Earthquakes and tsunamis are two of the ocean’s most powerful and destructive natural phenomena, and they are intrinsically linked. An earthquake is the sudden release of energy in the Earth’s lithosphere, generating seismic waves that cause the ground to shake. A tsunami is a series of ocean waves with extremely long wavelengths, often incorrectly called a tidal wave because tides play no role in their formation. The relationship is causal: an earthquake can generate a tsunami, but not all earthquakes possess the characteristics necessary to trigger one.
The Necessary Earthquake Condition
The vast majority of destructive tsunamis originate from earthquakes that occur beneath or very near the ocean floor. The type of crustal movement is the most important differentiator, more so than magnitude or location. Earthquakes caused by horizontal movement, such as those along a strike-slip fault, rarely generate significant water displacement. The key requirement for a tsunami is a massive and sudden vertical shift of the seafloor.
This vertical movement most commonly occurs at subduction zones, where one tectonic plate is forced beneath another. As the overriding plate is dragged downward, immense stress builds up. When the stress exceeds the locking strength, the plate suddenly snaps upward in a thrust fault earthquake. This abrupt, large-scale vertical displacement provides the initial energy pulse to the water above it.
The Mechanism of Water Displacement
When a thrust fault ruptures and the seafloor springs upward, the entire column of water resting on top of the displaced area is lifted or dropped. This transfer of massive amounts of stored tectonic energy into the water’s kinetic energy is the genesis of a tsunami wave. The initial disturbance is a gravitational wave that affects the ocean from the surface all the way down to the seabed, not merely a surface ripple.
The volume of water displaced, which can span an area of the seafloor hundreds of kilometers long, is what makes the resulting wave so powerful. Unlike a wind-generated wave, which only moves the water near the surface, a tsunami wave involves the total mass of the ocean water in the affected area. The resulting wave carries the momentum of the entire water column, allowing it to travel immense distances across ocean basins.
How Tsunami Waves Travel and Intensify
Once formed, the tsunami radiates outward from the source as a train of waves with extremely long wavelengths, sometimes exceeding 500 kilometers. In the deep ocean, where the average depth is around 4,000 meters, the waves can travel at speeds comparable to a jet aircraft, often reaching over 700 kilometers per hour. Despite this incredible speed, the wave’s amplitude, or height, is typically less than one meter, making it virtually unnoticeable to ships at sea.
The speed of a tsunami is directly governed by the water depth. This means that as the wave approaches a coastline and the seafloor begins to rise, the tsunami slows down dramatically. This process, known as shoaling, forces the massive amount of energy contained in the wave to compress.
As the speed decreases, the wavelength shortens, and the energy is channeled upward, causing the wave’s amplitude to grow rapidly. A wave that was barely a ripple in the deep ocean can pile up into a surge of water reaching tens of meters in height near the shore. The destructive force is the massive volume of water and the powerful current that rushes inland, which can persist for many minutes as the entire wave period washes ashore.
Tsunami Generation Without Earthquakes
While the majority of destructive, basin-wide tsunamis are seismically generated, the sudden displacement of a large volume of water can occur through other mechanisms. Any event that causes a massive, instantaneous shift in the ocean floor or water body can initiate a tsunami. These non-seismic sources are typically localized but can still produce devastating waves near their origin.
Submarine or coastal landslides are a notable cause, as the rapid movement of rock and debris into the water displaces the surrounding volume. Large volcanic eruptions can also trigger tsunamis, either through explosive displacement or by the collapse of a volcanic caldera into the ocean. Even extremely rare events, such as a large meteorite impact in the ocean, would produce a colossal tsunami. These non-seismic events follow the same fundamental principle: the sudden, large-scale transfer of potential energy into the water column.