Earthquakes represent the sudden shaking of the Earth’s surface caused by the release of energy in the Earth’s crust. Tsunamis are a series of powerful ocean waves generated by the displacement of a large volume of water. While both are natural phenomena, a specific type of earthquake can set in motion the process that forms a tsunami. This article explains how certain seismic events can generate these destructive ocean waves.
Understanding Earthquake Conditions
The Earth’s outer shell consists of large pieces called tectonic plates, which are constantly moving. Most tsunamis are triggered by earthquakes in subduction zones, where one tectonic plate slides beneath another. These zones are responsible for the most powerful earthquakes globally, often exceeding magnitude 8.0. This geological process can cause stress to build up over decades or centuries.
For an earthquake to generate a tsunami, it must primarily cause a large and sudden vertical displacement of the seafloor. Earthquakes causing horizontal movement of the seafloor do not generate tsunamis, as they do not significantly displace the overlying water column. While some research suggests horizontal movement can contribute to tsunami energy, vertical motion is the primary factor. Such tsunamigenic earthquakes have a magnitude of 7.0 or greater, with those above 7.5 producing destructive tsunamis.
The depth of the earthquake’s origin plays a role; shallow earthquakes, occurring less than 100 kilometers (62 miles) below the Earth’s surface, are more likely to generate tsunamis. This is because shallow ruptures lead to more substantial seafloor displacement, directly affecting the water above. Over 80% of the world’s tsunamis originate in the Pacific along its Ring of Fire subduction zones.
From Seafloor Shift to Ocean Wave
When a suitable earthquake occurs, the sudden vertical movement of the seafloor acts like a massive paddle, pushing the entire overlying water column upward or allowing it to subside. This displacement is not merely a surface ripple; it affects the full depth of the ocean, from the seafloor to the surface. The initial disturbance displaces a broad area of the sea surface, potentially over a radius of 100 kilometers or more.
Gravity then pulls this displaced water back down towards its equilibrium position, creating a series of powerful waves. These waves radiate outwards in all directions from the source, similar to ripples expanding from a stone dropped into a pond. In the deep ocean, tsunami waves possess a long wavelength, often hundreds of kilometers from crest to crest. Despite their immense wavelength, their wave height in the deep ocean is small, less than a meter, making them imperceptible to ships.
Tsunami Propagation Across Oceans
Once generated, a tsunami travels across vast ocean basins at high speeds, its velocity depending on the ocean’s depth. In deep ocean waters, tsunamis can move as fast as a jet plane, reaching speeds of over 800 kilometers per hour. For example, in water 6,100 meters (20,000 feet) deep, a tsunami can travel at 890 kilometers per hour. This allows a tsunami to cross an entire ocean basin in less than a day, such as traveling from Chile to Japan in under 24 hours.
Despite their high speed in the open ocean, tsunamis lose little energy as they propagate. This is due to their long wavelength, which allows their energy to be distributed throughout the entire water column, rather than just the surface. Because of their low height and long wavelength in deep water, mariners on ships rarely notice a tsunami passing beneath them. The passage feels like a gentle rise and fall, or is entirely unnoticed, as the ship simply rides over the subtle change in sea level.
Coastal Interaction and Impact
As a tsunami approaches shallower coastal waters, it undergoes a transformation known as shoaling. The wave’s speed decreases due to friction with the shallower seafloor. This reduction in speed causes the trailing part of the wave to catch up with the leading edge, resulting in an increase in wave height. A tsunami that was only a meter high in the deep ocean can grow to several meters or even tens of meters as it nears the coast.
Upon reaching the shore, a tsunami manifests as either a rapidly rising tide, a series of powerful breaking waves, or a bore. The maximum vertical height the water reaches on land above sea level is termed “run-up,” and any run-up exceeding one meter can be dangerous. The horizontal distance the water penetrates inland is called “inundation,” which depends on the run-up height and coastal elevation. Low-lying coastal areas, such as beaches and river deltas, are susceptible to extensive inundation.
Tsunamis consist of a series of waves, often referred to as a wave train, rather than a single wave. The first wave may not be the largest or most destructive, with subsequent waves arriving minutes or even hours later. The local topography of the coastline, including bays, harbors, and the slope of the beach, can influence and amplify the tsunami’s local impact. For example, enclosed bays can funnel the wave, increasing its height and destructive potential.