A tsunami is a powerful series of ocean waves, not just a single wave, that can travel across entire ocean basins. They possess significant destructive potential, causing widespread devastation upon reaching coastal areas. Their formation always involves the large-scale displacement of a substantial volume of water.
Formation by Underwater Earthquakes
Most tsunamis originate from large underwater earthquakes, typically those with a magnitude of 7.5 or greater. These powerful seismic events occur primarily in areas where tectonic plates interact, especially at subduction zones. In these regions, one oceanic plate is forced beneath another, leading to immense stress accumulation.
When accumulated stress along a fault line in a subduction zone exceeds rock strength, a sudden rupture occurs. This rupture causes a rapid, vertical displacement of the seafloor. This vertical movement directly lifts or drops the immense column of water above the affected area.
This abrupt vertical shift of the ocean floor then transfers energy from the seismic event into the overlying water. The sudden displacement creates a ripple effect, generating a series of waves that propagate outwards in all directions from the epicenter. This initial disturbance is the birth of a tsunami.
Tsunami Behavior from Deep Ocean to Coast
Once generated, a tsunami begins its journey across the ocean, exhibiting distinct characteristics in deep water. In the open ocean, tsunamis have long wavelengths, often hundreds of kilometers, but a small wave height, less than a meter. Despite their low height, they travel at high speeds, comparable to a jet plane, reaching up to 800 kilometers per hour. This combination makes them largely imperceptible to ships in deep water.
As a tsunami approaches shallower coastal waters, its behavior undergoes a significant transformation due to a phenomenon known as shoaling. The leading edge of the wave interacts with the seafloor, causing it to slow down due to friction. However, trailing waves continue to push forward, causing the wave to compress.
This compression, combined with energy conservation, leads to an increase in wave height. While wavelength decreases and speed reduces, the wave’s energy is conserved, causing its amplitude to build. What was once an imperceptible ripple in the deep ocean can transform into a large wave or a rapidly rising tide as it nears the shore. This process culminates in run-up, where the wave surges inland, inundating coastal areas.
Other Catalysts for Tsunami Generation
While earthquakes are the most common cause, other geological events can also generate tsunamis. Large underwater landslides, where rock or sediment suddenly slide down continental slopes or into deep ocean trenches, can displace water. This rapid displacement can initiate a tsunami, similar to how an earthquake-induced seafloor uplift creates one.
Volcanic eruptions also pose a risk for tsunami generation, particularly those that are explosive or involve the collapse of volcanic islands. An explosive eruption underwater can directly displace water, while the sudden collapse of a volcano’s flank or caldera into the ocean can create a large splash, generating waves. These events can create localized but destructive tsunamis.
Rare, large meteorite impacts into the ocean are theorized to be capable of generating global tsunamis of large scale. While not observed in modern times, geological evidence suggests such events have occurred in Earth’s distant past. Additionally, in specific polar regions, large-scale glacier calving, where chunks of ice break off into water bodies, can create smaller, localized tsunamis.