The powerful, destructive surges of water that crash onto coastlines are often popularly referred to as “tidal waves.” This term is scientifically inaccurate when describing these massive phenomena. The science behind these intense ocean events points to a different origin and mechanism, requiring precise terminology. This article clarifies the distinction between the commonly misused phrase and the actual forces that generate these formidable natural hazards, explaining their formation and coastal impact.
Defining the Terms: Tidal Wave Versus Tsunami
The term “tidal wave” refers to a specific, predictable ocean movement caused by the gravitational influence of celestial bodies. A true tidal wave is a shallow-water wave driven by the gravitational pull exerted by the Moon and the Sun, resulting in the regular, twice-daily rise and fall of sea level known as the tide. True tidal waves cause minor, predictable changes in water level and current, and they are not responsible for catastrophic coastal flooding.
The correct scientific term for a destructive wave sequence caused by a sudden displacement of water is a tsunami, a Japanese word meaning “harbor wave.” Tsunamis are a series of ocean waves with extremely long wavelengths that can span hundreds of kilometers. These waves are unrelated to the gravitational forces that create tides, which is why scientists avoid the older terminology. The sudden movement of the entire water column, from the ocean floor to the surface, defines a wave as a tsunami.
Tsunamis are impulsive events, created by a single, powerful action that displaces an immense volume of water. This contrasts sharply with wind-generated waves or tidal waves, which are continuous and cyclical phenomena. The energy released is distributed across the entire depth of the ocean, allowing the wave to propagate across vast ocean basins with minimal energy loss.
Triggers of Destructive Wave Events
The vast majority of destructive tsunamis are initiated by large-scale seismic activity beneath the ocean floor. The most common cause is a powerful submarine earthquake involving vertical movement of the crust, typically a thrust fault in a subduction zone. This sudden motion of the seafloor acts like a giant paddle, pushing the overlying water column and generating the initial wave disturbance.
For a substantial tsunami to form, the earthquake usually needs to be shallow and exceed a magnitude of 7.0 on the Richter scale. A magnitude of 8.0 or greater is often required to create a dangerous, distant tsunami. The earthquake must occur less than 100 kilometers below the Earth’s surface to effectively displace the seafloor and the water above it. Strike-slip faults, where plates slide horizontally, rarely generate significant tsunamis because they lack the necessary vertical water displacement.
While seismic activity is the primary trigger, other powerful geological events can also initiate tsunamis. Large submarine landslides, often triggered by smaller earthquakes, can displace enormous volumes of water, creating local tsunamis. Volcanic activity, such as the collapse of a caldera or a massive pyroclastic flow entering the sea, can also generate sudden displacement. In rare scenarios, an impact from a large meteor or asteroid striking the ocean could generate a mega-tsunami.
How Tsunamis Interact with Coastal Environments
In the deep open ocean, a tsunami travels at speeds comparable to a jet airliner, often exceeding 800 kilometers per hour (500 mph). Despite this incredible velocity, the wave height in deep water is often less than a meter, making it virtually undetectable by ships at sea. The immense power of the tsunami is hidden in its massive wavelength, which can stretch for hundreds of kilometers between crests.
This characteristic changes dramatically when the tsunami approaches the shore and the water depth decreases, a process known as shoaling. As the leading edge of the wave enters shallow water, friction with the seabed causes its speed to decrease significantly. The trailing portion of the wave continues moving quickly, causing the entire wave structure to compress.
This compression forces the wave’s energy upward, resulting in a rapid and dramatic increase in the wave’s amplitude, or height. When the tsunami finally reaches the coastline, it typically manifests as a rapidly rising tide or a fast-moving, turbulent wall of water that floods the land, rather than a typical breaking wave.
The maximum vertical height the water reaches above the normal sea level on shore is called the run-up, which can be tens of meters high depending on the coastal topography. Tsunamis are a series of waves, and the initial surge is often not the largest; subsequent waves can arrive minutes or even hours later, sometimes with greater destructive force.