How Tsunamis Are Generated
Most of the tsunami are generated by earthquakes that cause displacement of the seafloor, but, as we shall see, tsunami can be generated by volcanic eruptions, landslides, underwater explosions, and meteorite impacts.
- Earthquakes: Earthquakes cause tsunami by causing a disturbance of the seafloor. Thus, earthquakes that occur along coastlines or anywhere beneath the oceans can generate tsunami. The size of the tsunami is usually related to the size of the earthquake, with larger tsunami generated by larger earthquakes. But the sense of displacement is also important. Tsunami is generally only formed when an earthquake causes vertical displacement of the seafloor. Because of this, most tsunami are generated by earthquakes that occur along the subduction boundaries of plates, along the oceanic trenches. Since the Pacific Ocean is surrounded by plate boundaries of this type, earthquakes around the margins of the Pacific Ocean frequently generate tsunamis.
- Volcanic Eruptions: Volcanoes that occur along coastal waters, like in Japan and island arcs throughout the world, can cause several effects that might generate a tsunami. Explosive eruptions can rapidly emplace pyroclastic flows into the water; landslides and debris avalanches produced by eruptions can rapidly move into water, and collapse of volcanoes to form calderas can suddenly displace the water.
- Landslides: Landslides moving into oceans, bays, or lakes can also generate tsunami. Earthquakes or volcanic eruptions generate most such landslides.
- Underwater Explosions: Nuclear testing by the United States in the Marshall Islands in the 1940s and 1950s generated tsunami.
- Meteorite Impacts: While no historic examples of meteorite impacts are known to have produced a tsunami, the apparent impact of a meteorite at the end of the Cretaceous Period, about 65 million years ago near the tip of what is now the Yucatan Peninsula of Mexico, produced tsunami that left deposits all along the Gulf coast of Mexico and the United States.
Mitigation Of Risks And Hazards
The main damage from tsunami comes from the destructive nature of the waves themselves. Secondary effects include the debris acting as projectiles which then run into other objects, erosion that can undermine the foundations of structures built along coastlines, and fires that result from disruption of gas and electrical lines. Tertiary effects include loss of crops and water and electrical systems, which can lead to famine and disease.
Prediction And Early Warning
For areas located at great distances from earthquakes that could potentially generate a tsunami there is usually plenty of time for warnings to be sent and coastal areas evacuated, even though tsunami travel at high velocities across the oceans. Hawaii is a good example of an area located far from most of the sources of tsunami, where early warning is possible and has saved lives. For earthquakes occurring anywhere on the subduction margins of the Pacific Ocean there is a minimum of 4 hours of warning before a tsunami would strike any of the Hawaiian Islands. The National Oceanic and Atmospheric Administration (NOAA) has set up a Pacific warning system for areas in the Pacific Ocean, called the Pacific Tsunami Warning Center.