An underwater earthquake, originating beneath the seafloor, presents a vastly different experience than a quake felt on land. The familiar violent shaking associated with surface earthquakes is fundamentally altered by the ocean water column. How a person experiences this event—whether on a boat or submerged—depends on the properties of the water and how it interacts with the energy released from the seabed. Seismic wave propagation through liquid dictates that the direct impact of the initial rupture is often muted, but the water’s response can lead to a far greater hazard.
The Transmission of Seismic Waves Through Water
Earthquakes generate two primary types of body waves: Primary (P-waves) and Secondary (S-waves). P-waves are compressional waves that move through a medium by pushing and pulling the material in the direction of travel, similar to sound waves. Since all states of matter resist compression, P-waves are effectively transmitted through solids, liquids, and gases. In water, P-waves travel at approximately 1,500 meters per second, manifesting as rapid changes in pressure rather than physical shaking.
Conversely, S-waves are shear waves that move the medium perpendicular to the direction of propagation. This shearing motion requires a rigid medium, meaning S-waves can only travel through solids like rock and cannot pass through water. This explains why the intense, side-to-side shaking characteristic of S-waves on land is completely absent in the ocean environment. The water column effectively filters out the most destructive form of direct seismic energy.
Experiencing the Earthquake Aboard a Vessel
A ship on the ocean surface is largely isolated from the direct seismic energy originating from the seafloor. The vessel floats on a cushion of water, which acts as a mechanical buffer, decoupling the hull from the seabed’s intense movement. In deep water, most ships will not notice an earthquake at all, even a significant one, because the water depth attenuates the faint ground motion.
If the ship is very close to the epicenter of a strong, shallow quake, the crew may perceive a phenomenon known as a “seaquake.” This is felt as a slight, rapid vertical jolt or a sudden thump against the hull, caused by the initial arrival of the P-waves. In rare cases, the acoustic energy can be so intense that the vessel is momentarily rocked, leading some to mistakenly believe they have struck a shoal.
Sensing the Event While Submerged
For a person submerged, such as a diver, the sensation of an earthquake is markedly different from the experience on a vessel. Since P-waves propagate easily through liquid, the most distinct and immediate sign of an underwater earthquake is auditory. The seismic energy is transferred into the water as sound, which can be heard as a loud, low-frequency rumble or bang.
The body is fully immersed in the medium transmitting the P-waves, leading to the sensation of rapid pressure changes. A diver may feel these compressional waves directly against their eardrums and body. Although the water moves in response to seabed displacement, a free-floating diver would likely perceive this motion as a subtle push or current rather than the physical shaking felt on land.
The Significant Result: Tsunami Generation
While the direct feeling of the earthquake is minimal, the most profound consequence of a large underwater quake is the generation of a tsunami. This destructive wave is caused not by shaking, but by the sudden, large-scale vertical displacement of the seafloor. This displacement, often occurring in subduction zones, transfers immense energy to the entire overlying water column.
The water column is lifted or dropped out of its equilibrium position, and gravity immediately attempts to restore the balance, initiating a series of long-wavelength waves. In the open ocean, a tsunami has a very long wavelength and a low amplitude, often less than one meter in height, allowing it to pass completely unnoticed beneath a ship. As the wave approaches the coast and enters shallow water, friction with the seabed causes the wave’s speed to decrease and its height to increase dramatically, resulting in its destructive power.