sense because a tsunami wave is so long (horizontally from crest to crest) by comparison.
The reason for the Japanese name “harbour wave” is that sometimes a village’s fishermen would sail out, and encounter no unusual waves while out at sea fishing, and come back to land to find their village devastated by a huge wave.
As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its speed decreases below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12 mi) and its amplitude grows enormously – in accord with Green’s law. Since the wave still has the same very long period, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not break, but rather appears like a fast-moving tidal bore. Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with a steep-breaking front.
When the tsunami’s wave peak reaches the shore, the resulting temporary rise in sea level is termed run up. Run up is measured in metres above a reference sea level. A large tsunami may feature multiple waves arriving over a period of hours, with significant time between the wave crests. The first wave to reach the shore may not have the highest run-up.
About 80% of tsunamis occur in the Pacific Ocean, but they are possible wherever there are large bodies of water, including lakes. They are caused by earthquakes, landslides, volcanic explosions, glacier calvings, and bolides.
All waves have a positive and negative peak; that is, a ridge and a trough. In the case of a propagating wave like a tsunami, either may be the first to arrive. If the first part to arrive at the shore is the ridge, a massive breaking wave or sudden flooding will be the first effect noticed on land. However, if the first part to arrive is a trough, a drawback will occur as the shoreline recedes dramatically, exposing normally submerged areas. The drawback can exceed hundreds of metres, and people unaware of the danger sometimes remain near the shore to satisfy their curiosity or to collect fish from the exposed seabed.
A typical wave period for a damaging tsunami is about twelve minutes. Thus, the sea recedes in the drawback phase, with areas well below sea level exposed after three minutes. For the next six minutes, the wave trough builds into a ridge which may flood the coast, and destruction ensues. During the next six minutes, the wave changes from a ridge to a trough, and the flood waters recede in a second drawback. Victims and debris may be swept into the ocean. The process repeats with succeeding waves.
Natural warning signs
Should you find yourself in the unlikely situation of a tsunami where there is no warning infrastructure, there are usually what are referred to as “natural warning signs” – nature itself telling you that a tsunami is coming. If an earthquake occurs right on the coast you may only have a matter of minutes to escape, so don’t hesitate to evacuate. Some noticeable natural warning signs are:
Strange behavior in animals such as restlessness (may or may not occur)
Water receding and exposing ocean floor that is usually under water at all times (this is a dead giveaway that the tsunami is imminent and you have, at best, a couple of minutes to flee from this point on)
Notable shocks of an earthquake off or close to the coast (if you are at the coast and feel an earthquake, don’t wait for any official warnings, but head inland or to higher ground immediately)
Loud or unusual noises coming from the sea
Warnings and predictions
Drawbacks can serve as a brief warning. People who observe drawback (many survivors report an accompanying sucking sound), can survive only if they immediately run for high ground or seek the upper floors of nearby buildings. In 2004, ten-year-old Tilly Smith of Surrey, England, was on Maikhao beach in Phuket, Thailand with her parents and sister, and having learned about tsunamis recently in school, told her family that a tsunami might be imminent. Her parents warned others minutes before the wave arrived, saving dozens of lives. She credited her geography teacher, Andrew Kearney.
In the 2004 Indian Ocean tsunami drawback was not reported on the African coast or any other east-facing coasts that it reached. This was because the wave moved downwards on the eastern side of the fault line and upwards on the western side. The western pulse hit coastal Africa and other western areas.
A tsunami cannot be precisely predicted, even if the magnitude and location of an earthquake is known. Geologists, oceanographers, and seismologists analyse each earthquake and based on many factors may or may not issue a tsunami warning. However, there are some warning signs of an impending tsunami, and automated systems can provide warnings immediately after an earthquake in time to save lives. One of the most successful systems uses bottom pressure sensors, attached to buoys, which constantly monitor the pressure of the overlying water column.
Regions with a high tsunami risk typically use tsunami warning systems to warn the population before the wave reaches land. On the west coast of the United States, which is prone to Pacific Ocean tsunami, warning signs indicate evacuation routes. In Japan, the community is well-educated about earthquakes and tsunamis, and along the Japanese shorelines the tsunami warning signs are reminders of the natural hazards together with a network of warning sirens, typically at the top of the cliff of surroundings hills.
The Pacific Tsunami Warning System is based in Honolulu, Hawaiʻi. It monitors Pacific Ocean seismic activity. A sufficiently large earthquake magnitude and other information triggers a tsunami warning. While the subduction zones around the Pacific are seismically active, not all earthquakes generate a tsunami. Computers assist in analysing the tsunami risk of every earthquake that occurs in the Pacific Ocean and the adjoining land masses.
As a direct result of the Indian Ocean tsunami, a re-appraisal of the tsunami threat for all coastal areas is being undertaken by national governments and the United Nations Disaster Mitigation Committee. A tsunami warning system is being installed in the Indian Ocean.
Computer models can predict tsunami arrival, usually within minutes of the arrival time. Bottom pressure sensors can relay information in real time. Based on these pressure readings and other seismic information and the seafloor’s shape (bathymetry) and coastal topography, the models estimate the amplitude and surge height of the approaching tsunami. All Pacific Rim countries collaborate in the Tsunami Warning System and most regularly practise evacuation and other procedures. In Japan, such preparation is mandatory for government, local authorities, emergency services and the population.
Some zoologists hypothesise that some animal species have an ability to sense subsonic Rayleigh waves from an earthquake or a tsunami. If correct, monitoring their behaviour could provide advance warning of earthquakes, tsunami etc. However, the evidence is controversial and is not widely accepted. There are unsubstantiated claims about the Lisbon quake that some animals escaped to higher ground, while many other animals in the same areas drowned. The phenomenon was also noted by media sources in Sri Lanka in the 2004 Indian Ocean earthquake. It is possible that certain animals (e.g., elephants) may have heard the sounds of the tsunami as it approached the coast. The elephants’ reaction was to move away from the approaching noise. By contrast, some humans went to the shore to investigate and many drowned as a result.
Along the United States west coast, in addition to sirens, warnings are sent on television and radio via the National Weather Service, using the Emergency Alert System.
Forecast of tsunami attack probability
Kunihiko Shimazaki (University of Tokyo), a leading member of the Earthquake Research Committee at The Headquarters for Earthquake Research Promotion in Japan, has mentioned an idea for instituting a system of public education regarding the probability of tsunami risk; such a system was announced by Shimazaki at the Japan National Press Club in May 2011. The forecast would include a detection for environmental risk, including proposed tsunami height, danger areas prone to tsunamis, and overall occurrence probability. The forecast would integrate the scientific knowledge of recent interdisciplinarity with information gathered from the aftermath of the 2011 Tōhoku earthquake and tsunami. Per the announcement, a plan was due to be put in place by 2014; however, reliable forecasting of earthquake and tsunami probability is still unavailable. Shimazaki acknowledged that, given the current literature on the topic, tsunami probability warnings are just as, if not more, difficult to predict than earthquake risk probability.
During a tsunami
During a tsunami warning, follow the escape routes and pay close attention to what the authorities are saying as they are usually (especially in countries with a Pacific coast line) well aware of the nature of a tsunami and have refined their emergency plans with earlier tsunamis. The most important thing during a tsunami is obviously to get to high ground. However, trees and even the roofs of houses are a sub-par alternative to mountains as the enormous force of the tsunami may simply destroy the tree or even the house you are seeking shelter in (or on the top of).
Under no circumstances should you count on your swimming ability against such a powerful current! If the absolute worst happens and you are swept away by the water, try to hold onto a floating piece of debris and await rescue.
Do not go to the coast to sight-see or watch for a tsunami! During the tsunami of March 2011 triggered by the Tōhoku earthquake, a 25-year-old man was swept into the Pacific Ocean near the Klamath River in Northern California. The man and some friends had travelled to the shoreline to take photos of the incoming waves – his friends survived, but the man’s body was never found.
After a tsunami
Be aware that there may be more than one tsunami wave and they can occur up to 24 hours after the initial wave, so remain evacuated until you are told by officials that it is safe to return.
When returning to buildings, be aware that floodwaters may have caused serious damage to infrastructure. Floodwater may also be contaminated with sewage.
Avoid sightseeing and only travel if necessary. If you have made travel arrangements with an airline or otherwise, it is best to contact them for advice.