An earthquake is the sudden shaking of the Earth’s surface resulting from a rapid release of energy in the lithosphere that generates seismic waves. A tsunami is a series of exceptionally long waves propagating across the ocean, typically caused by a massive, sudden disturbance of the sea floor. While these two phenomena are often linked, the majority of earthquakes do not create tsunamis. Specific geological conditions and physical mechanisms are required for an underwater earthquake to generate a destructive tsunami wave.
The Critical Link: Subduction Zones and Thrust Faults
Most earthquakes, such as those occurring far inland or those involving purely horizontal movement, do not generate tsunamis. Tsunami generation requires an underwater fault that causes a large-scale vertical displacement of the seafloor. The most destructive tsunamis originate from mega-thrust earthquakes within subduction zones, where one tectonic plate is forced beneath another.
At a subduction boundary, the two plates become locked together due to friction, even as the rest of the plates continue to move. This locking causes stress to build up over decades or centuries, deforming the overriding plate like a compressed spring. When the stress exceeds the fault’s strength, the overriding plate suddenly snaps upward and seaward, a process governed by the elastic-rebound theory.
This sudden movement occurs on a reverse or thrust fault. Earthquakes on these faults, typically with a magnitude greater than 7.5 and occurring at a shallow depth near the ocean floor, are the most effective at generating tsunamis. In contrast, strike-slip faults, where movement is predominantly horizontal, rarely displace enough water to create a tsunami.
The Tsunami Generation Mechanism
The sudden vertical movement of the seafloor acts like a giant paddle, pushing the entire water column above it. This vertical displacement is the direct cause of the tsunami. The entire column of water, from the seabed to the surface, is disturbed simultaneously, unlike wind waves which only affect the surface layers.
The massive volume of displaced water is immediately acted upon by gravity, which attempts to pull the water back toward a state of equilibrium. This gravitational restoration initiates the series of waves known as the tsunami, which are classified as long gravity waves. The initial disturbance can involve a sudden uplift of the seafloor, pushing the water up, or a sudden drop, causing the water to subside and create a trough.
The resulting wave in the deep ocean is often barely noticeable, but it possesses an enormous wavelength that can span tens to hundreds of kilometers. This vast wavelength and the immense volume of water involved allow the tsunami to carry energy across entire ocean basins with very little loss. The energy transfer from the seismic event into the water column is what gives the tsunami its destructive potential, rather than the height of the initial disturbance.
From Deep Ocean to Coastline
Once generated, the tsunami wave propagates across the deep ocean at speeds comparable to a jet airliner. The speed of the wave is directly related to the water depth; in the deep ocean, the wave travels very fast because there is little friction with the seabed. As the tsunami approaches the continental shelf and shallower coastal waters, a process known as shoaling begins.
The shallower water increases friction with the seabed, causing the leading edge of the wave to slow down. The trailing waves are often still moving rapidly in deeper water, causing the wavelength to decrease and the waves to compress. This compression forces the massive volume of water to pile up, converting the wave’s kinetic energy into potential energy, resulting in an increase in wave height.
The wave that was a low, fast-moving swell in the deep ocean transforms into a high surge or a fast-moving tidal bore near the coast. The final height the water reaches as it rushes onto the land is known as the run-up, which can be magnified by the coastal bathymetry and the shape of the shoreline. The destructive power at the coast comes not just from the height of the wave, but from the strong currents and the sheer volume of water that continues to flow inland for many minutes.