An earthquake is the shaking of the Earth’s surface resulting from the sudden release of energy in the lithosphere, typically caused by movement along a geological fault. A tsunami, in contrast, is a series of powerful waves in a body of water, most often the ocean, generated by the large-scale displacement of the entire water column. While the vast majority of large earthquakes occur beneath the ocean floor, only a small fraction of them produce a tsunami. The generation of a destructive ocean wave depends on specific geological and mechanical conditions that must be met by the originating seismic event.
The Necessary Condition: Vertical Water Displacement
The fundamental physical requirement for an earthquake to generate a tsunami is the abrupt, vertical displacement of the seafloor. This motion must be significant enough to push up or pull down the massive column of water above it, from the ocean floor all the way to the surface. The water is forced out of its equilibrium position, and gravity immediately acts to pull it back down, setting off a series of waves that travel outward across the ocean basin.
This mechanism is why the everyday shaking from a submarine earthquake is not sufficient to create a tsunami. The seismic waves that cause the ground to shake only generate typical surface waves in the water, which are short-lived and dissipate quickly.
A true tsunami requires the seafloor itself to act as a giant paddle, instantly lifting or dropping the overlying water. Because water is nearly incompressible, the movement of the solid seafloor is directly and efficiently transferred to the water column. The resulting wave involves the full depth of the ocean, giving it the immense energy and long wavelength that allows it to travel thousands of miles without losing much of its destructive power.
Geophysical Cause: Fault Type and Tectonic Setting
The type of fault that ruptures during an earthquake is the primary factor determining whether the necessary vertical displacement will occur. Most large tsunamis originate from earthquakes on thrust faults, which are most commonly found in subduction zones. A subduction zone is a tectonic setting where one massive oceanic plate slides beneath another, often a continental plate.
As the plates converge, they do not slide smoothly; instead, the plates lock together, causing immense strain to build up over decades or centuries. The overriding plate is slowly dragged downward and compressed at its leading edge. When the stress exceeds the fault’s strength, the overriding plate suddenly snaps back into position, rapidly uplifting the seafloor across a large area.
This sudden upward thrust, which can involve a section of seafloor hundreds of kilometers long, is the most effective mechanism for displacing the massive volume of water required to form a destructive tsunami. The rapid release of strain energy directly translates into vertical movement of the crust.
Conversely, earthquakes that occur on strike-slip faults rarely generate tsunamis. In this fault type, the tectonic plates slide past each other predominantly in a horizontal direction, with very little vertical motion. Since the seafloor is not significantly lifted or dropped, the water column remains largely undisturbed. The San Andreas Fault in California, for instance, is a major strike-slip fault, which is why even large earthquakes along its offshore segments are unlikely to produce a major tsunami.
Energy Requirements: Magnitude and Hypocenter Depth
Even with the correct fault type, an earthquake must meet certain quantitative energy requirements to generate a significant tsunami. The magnitude of the earthquake is a direct measure of the energy released and is a clear indicator of tsunami potential. An earthquake must register at least a magnitude 7.0 to create a noticeable tsunami, but destructive, ocean-basin-crossing tsunamis are associated with events of magnitude 8.0 or greater.
This magnitude threshold relates to the size of the fault rupture area. A larger magnitude earthquake ruptures a longer segment of the fault, resulting in a wider area of seafloor displacement. A massive area of uplift is required to affect a large enough volume of the ocean to sustain a powerful wave across great distances.
The hypocenter depth, or the point where the rupture begins beneath the surface, is also a highly important factor. Earthquakes that occur close to the surface are far more efficient at transferring their energy to the seafloor. Shallow earthquakes maximize the vertical displacement of the crust.
In contrast, an earthquake with a deep hypocenter may release enormous energy, but much of that energy dissipates within the Earth’s crust. A very deep earthquake, even one with a high magnitude, is unlikely to generate a destructive tsunami because the distance and intervening rock layers dampen the vertical movement that reaches the ocean floor.