Tsunamis are among nature’s most destructive phenomena, representing an immense transfer of energy from the Earth’s crust into the ocean. The sheer speed and volume of water involved allow these wave trains to inundate coastal areas with catastrophic force, often with little warning. The Japanese term “tsunami” translates to “harbor wave.” To understand the risk, it is necessary to identify the geological forces capable of generating such powerful waves, particularly the mechanism responsible for the vast majority of these events.
What Defines a Tsunami Wave?
A tsunami is not a normal, wind-driven surface wave but a deep-water wave that affects the entire column of water from the surface to the seafloor. Unlike typical ocean waves, a tsunami’s wavelength can span hundreds of miles in the deep ocean. This long wavelength allows the wave to travel across entire ocean basins with minimal energy loss. In water depths averaging 13,000 feet, a tsunami can reach speeds comparable to a jet airplane, often exceeding 500 miles per hour.
In the open ocean, the wave height may be less than three feet, making it virtually unnoticeable to a ship passing overhead. The dramatic change occurs as the wave approaches the coast and enters shallower water, a process known as shoaling. As the water depth decreases, the wave’s speed diminishes significantly, slowing to the speed of a car. However, the conservation of energy forces the wave height to increase dramatically, resulting in the massive, surging wall of water or rapidly rising tide known as run-up.
The Dominant Cause: Submarine Earthquakes
The vast majority of large and destructive tsunamis are caused by a specific type of submarine earthquake that creates a rapid and massive vertical displacement of the seafloor. This necessary vertical motion is most commonly produced by megathrust earthquakes occurring in subduction zones. Subduction zones are convergent plate boundaries where one tectonic plate is forced beneath another, a process that builds up immense stress over centuries.
The locked section of the overriding plate is slowly dragged downward until the stress overcomes the friction, causing the plate to suddenly snap back into position. This abrupt release of energy involves movement along a thrust fault, a type of fault where the overriding block moves upward relative to the block beneath it. The vertical movement of the seafloor acts like a giant paddle, instantaneously pushing the entire water column above it upward or allowing it to subside. Gravity then pulls this displaced volume of water back to equilibrium, generating the series of waves that radiate outward as a tsunami.
Not all underwater earthquakes are effective at generating tsunamis; the motion must be predominantly vertical. Earthquakes occurring on strike-slip faults, where the movement is primarily horizontal, rarely generate tsunamis of significant size. The lateral sliding of the seafloor does not effectively displace the enormous volume of water required to start a large, transoceanic tsunami. The dominant mechanism remains the immense vertical uplift from a subduction zone thrust fault, which explains why approximately 90% of all major tsunamis originate in the subduction zones encircling the Pacific Ocean, often called the Ring of Fire.
Secondary and Rare Tsunami Triggers
While seismic activity is the primary driver, other geological events are also capable of displacing enough water to generate a tsunami. These secondary causes are generally responsible for more localized, though still devastating, events.
Landslides
Large-scale submarine and coastal landslides are a significant non-seismic trigger, often occurring in conjunction with or immediately following an earthquake. A massive slump of sediment and rock sliding rapidly down a continental slope or into a body of water displaces the water ahead of it, creating a powerful, impulsive wave. A historic example in Lituya Bay, Alaska, involved an earthquake-triggered rockslide that generated a wave that ran up a slope 1,722 feet high.
Volcanic Activity
Volcanic activity can also cause tsunamis through several mechanisms, including the collapse of a volcano’s flank, a violent underwater explosion, or a fast-moving pyroclastic flow entering the sea. The 1883 eruption of Krakatoa in Indonesia involved a catastrophic caldera collapse and pyroclastic flows that generated waves reaching 135 feet, killing tens of thousands of people in the immediate region.
Meteorite Impact
The rarest and most globally destructive cause would be a large meteorite impact into the ocean. This event would create a momentary crater that displaces an immense volume of water, though no such event has been recorded in human history.