Imagine standing on the beach on a sunny day. Suddenly, you notice that the water has become shallow; the beach appears wider because the water has withdrawn. Fish flop helplessly on the sand, and the people on the beach look worried. Something big is about to happen. These are signs that a tsunami is approaching.
What, though, are these phenomena, and where do they come from?
What are Tsunamis?
Tsunamis are giant, often destructive, ocean waves caused by displacement of water by earthquakes or volcanic eruptions on the ocean floor. For this reason, they are also called seismic sea waves. They can also be caused by falling glaciers, landslides, explosions under the sea, and meteorite impacts.
These waves move very quickly through deep ocean waters, often as fast as a jet airplane. It would be difficult to notice a tsunami far out in the ocean, though. Why? Although the wave is traveling rapidly, it does not begin to build in height until it reaches more shallow waters and starts to slow down. At sea, the tsunami usually only raises the ocean level by about 300 milimeters, or 1 foot. When the waves reach land, however, they have been known to grow to heights of 14 meters (46 feet). This change in height and appearance is called wave shoaling. Some are much taller than normal, reaching hundreds of meters, and these are called megatsunamis.
From the shore, the first sign of an incoming tsunami is called drawback. The water can be seen to recede from its normal position, often leaving fish stranded on the exposed seabed. The wave then forms a ridge, the characteristic tall wave most often associated with tsunamis. When the ridge reaches the shore, the water may flow inland for hundreds of meters, destroying or damaging structures in its path. Finally, the wave forms a trough, and the flood waters return to the sea. The span of time from drawback to trough is typically about twelve minutes, and another wave may follow each trough.
Today, most scientists prefer to call these waves tsunamis rather than tidal waves, because these waves have nothing to do with the normal shifting of the tides. Scientists also try to predict when tsunamis will occur so that people can be warned through the media or by warning sirens. In places where tsunamis are common, people are generally well educated that when they see the drawback phase, they need to take immediate shelter on high ground on on the upper floors of nearby buildings.
Landmark Tsunamis throughout History
Dozens of major tsunamis have been recorded throughout history, and nearly fifty took place within the last 100 years. Below are some of the most notable.
- Sicily, circa 6,000 B.C. - The oldest tsunami occurred before people began keeping records. Archaeologists and geologists have found evidence of ancient tsunamis dating back more than 8,000 years. One such event was caused by an avalanche in Sicily, and the wave spread across the Mediterranean Sea, from one end to the other. It is estimated that the wave was taller than a ten story skyscraper.
- Santorini, Greece, circa 1,600 B.C. - The oldest recorded tsunami, however, struck Santorini, Greece around 1600 BCE. This tidal wave was caused by a volcano and damaged several cities.
- Lituya Bay, Alaska - The tallest tsunami ever recorded took place in 1958 in Lituya Bay, Alaska. This tsunami was caused by a magnitude 7.8 earthquake and the resulting landslide of 90 million tons of rock. The wave reached a height of 524 meters (1,719 feet).
- Indian Ocean, 2004 - The deadliest tsunami occurred in the Indian Ocean in 2004. The 51 meter (167 foot) wave made landfall in the Sri Lanka, India, Thailand, the Maldives, Malaysia, Madagascar, Somalia, Kenya, Tanzania, and South Africa. Between 230,000 and 280,000 people lost their lives.
- Japan and the Pacific Rim, 2011 - The widest affected area may have been caused by the Tohuku tsunami in 2011. In addition to widespread damage to Japan and the Pacific Rim, the 40.5 meter (133 ft) wave precipitated a nuclear meltdown at a nuclear power plant in Fukushima, Japan. Radiation from the meltdown and explosions were carried by prevailing winds and have been detected at locations all over the world.