A tsunami is a series of ocean waves generated by the sudden displacement of a massive volume of water, typically caused by an underwater earthquake, landslide, or volcanic eruption. Unlike normal wind-driven waves that only affect the surface, a tsunami involves the entire water column from the seabed to the surface. The visual experience is not a single, giant, cresting surf wave but a rapidly unfolding sequence of events before, during, and after impact.
The Critical Precursors: What the Ocean Looks Like Before Impact
The ocean often provides the first warning sign of an approaching tsunami: a sudden and extreme drop in sea level known as “drawback.” This occurs when the trough, or lowest point of the wave, reaches the shore first, sucking the water away from the coast. The water can recede hundreds of meters, exposing the seabed, reefs, and marine life, resembling an unusually low tide.
This exposed area signals that a massive volume of water is about to rush back in. An auditory warning is also common, often described as a loud, deep roar coming from the ocean just before the wave is visible. This sound is similar to a jet aircraft or a freight train and is caused by air being violently displaced by the turbulent front of the incoming wave.
The Tsunami Wave: Appearance and Behavior
As the immense volume of water approaches shallow coastal areas, the wave begins shoaling: its speed decreases while its height rapidly increases. The wave slows from hundreds of miles per hour in the deep ocean to 30 to 50 miles per hour near the shore. The approaching tsunami does not typically look like a towering, curling wave with a white crest, but rather a rapidly rising tide or a turbulent wall of water.
The most common visual is a “bore,” a vertical wall of water with a steep front that pushes aggressively inland. This bore is often dark, opaque, and dense due to the sediment, sand, and mud scoured from the ocean floor. In industrialized areas, the water can become a “black tsunami,” heavily contaminated with pollutants. The sheer mass of the water, extending to the ocean floor, is the primary destructive force, making the wave appear relentless and solid.
Coastal Inundation: The Reality of the Flooding
Once the tsunami wave reaches the coastline, the visual event transforms into a chaotic, churning flood known as inundation. This is a horizontal surge of water that travels far inland, often covering low-lying areas more than a mile from the original shoreline. The water moves with overwhelming hydrodynamic force, capable of lifting, crushing, and sweeping away massive objects like cars, trees, and entire buildings.
The flow appears as a relentless, murky torrent, carrying a lethal mixture of debris that compounds the destruction. This debris mixes into the water, creating a turbulent, hazardous slurry. The water continues its destructive path inland for several minutes, sometimes for hours, as successive waves may follow the first. The final visual is the retreat of the water, which pulls an equally devastating volume of water and wreckage back out to sea in a powerful, sucking current, leaving behind widespread chaos and structural devastation.