A tsunami is a series of powerful ocean waves generated by the significant and rapid displacement of a large volume of water. These waves differ from typical wind-driven waves. Understanding how they form and dramatically increase in height as they approach land is key to their destructive potential. This process involves the initial disturbance of the ocean, the wave’s journey across the deep ocean, and its transformation in shallow coastal waters.
How Tsunami Waves Begin
Tsunami waves primarily begin with large-scale underwater geological events that cause sudden, vertical movement of the seafloor. The most frequent cause is a submarine earthquake, particularly those at thrust faults where one tectonic plate is forced beneath another, leading to an abrupt shift of the ocean floor. This sudden displacement pushes the overlying water column out of its equilibrium, initiating the tsunami.
Other phenomena can also generate tsunamis by rapidly displacing a substantial amount of water. These include large submarine landslides (which can be triggered by earthquakes or occur independently), violent volcanic eruptions beneath the ocean (or significant amounts of volcanic material entering the sea), and less commonly, meteorite impacts in the ocean.
Tsunami Waves in the Deep Ocean
When a tsunami travels across the deep ocean, its characteristics differ from those observed near the coast. In deep water, tsunami waves possess extremely long wavelengths, often spanning hundreds of kilometers. Their wave height is relatively small, typically less than a meter, making them imperceptible to ships overhead.
These waves travel at incredible speeds across vast ocean basins, comparable to a jet airplane, often reaching 800 kilometers per hour (about 500 miles per hour) or more. Their ability to travel such great distances with minimal energy loss is due to their long wavelengths and deep water conditions, allowing them to cross an entire ocean in less than a day.
Why Water Rises Dramatically Near the Coast
The dramatic increase in water height as a tsunami approaches the coast is due to “shoaling.” As the wave moves from the deep ocean into shallower coastal waters, its speed significantly decreases because wave speed relates to water depth. The leading edge slows, but the trailing part continues moving faster, causing the wave to compress.
This compression shortens the wave’s wavelength. Simultaneously, to conserve total energy, the wave’s amplitude, or height, must increase. Imagine a long, fast-moving train entering a crowded station; as the front cars slow, the cars behind them bunch up, and the train becomes much taller. This piling up of water transforms the once-imperceptible wave into a towering surge.
What Influences a Tsunami’s Coastal Impact
Several factors beyond shoaling can modify a tsunami’s impact and the extent of water rise along a coastline. The shape and configuration of the coastline itself play a significant role. Bays, harbors, and inlets can funnel incoming waves, amplifying their height and destructive power as water concentrates into a smaller area.
The underwater topography, or bathymetry, immediately offshore also dictates how a tsunami interacts with the land. A gently sloping seafloor can allow the wave to build higher, while a steeper shoreline might experience a more sudden, forceful impact. Coastal features like natural barriers such as reefs or estuaries can influence wave energy and run-up. Sometimes, before the main wave arrives, the sea may recede unusually far from the shore, exposing the seabed in a phenomenon called “drawback,” a natural warning sign.