A tsunami is a series of ocean waves with extremely long wavelengths, initiated by the rapid, massive, vertical displacement of a large volume of water. The term originates from the Japanese words tsu (harbor) and nami (wave), accurately describing their destructive impact upon reaching shorelines. Unlike typical wind-driven waves that only affect the ocean’s surface, a tsunami involves the entire water column, from the seabed to the surface, carrying immense energy across ocean basins. This phenomenon is often mistakenly called a “tidal wave,” but it has no connection to the gravitational pull of the moon or sun that causes tides.
Creation by Seismic Activity
The majority of devastating tsunamis, approximately 80 percent, are generated by sudden movements of the Earth’s crust beneath the ocean floor. This mechanism is most common along subduction zones, which are boundaries where one tectonic plate is forced beneath another. Over long periods, the plates become locked together, slowly building up enormous stress at the interface.
When the built-up stress exceeds the friction holding the plates, a megathrust earthquake occurs, causing the overriding plate to suddenly spring upward and seaward. This abrupt motion is the direct cause of the tsunami, displacing a section of the seafloor hundreds of kilometers long. The vertical displacement of the seafloor acts like a giant paddle, instantly pushing the entire overlying column of water up or pulling it down.
This transfer of energy into the water is extremely efficient and sets the wave train in motion. The resulting wave then propagates outward in all directions from the source area. Only earthquakes involving significant vertical movement, typically along thrust or reverse faults, can generate destructive tsunamis.
In contrast, earthquakes that feature predominantly horizontal motion, known as strike-slip faults, generally do not create tsunamis because they fail to displace the overlying water column vertically. Magnitude is a strong factor; a shallow-focus earthquake usually needs to be at least magnitude 7.0 to produce a potentially destructive tsunami. The most hazardous events are caused by great earthquakes, those exceeding magnitude 8.0, which can rupture a large enough area of the seafloor to generate waves that travel across entire oceans.
Creation by Subaqueous Mass Movements
A second major mechanism for tsunami generation involves the sudden movement of large masses of material beneath the water, known as subaqueous mass movements. These events, often referred to as submarine landslides or slumps, occur when unstable sediment on continental slopes or shelves collapses. The movement is gravity-driven, distinct from the tectonic movement of the Earth’s crust itself.
Submarine landslides can be triggered by an earthquake, volcanic activity, or the gradual buildup of sediment instability. When a massive volume of sediment rapidly slides down a slope, the moving material displaces the water above it. This displacement process converts the kinetic energy of the landslide into wave energy.
The characteristics of the resulting tsunami are primarily determined by the volume, initial acceleration, and velocity of the sliding mass. While these tsunamis tend to be more localized than those caused by megathrust earthquakes, they can be exceptionally destructive near the source. For example, the 1998 Papua New Guinea tsunami is believed to have been generated by a large underwater slump triggered by a nearby earthquake.
The energy from a landslide-generated tsunami dissipates more quickly than seismic tsunamis, meaning they pose a reduced threat to distant coastlines. However, they can produce massive wave run-ups in confined areas, such as the 1958 event in Lituya Bay, Alaska, where a rock fall into the narrow bay created a wave that surged hundreds of meters up the opposite slope.
Creation by Volcanic Events and Cosmic Impacts
Less frequent, but potentially catastrophic, tsunamis can be generated by volcanic activity. A violent submarine volcanic eruption can create an impulsive force that rapidly pushes the water column upward. More commonly, volcanic tsunamis are caused by secondary effects, such as the massive collapse of a volcano’s flank or the rapid entry of large pyroclastic flows into the ocean.
One of the most famous examples is the 1883 eruption of Krakatoa, where the collapse of the volcano’s caldera and the resulting pyroclastic flows displaced enormous amounts of water. The resulting waves can be extremely high and destructive in the immediate vicinity of the volcano.
In the realm of theoretical but global hazards, a large cosmic impact event, such as an asteroid or comet striking the ocean, could generate a colossal tsunami. The sheer kinetic energy of a large extraterrestrial object impacting the water would instantly convert into wave energy, resulting in a displacement of water far exceeding any terrestrial event. While the likelihood of such an event is extremely low, the resulting tsunami would likely be a global phenomenon, affecting coastlines across all ocean basins.