What is tsunami?
Tsunami is a Japanese word with the English translation, “harbor wave.” The phenomenon we call tsunami is a series of large waves of extremely long wavelength and period usually generated by a violent, impulsive undersea disturbance or activity near the coast or in the ocean. When a sudden displacement of a large volume of water occurs, or if the sea floor is suddenly raised or dropped by an earthquake, big tsunami waves can be formed by forces of gravity. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis. Tsunamis can savagely attack coastlines, causing devastating property damage and loss of life.
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 of earthquakes that 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.
What happens when a tsunami encounters land?
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 meters 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 30 meters.
Speed of tsunami
Tsunami wave can travel at the speed of a commercial jet plane, over 800 km/h. They can move from one side of the Pacific Ocean in less than a day. The waves can be extremely dangerous and damaging when they reach the shore.
What should you do?
- If you are at home and hear there is a tsunami warning, you should make sure your entire family is aware of the tsunami. Your family should evacuate your house if you live in a tsunami evacuation zone.
- If you are at the beach or near the ocean and you feel the earth shake, move immediately to higher ground. Do not wait for a tsunami warning to be announced.
- If you are on a ship or boat, do not return to port if you are at sea and a tsunami warning has been issued for your area. Tsunami can cause rapid changes in water level and unpredictable dangerous current in harbors and ports.
|Local tsunami||A tsunami from a nearby source for which its destructive effects are confined to coasts within 100 km of the source. A local tsunami is usually generated by an earthquake, but can also be caused by a landslide or a pyroclastic flow from a volcanic eruption.|
|Distant / teleseismic tsunami||A tsunami originating from a far away source, generally more than 1,000 km away. Less frequent, but more hazardous than regional tsunamis. Usually starting as a local tsunami that causes extensive destruction near the source, these waves continue to travel across an entire ocean basin with sufficient energy to cause additional casualties and destruction on shores more than 1,000 kilometers from the source.|
Tsunami Safety Rules
- Not all earthquakes cause tsunamis but many do. When you hear that an earthquake has occurred, stand by for a tsunami emergency message.
- An earthquake in your area is a natural tsunami warning. Do not stay in low-lying coastal area after a strong earthquake has been felt.
- A tsunami is not a single wave but a series of waves. Stay out of danger areas until an “all-clear” is issued by a competent authority.
- Approaching tsunamis are sometimes preceded by a noticeable rise or fall of coastal water. This is nature’s tsunami warning and should be heeded.
- A small tsunami at one point on the shore can be few miles away. Don’t let the modest size of one make you lose respect for all.
- All warnings to the public must be taken very seriously, even if some are for non-destructive events. The tsunami of May, 1960 killed 61 people in Hilo, Hawaii because some thought it was just another false alarm.
- All tsunamis like typhoons are potentially dangerous though they may not damage every coastline they strike.
- Never go down to the shore to watch for a tsunami. When you can see the wave you are too close to escape it. Never try to surf a tsunami; most tsunamis are like a flash flood full of debris and they do not curl or break like surfing waves.
- During a tsunami local civil defence, police and other emergency organizations will try to save your life. Give them your fullest cooperation.