A tsunami is a series of powerful waves generated by the sudden displacement of a large volume of water, typically involving the entire water column from the seafloor to the surface. While many people associate these phenomena with massive earthquakes, the underlying physics reveals surprising characteristics. These long-wavelength waves travel across entire ocean basins, often unnoticed, only transforming into devastating surges upon reaching coastlines.
Unexpected Origins Beyond Earthquakes
While the majority of large tsunamis are triggered by significant underwater earthquakes, any event that rapidly moves a massive amount of water can create one. Non-seismic origins often produce highly localized but destructive waves. Underwater landslides, or “slumps,” are a threat because they can occur without a preceding earthquake and displace water close to the coast. Volcanic activity also generates tsunamis, particularly through the collapse of a volcano’s flank or caldera into the ocean, such as the 1883 eruption of Krakatoa. The rapid entry of debris or pyroclastic flows into the water acts as a powerful displacement mechanism.
The Illusion of Speed in Deep Water
A counter-intuitive fact about tsunamis is their speed in the deep ocean, where they are almost impossible to detect. Unlike wind-driven surface waves, a tsunami is a shallow-water wave, meaning its speed is governed by the water’s depth. In the deep ocean, where depths can exceed 20,000 feet, a tsunami can travel at speeds approaching 500 to 550 miles per hour. At these high speeds, the wave’s height is typically less than one meter, appearing as a subtle, long swell that passes harmlessly beneath ships. As the wave approaches shallower waters, the process known as “shoaling” begins. The decrease in water depth causes the wave to slow dramatically, often reaching 20 to 30 miles per hour near the coast. This reduction in speed forces the wave’s energy into a smaller volume, causing the wave height to increase dramatically.
Record-Breaking Scale and Height
The destructive power of a tsunami is measured by its run-up—the maximum vertical height the water reaches above sea level on the shore or coastal slope. The largest recorded run-up in modern history occurred in Lituya Bay, Alaska, on July 9, 1958. An earthquake triggered a massive rockslide into the narrow fjord, creating a wave that surged up the opposite mountainside. The water scoured trees and soil to an elevation of 1,720 feet (524 meters), marking the highest run-up ever documented. Other events demonstrate the global reach of these waves. For example, the 2004 Indian Ocean tsunami traveled across multiple oceans, impacting coasts in 17 countries thousands of miles from its origin.
Common Misconceptions
The term “tidal wave” is inaccurate, as tsunamis have no connection to the astronomical tides caused by the gravitational pull of the Moon and Sun. Tsunamis are purely the result of rapid water displacement. Another common misunderstanding is that a tsunami is a single, towering wall of water. In reality, a tsunami is a “wave train,” a series of waves separated by long periods that can range from minutes to hours. The first wave to reach the shore is often not the largest, and subsequent waves may be significantly more powerful. The most important natural warning sign is “drawback,” where the ocean water recedes dramatically, exposing the seabed far beyond the normal low tide mark. This recession occurs when the trough of the wave train arrives first, signaling that the massive crest of the wave is approaching and immediate movement to higher ground is necessary.