Earthquakes commonly occur on land, but many also happen beneath the ocean. The experience of an underwater earthquake differs significantly from one on land due to how seismic energy interacts with water. Water, a fluid medium, transmits seismic waves differently than solid ground.
Understanding Underwater Seismic Waves
Seismic waves propagate through the Earth’s crust. These include P-waves (primary waves) and S-waves (secondary waves). P-waves are compressional waves, causing particles to move back and forth in the direction of wave travel, similar to sound waves. They travel through solids, liquids, and gases. S-waves are shear waves, causing particles to move perpendicular to wave propagation. S-waves cannot travel through liquids. When an earthquake occurs beneath the ocean, P-waves transmit energy into the water column as pressure waves. These pressure changes travel through the water. S-waves are blocked by liquid water and do not propagate through the ocean itself. The seafloor’s shaking still generates these waves, but their energy is not directly transferred into the water like P-waves.
Direct Human Sensation in the Ocean
A person in the ocean during an underwater earthquake is unlikely to feel the direct ground shaking experienced on land. Instead, the sensation would likely involve subtle pressure changes or low-frequency sounds. P-waves, as pressure waves, cause these changes in water pressure, which might be felt by divers or those submerged. Some divers have reported feeling intense vibrations, pressure changes, or a loud, roaring sound during underwater earthquakes. This auditory or pressure-based sensation is distinct from the violent shaking one might feel on solid ground. While the seafloor might be heaving and sand stirred up, a person floating in the water would largely be insulated from the ground’s direct physical motion. The extent of what is felt depends on the earthquake’s magnitude, proximity, and water depth.
Tsunamis: A Powerful Consequence
Tsunamis are not typical ocean waves caused by wind or tides; they are powerful ocean waves resulting from the displacement of a large volume of water. This displacement is most commonly triggered by underwater earthquakes, particularly those causing significant vertical movement of the seafloor. Earthquakes strong enough to generate destructive tsunamis typically have magnitudes exceeding 7.0, and often occur in subduction zones where one tectonic plate is forced beneath another. Such events cause the seafloor to suddenly rise or fall, displacing the overlying water. In the deep ocean, tsunamis possess a very long wavelength (hundreds of kilometers) but a relatively small wave height (less than a meter). They travel at immense speeds, comparable to a jet plane (over 800 kilometers per hour), making them barely noticeable to ships in open water. As a tsunami approaches shallower coastal waters, its speed decreases, but its wave height dramatically increases due to shoaling, where the wave’s energy is compressed. This transformation can result in towering waves, causing widespread destruction upon reaching land.
Scientific Detection of Ocean Earthquakes
Scientists employ various methods to detect and monitor underwater earthquakes. Seismographs on land record seismic waves, but specialized instruments are needed for direct seafloor measurement. Ocean Bottom Seismometers (OBS) are placed on the ocean floor to record seismic data, including ground motion and acoustic events. These instruments withstand immense deep-sea pressures and record data for extended periods. The Deep-ocean Assessment and Reporting of Tsunamis (DART) buoy network plays a significant role in early warning systems. DART stations consist of a seafloor bottom pressure recorder that detects changes in water pressure caused by tsunamis and a surface buoy that transmits this data via satellite to warning centers. This system allows for real-time detection and reporting of tsunami waves in the open ocean, providing valuable time for coastal communities to prepare. Emerging technologies, such as using existing submarine fiber optic cables as seismic sensors, show promise for expanding underwater earthquake detection capabilities.