The word “tsunami” comes from the Japanese language, describing very long, low seismic sea waves. They are are triggered by seismic disturbances - coastal earthquakes, volcanic eruptions, or undersea landslides - that jolt the ocean floor. During this tremor, the crust is likened to a giant paddle, smacking the water at the site of the disturbance. Oftentimes, earth tremors will displace ground surface, sending an entire column of water in motion outward from an earthquake region. The result is a deep wave that reaches from the sea’s surface to the floor and travels horizontally at speeds up to 500 miles per hour and reaches heights between 50 and 100 feet.
The great pulse of water usually passes unnoticed beneath ships at sea. The height from crest to trough is usually only a few feet, and the distance between two crests more than 100 miles. The wave’s speed equals the square root of the product of acceleration and the water depth. In other words, the waves travel faster in deeper water.
When it approaches shore, the wave rises. It is the sharp elevation of the ocean floor near the coast that slows the bottom of the wave while the top keeps moving at the original speed. Vast quantities of water are then piled up into a vertical wall, 100 feet or higher, that finally crashes over the shore with amazing force.
In the open sea, tsunamis are only about a meter high, but when they reach a continental shoreline, they can be taller than a house and weigh millions of tons. Large tsunamis can carry ships quite a distance inland, as well as drown out crops and villages. Seismologist Nicholas Heck wrote of tsunamis, “It is the most spectacular and appalling of all earthquake phenomena.”
The first sign of an approaching tsunami is the sea tide draining away from the shore. The sea bottom is exposed for a large distance out. Then, the ocean water flows back in, higher and faster than before. These changes may be repeated several times before the tsunami itself roars in to land.
A tsunami travels radially outward from its “epicenter” in concentric circles, like pond ripples. Waves will also travel long distances without losing any of its original power. Even if islands or reefs stand in the tsunami’s path, its force is not significantly reduced. Usually the waves don’t dissipate until they strike a continental land mass, and some waves even “bounce,” traveling back and forth across the Pacific Ocean for over a week.
There is a rather unpredictable correlation between land earthquakes and tsunamis. Minor land tremors can trigger destructive waves. Anyone Pacific coast-dwelling citizens at elevations below 50 feet are at risk. However, the United States has sustained a warning system since 1960, with underwater detection devices that would warn of approaching waves.