An earthquake is a sudden release of stored energy in the Earth’s crust, generating waves that travel through the planet. When this event occurs beneath the ocean floor, the energy transfer into the water creates a sensory experience vastly different from the violent shaking felt on land. Earthquakes can indeed be “felt” in the ocean, but perception depends on whether a person is on the surface or deep underwater. The water column acts as a filter, altering the seismic energy into either subtle surface movements or intense acoustic signals.
Surface Perception of Seismic Waves
A person on a boat or ship at the ocean’s surface will rarely experience the sharp, violent jolting associated with a land-based tremor. This lack of intense shaking is due to the difference between solid ground and a liquid medium. The water column acts as a natural damper, absorbing and dissipating the high-frequency energy components of the seismic waves. These waves cause the rapid, destructive vibrations on land.
Instead of a sharp jolt, a vessel on the surface feels an unusual, drawn-out rocking motion, especially if the earthquake is distant. This sensation is caused by long-period surface waves, which travel across the ocean floor and slightly perturb the water above. If the earthquake is located directly beneath the vessel, the experience may be more abrupt, like a sudden drop or lift of the water surface. However, the water’s ability to flow prevents the ship from being subjected to the rigid, destructive forces that structures on land must endure.
Acoustic and Pressure Signatures
The most profound way an underwater earthquake is perceived within the water column is through the conversion of seismic energy into sound. As the initial compressional seismic waves (P-waves) encounter the water-rock interface on the seafloor, they couple their energy into the water. This process creates hydroacoustic waves, which seismologists refer to as T-phases, or Tertiary phases.
These acoustic waves travel through the ocean at 1,500 meters per second, which is significantly slower than the P-waves traveling through the solid rock of the crust. This speed difference allows the acoustic waves to be the third arrival detected by monitoring stations, following the faster P and S waves. The conversion of energy from the rock to the water is most efficient where there are steep bathymetric features, such as along seamounts and continental slopes.
For marine life and divers, this energy is perceived as a sudden, intense pressure change or a loud, low-frequency sound. The majority of the acoustic energy in a T-phase is concentrated in the low-frequency range, between 2 and 20 Hertz. This signal can travel vast distances through the deep ocean sound channel, or SOFAR channel, which acts as a waveguide, propagating the acoustic energy over thousands of kilometers. This acoustic signature is the primary way that modern hydrophone networks detect and locate underwater seismic events across ocean basins.
The Role of Seafloor Movement in Water Displacement
Beyond the transmission of acoustic energy, a powerful underwater earthquake causes displacement of the water column. This occurs when the fault rupture involves significant vertical movement of the seafloor, such as during a thrust-fault earthquake in a subduction zone. In these events, one tectonic plate is forced beneath another, and the leading edge of the overriding plate can suddenly slip and spring upward or drop downward.
This instantaneous vertical shift of the crust acts like a colossal paddle, lifting or dropping the water directly above the deformed region. This vertical displacement of the seafloor is the primary mechanism for initiating a long-wavelength gravity wave. A strike-slip or lateral-movement fault, which causes side-to-side motion, does not create the necessary vertical lift to displace enough water. The resulting gravity wave moves outward from the source, representing the large-scale physical consequence of the seafloor deformation.