Most of us have dropped a rock into the water and watched waves radiate outward. Tsunami waves are also formed from a disturbance and waves that radiate out from the source. Imagine the amount of energy that was required to start the massive movement of water that became the deadly March 2011 Honshu, Japan tsunami.
A tsunami is a series of ocean waves started by a sudden displacement of ocean water, usually by an earthquake. Like the waves spreading out from a dropped rock, a tsunami's waves rapidly travel in all directions away from the disturbance and can spread across entire ocean basins. For example, the tsunami triggered by the 9.0 earthquake near Japan, swept across the Pacific. Within minutes thirteen meter high waves rushed onto the nearby Japanese coast. In less than twenty hours two meter high waves washed onto South America over 6000 miles away.
Although most tsunamis are generated by earthquakes on converging tectonic plate boundaries, they can also be caused by volcanic eruptions, landslides, undersea slumps, or even meteorites. Unlike ordinary wind-driven waves, out in the open ocean the wave crests of a tsunami can be over 100 miles apart and can travel at 600 mph. When in the deep-ocean, a typical tsunami is barely noticeable, being only a few inches high and posing no real danger to ships.
Only when they approach land do tsunamis become hazardous. The speed at which tsunamis travel depends on the ocean depth. As a tsunami enters shallow water it slows to 20 or 30 mph and becomes compressed, causing it to grow in height to more than 100 feet. In the best of cases, the tsunami comes onshore like a quickly rising tide causing a gentle flooding of low-lying coastal areas. In the worst cases, a wall of turbulent water rushes onshore with tremendous destructive power sweeping away almost anything in its path. Minutes later, as the trough of the wave arrives, the water drains away carrying with it debris and victims and sometimes exposing great patches of the sea floor. This destructive cycle may repeat several times as other waves arrive causing additional damage.
Tsunamis typically are highest and cause the most severe damage and casualties very near their source where there is little time for warning. The largest tsunamis, however, can cause destruction and casualties over a wide area, sometimes across an entire ocean basin. Of the 230,000 people killed by the 2004 Indonesian tsunami, over 55,000 were on distant shores.
Predicting when and where the next tsunami will strike is currently impossible. However, once a tsunami has been generated, forecasting approximate arrival times is now possible. Recently developed and deployed, real-time deep ocean tsunami detectors provide the data necessary to make these forecasts possible, giving coastal communities the tools needed to reduce the impact of future tsunamis.
Students can investigate tsunamis to discover the impacts of Earth's systems on humans. These deadly waves and other natural disasters can be used to bring relevance to science concepts such as plate tectonics, acceleration and speed, force and motion, energy transfer and the physics of waves. In addition, many schools, homes and business are located in tsunami hazard zones. Teaching students about these destructive waves may also save lives by preparing them to be Tsunami Ready.
Page last updated June 2011.