A tsunami is a series of powerful ocean waves caused by large-scale disturbances of the sea. While often mistakenly called tidal waves, they are unrelated to tides, which are influenced by the moon and sun. These immense waves can be triggered by various events, including volcanic eruptions or underwater landslides. However, the most frequent cause of destructive tsunamis is a powerful earthquake beneath or near the ocean floor. Not all earthquakes lead to tsunamis, as the relationship between earthquake size and tsunami generation is intricate, involving more than just magnitude.
How Earthquakes Generate Tsunamis
Earthquakes generate tsunamis primarily through vertical displacement of the seafloor. When sections of the Earth’s crust suddenly move upwards or downwards beneath the ocean, they push or pull the entire water column above them. This rapid displacement of a large volume of water initiates the formation of tsunami waves. The water, seeking to regain its equilibrium, oscillates and creates a series of waves that propagate outwards across the ocean.
Unlike typical wind-driven waves that only affect the ocean’s surface, tsunamis involve the movement of the entire water column from the seafloor to the surface. This deep-water involvement gives tsunamis their immense energy and destructive potential. The most effective earthquakes for generating tsunamis occur in subduction zones, where one tectonic plate slides beneath another. In these regions, the sudden release of built-up stress can cause significant vertical shifts in the ocean floor, directly transferring energy to the overlying water.
Critical Factors Beyond Magnitude
While earthquake magnitude is a factor, it is not the sole determinant of a tsunami’s size or whether one will form.
Earthquake Magnitude
A significant earthquake, magnitude 7.0 or higher, is needed to generate a tsunami. For a widespread and dangerous tsunami capable of traveling across an ocean basin, the earthquake needs to exceed magnitude 8.0. Earthquakes below magnitude 6.5 rarely trigger tsunamis; those between 6.5 and 7.5 produce only localized, non-destructive sea level changes.
Fault Movement
The type of fault movement is crucial. Only certain types of fault motion cause the necessary vertical displacement of the seafloor. Thrust faults, also known as reverse faults, are particularly effective because they involve one block of crust moving up and over another. Megathrust earthquakes, which occur in subduction zones, are a prime example of this type of movement, leading to the most powerful tsunami-generating events. In contrast, strike-slip faults, where crustal blocks slide horizontally past each other, do not produce significant tsunamis because they cause minimal vertical water displacement.
Earthquake Depth
The depth of the earthquake’s origin plays a role in tsunami generation. Shallow earthquakes, occurring less than 100 kilometers (62 miles) below the Earth’s surface, are far more effective at displacing the ocean floor and the overlying water column. Even a very large magnitude earthquake originating deep within the Earth is unlikely to generate a significant tsunami because the energy dissipates before reaching the seafloor.
Earthquake Location
The location of the earthquake is essential. A tsunami-generating earthquake must occur under or very near the ocean to directly affect the water column. The amount of water above the rupture zone also influences the resulting tsunami. An earthquake occurring far inland, regardless of its magnitude, will not generate a tsunami.
Global Tsunami Hotspots
Tsunami-generating earthquakes are not randomly distributed across the globe; they are concentrated in specific geographical regions linked to tectonic plate boundaries. The vast majority of tsunamis occur in areas where tectonic plates converge and one slides beneath another, a process known as subduction. These subduction zones are characterized by high seismic activity and are responsible for the most powerful earthquakes and subsequent tsunamis.
The Pacific Ring of Fire is the most prominent tsunami hotspot, accounting for approximately 80% of the world’s tsunamis. This arc of intense seismic and volcanic activity encircles the Pacific Ocean, extending from Chile, up through Central America, Mexico, the west coast of North America, Alaska, the Aleutian Islands, and down through Japan, the Philippines, Indonesia, and New Zealand. Other notable subduction zones prone to tsunamis include parts of the Indian Ocean and the Cascadia Subduction Zone off the Pacific Northwest of North America.