While the largest, most destructive tsunamis are confined to specific geological boundaries, the fundamental mechanism that creates a tsunami can occur in nearly any body of water. A tsunami is a series of ocean waves caused by the displacement of a large volume of water. This is distinct from regular wind-driven waves, as a tsunami involves the movement of the entire water column, from the surface to the seafloor.
The Essential Ingredients for Tsunami Formation
The creation of a tsunami requires a large, rapid, and vertical disturbance of the water column. The most common source for the largest tsunamis is a sudden shift in the ocean floor’s elevation, typically triggered by powerful, shallow earthquakes beneath the sea. The specific type of tectonic activity is crucial. Earthquakes involving horizontal sliding (strike-slip faults) are generally ineffective. The most productive source is vertical movement, such as the sudden uplift or drop of the seabed along a thrust fault in a subduction zone. When one tectonic plate is forced beneath another and the locked plates suddenly slip, the overlying crust snaps upward, displacing a large volume of water and generating a tsunami.
Once generated, a tsunami’s speed is controlled solely by the depth of the water. In the deep ocean (around 4,000 meters deep), tsunami waves can travel at speeds comparable to a jet airliner, approximately 700 to 800 kilometers per hour. The wave height in the deep sea may be less than a meter, making it imperceptible to ships. This high speed allows the tsunami to cross entire ocean basins in less than a day, carrying its destructive potential thousands of kilometers from its source.
Global Risk Mapping: Geographic Hotspots and Low-Risk Zones
The majority of the world’s large, basin-crossing tsunamis originate in the Pacific Ocean, specifically within the high-risk area known as the Ring of Fire. This region is almost entirely encircled by subduction zones, where the collision of tectonic plates creates the thrust faults necessary for generating powerful seismic tsunamis. Approximately 80% of all recorded tsunamis occur here. The size and depth of the Pacific allow these waves to maintain energy as they propagate toward distant shorelines.
In contrast, the Atlantic Ocean and the Gulf of Mexico are considered low-risk zones because they lack the extensive subduction zones found in the Pacific. The Atlantic’s primary tectonic feature is a mid-ocean ridge, a divergent boundary where plates move away from each other, resulting in less vertical seafloor movement. While the 1755 Lisbon earthquake generated a significant tsunami that reached the Caribbean, such events are rare. Underwater topographic features, such as the Mid-Atlantic Ridge, also help scatter wave energy, protecting the United States East Coast from distant basin-wide tsunamis.
The Gulf of Mexico is even more protected from distant seismic tsunamis due to its enclosed nature and the absence of a nearby subduction zone. While the threat from large, distant earthquakes is minimal, these low-risk areas remain vulnerable to localized tsunamis caused by other mechanisms. The primary hazard for coastal areas along the Atlantic and Gulf coasts comes from potential local submarine landslides. These slides, sometimes triggered by smaller, local earthquakes, can generate a significant tsunami that strikes the coast within minutes.
Tsunamis in Unexpected Places
Tsunamis are not exclusive to large, seismically active oceans, as the principle of water displacement applies everywhere. Landslide-generated tsunamis are a significant, localized hazard that can occur in coastal areas, deep fjords, and large lakes. The 1958 event in Lituya Bay, Alaska, where a rockfall into a narrow inlet created a wave that splashed up 524 meters, demonstrates the power of this non-seismic trigger. Similar events have devastated communities in Norway’s fjords, such as the 1934 Tafjord disaster.
Volcanic activity provides another non-seismic source, often resulting in destructive local waves. The collapse of a volcano’s flank or an underwater eruption can displace enough water to create a tsunami. The 2018 Sunda Strait tsunami in Indonesia was caused by the flank collapse of the Anak Krakatau volcano, a sudden mass movement that killed hundreds of people. Even a volcanic explosion, like the 2022 Tonga event, can displace vast amounts of water and generate waves that travel across the Pacific.
Another form of non-seismic wave is the meteotsunami, driven by rapid changes in atmospheric pressure associated with fast-moving weather systems like severe squalls. These weather-driven waves mimic seismic tsunamis, involving the entire water column and amplifying as they enter shallow bays or harbors. The Great Lakes, particularly Lake Michigan, experience these events frequently, averaging over 100 occurrences a year across the entire system, with destructive examples recorded in Chicago and other coastal towns.