Tsunamis range from barely noticeable ripples to waves that reshape entire coastlines, and the upper limit depends heavily on what triggers them. Ordinary earthquake-generated tsunamis along populated coasts typically reach heights of 10 to 30 meters, but landslide-driven waves have been recorded crashing more than 500 meters up mountainsides. Go back far enough in geological time and the scale becomes almost incomprehensible: the asteroid that ended the age of the dinosaurs launched a tsunami roughly 30,000 times more energetic than the devastating 2004 Indian Ocean event. The story of how big tsunamis can get is really several different stories, depending on whether you’re asking about the ocean, a fjord, a lake, or another planet entirely.
Earthquake Tsunamis and Their Typical Ceiling
Most tsunamis that threaten coastal cities are generated by undersea earthquakes along subduction zones, where one tectonic plate dives beneath another. The sudden vertical displacement of the seafloor shoves a column of water upward, sending waves outward in every direction. For any given stretch of coast near the quake, the maximum wave height is roughly proportional to how much the seabed shifted vertically during the rupture itself.1Earth and Planetary Science Letters. Tsunami threat in the Indian Ocean from a future megathrust earthquake west of Sumatra That proportionality sets a natural cap: the biggest subduction-zone earthquakes can only lift the seafloor so far, typically a handful of meters to perhaps 20 meters in the most extreme ruptures.
Modern modeling of worst-case scenarios along the Sunda megathrust south of Indonesia, for instance, projects maximum tsunami heights of around 20 to 34 meters along the coasts of Java and southern Sumatra.2Natural Hazards. On the potential for megathrust earthquakes and tsunamis off the southern coast of West Java and southeast Sumatra, Indonesia 3Scientific Reports. Implications for megathrust earthquakes and tsunamis from seismic gaps south of Java Indonesia Those numbers are terrifying enough for anyone living on a low-lying coast, but they represent roughly the upper end of what tectonic tsunamis can produce. To get bigger, you need a different kind of trigger.
Landslide Megatsunamis
The tallest tsunami wave ever recorded in the modern era didn’t come from an earthquake. On July 9, 1958, an earthquake-triggered rockslide dumped roughly 30 million cubic meters of rock into Lituya Bay, a narrow inlet in southeastern Alaska. The resulting wave stripped trees and soil from the opposite mountainside to a run-up height of 524 meters, taller than the Empire State Building.4PubMed Central. Landslides and tsunamis predicted by incompressible smoothed particle hydrodynamics (SPH) with application to the 1958 Lituya Bay event and idealized experiment That number often shocks people, because it sounds impossible for a wave to reach that high. The key is geometry: Lituya Bay is an extremely narrow, steep-walled fjord, and the massive rockslide entered the water at high speed. The wave had nowhere to spread out, so all that energy was focused upward along the opposite slope.
Lituya Bay is not a freak one-off. Landslide-generated tsunamis in confined waterways can achieve extraordinary heights precisely because of how the surrounding terrain channels the energy. A 2015 landslide in Taan Fiord, Alaska, produced a wave with run-up exceeding 190 meters in a similar glacial setting.5Scientific Reports. The 2015 landslide and tsunami in Taan Fiord, Alaska And in September 2023, a rock-ice avalanche of about 25 million cubic meters plunged into Dickson Fjord in East Greenland, triggering a tsunami with a 200-meter initial wave that then sloshed back and forth in the fjord for nine days, producing a seismic signal that instruments detected worldwide.6PubMed. A rockslide-generated tsunami in a Greenland fjord rang Earth for 9 days That persistent sloshing, technically called a seiche, generated a monotone vibration at a frequency so low it was picked up by seismometers on every continent.
Volcanic Flank Collapses and Open-Ocean Giants
Landslides into fjords produce spectacular local heights, but their energy dissipates quickly once the wave escapes the confined waterway. To generate a megatsunami in the open ocean, you need something larger: the catastrophic collapse of a volcanic island’s flank. Steep oceanic volcanoes are inherently unstable structures. They grow rapidly, their flanks are riddled with weak layers of ash and lava, and eventually gravity wins. When a large section of the volcano slides into the sea in a single fast event, it can displace enough water to generate waves hundreds of meters high even at considerable distance.
The clearest geological evidence for such an event comes from the Cape Verde Islands in the Atlantic. About 73,000 years ago, a massive flank of Fogo volcano collapsed into the ocean. On nearby Santiago Island, researchers found tsunami deposits at elevations exceeding 270 meters above the sea level of the time.7PubMed Central. Hazard potential of volcanic flank collapses raised by new megatsunami evidence On Maio Island, about 120 kilometers from the source, the same event produced run-ups above 60 meters.8Sedimentology. A geological record of multiple Pleistocene tsunami inundations in an oceanic island: The case of Maio, Cape Verde Those numbers suggest that volcanic flank collapses are capable of launching waves that remain destructive across open stretches of ocean, not just within enclosed bays.
This is the scenario that occasionally makes headlines in breathless “mega-tsunami could wipe out the East Coast” articles, usually in reference to the Cumbre Vieja volcano on La Palma in the Canary Islands. The reality is debated: whether a single fast collapse of the necessary volume would occur, or whether any failure would be slower and piecemeal, is an open question. But the Cape Verde evidence shows that at least some flank collapses in the deep past were indeed fast, voluminous, and tsunamigenic on a terrifying scale.
The Chicxulub Impact and the Upper Extreme
If volcanic collapses represent the upper end of what Earth’s own geology can produce, asteroid impacts blow past that ceiling entirely. The best-studied case is the Chicxulub impact, the roughly 10-kilometer asteroid that struck what is now the Yucatán Peninsula about 66 million years ago and triggered the mass extinction that ended the reign of non-avian dinosaurs. A 2022 global simulation of the resulting tsunami found that it was up to 30,000 times more energetic than the 2004 Indian Ocean tsunami.9AGU Advances. The Chicxulub Impact Produced a Powerful Global Tsunami
The simulation modeled the impact’s displacement of water, sediment, and crust over the first ten minutes using a hydrocode, then switched to an ocean-propagation model. The resulting waves would have been globe-spanning, reaching every coastline on Earth. Geological deposits consistent with massive tsunami inundation have been found on multiple continents, lending physical evidence to the modeling. In practical terms, the Chicxulub event represents the approximate upper bound for what a single tsunami-triggering event on this planet could do: an energy scale that dwarfs everything else in the geological record by orders of magnitude.
Why Tsunamis Grow as They Approach Shore
A common misconception is that a tsunami is a single towering wall of water crossing the deep ocean. In reality, a tsunami in the open sea is a broad, low swell, often less than a meter high, traveling at the speed of a jet airliner. Ships at sea typically don’t even notice one passing beneath them. The wave only becomes dangerous as it approaches shallower water near the coast. As the ocean depth decreases, the wave slows down, and that lost kinetic energy converts into height: the amplitude shoots up.10Journal of Ocean Engineering and Science. Mathematical modeling of tsunami wave propagation at mid ocean and its amplification and run-up on shore
This process, called shoaling, is why the same tsunami can be invisible in mid-ocean and devastating at the coast. It’s also why the shape of the coastline matters enormously. A broad, gently sloping continental shelf amplifies a tsunami differently than a steep drop-off. Funnel-shaped bays, narrow inlets, and V-shaped harbors can concentrate wave energy and push run-up heights far above what the open coast experiences. The 524-meter Lituya Bay event is an extreme example of this effect, but even at ordinary scales, local geography can triple or quadruple the wave height at one stretch of coast compared to another just a few kilometers away.
Tsunamis in Lakes
You don’t need an ocean to have a tsunami. Any enclosed body of water can experience one if enough material suddenly enters it or if the basin floor shifts. Lake Geneva, straddling France and Switzerland, has a 4,000-year record of mass-movement events preserved in its sediment cores. Numerical simulations of the deposits show that all of the recorded underwater landslides in the lake were large enough to have generated waves at least a meter high.11Sedimentology. Reconstructing 4000 years of mass movement and tsunami history in a deep peri‐Alpine lake (Lake Geneva, France‐Switzerland)
A more dramatic example comes from Lake Lucerne in central Switzerland. In 1601, a moderate earthquake triggered multiple underwater landslides and a rockfall into the lake, generating tsunami waves with run-up heights of up to 4 meters and inundation extending several hundred meters along the low-lying lakeshore.12Geochemistry, Geophysics, Geosystems. Shallow‐Water Tsunami Deposits: Evidence From Sediment Cores and Numerical Wave Propagation of the 1601 CE Lake Lucerne Event Four meters might not sound like much compared to oceanic events, but for communities built along a lakeshore with no expectation of tsunami risk, it’s more than enough to destroy buildings and take lives. Lake tsunamis are a genuinely underappreciated hazard, especially in mountainous regions where steep slopes surround deep water.
Meteotsunamis and Other Atmospheric Triggers
There’s a category of tsunami-like wave that doesn’t involve any geological trigger at all. Meteotsunamis are generated by atmospheric disturbances: rapidly moving pressure changes from thunderstorm outflow boundaries, squall lines, frontal passages, or atmospheric gravity waves. The initial sea-level disturbance is tiny, on the order of a few centimeters, but when the atmospheric disturbance travels at the same speed as long ocean waves in that depth of water, a resonance effect can amplify the wave dramatically.13PubMed Central. Are meteotsunamis an underrated hazard?
As the wave approaches the coast, further amplification from shoaling and harbor geometry can push heights to several meters. Meteotsunamis have caused fatalities and significant damage in the Mediterranean, the Great Lakes, the East Coast of the United States, and southwest Australia. They’re especially insidious because they arrive without the seismic warning that precedes earthquake tsunamis. A sudden surge of water floods a harbor on a day with no earthquake, no volcanic eruption, and no obvious explanation, just unusual weather.14Natural Hazards and Earth System Sciences. Meteotsunamis: atmospherically induced destructive ocean waves in the tsunami frequency band The waves themselves look and behave like small tsunamis, with long periods and the same flooding-and-retreat pattern, but their cause is entirely atmospheric.
How Scientists Read the Geological Record
Estimating the size of ancient tsunamis is a detective exercise. One of the most useful clues is the presence of large boulders deposited far from where waves could normally move them. Researchers distinguish between boulders transported by storm waves and those moved by tsunamis by calculating the maximum size of rock that local storm conditions could plausibly shift. Boulders heavier than that storm-wave threshold are attributed to tsunami activity.15Island Arc. Numerical identification of tsunami boulders and estimation of local tsunami size at Ibaruma reef of Ishigaki Island, Japan Using this approach on the coral reefs of Ishigaki Island, Japan, researchers estimated that the devastating 1771 Meiwa tsunami had wave amplitudes of roughly 6 to 7 meters and a period of about 4 to 5 minutes.
A newer method flips the logic: instead of using boulders to estimate past tsunami sizes, researchers use the boulders that storms have already deposited as a constraint on how large past tsunamis could not have been. If a storm-derived boulder is still sitting in its original position, then no tsunami since that storm was large enough to move it. This approach provides an upper bound on past tsunami sizes in a given area.16Earth and Planetary Science Letters. Numerical estimation of maximum possible sizes of paleo-earthquakes and tsunamis from storm-derived boulders Between these two methods, geologists are gradually building a more complete picture of tsunami sizes stretching back thousands of years, well beyond the written historical record.
Climate Change and Increasing Landslide Risk
If the largest tsunamis on record are landslide-driven, then anything that destabilizes mountain slopes near water increases the risk. Climate change is doing exactly that in polar and high-mountain regions. Glacial retreat removes the buttressing support that ice provides to steep rock faces. Permafrost degradation weakens the bonds holding fractured rock together. The result is that slopes that were stable for millennia are becoming prone to sudden failure.
The 2015 Taan Fiord event in Alaska, which generated run-up exceeding 190 meters, was directly linked to glacial thinning that destabilized the adjacent rock slope.5Scientific Reports. The 2015 landslide and tsunami in Taan Fiord, Alaska The 2023 Dickson Fjord event in Greenland was triggered by a rock-ice avalanche that researchers attributed to glacial thinning.6PubMed. A rockslide-generated tsunami in a Greenland fjord rang Earth for 9 days In West Greenland, two separate landslides at Paatuut, occurring decades apart, have been linked to permafrost degradation since roughly the mid-twentieth century, and the two events produced tsunamis of substantially different magnitudes despite involving similar slope failures.17Landslides. Two similar permafrost degradation landslides at Paatuut, West Greenland, caused tsunamis of substantially different magnitudes That unpredictability is part of the problem: even when you can identify an unstable slope, the size of the resulting tsunami depends on how quickly the mass enters the water, the geometry of the waterway, and a host of other variables that are difficult to forecast.
The practical upshot is that Arctic and subarctic fjords, alpine lakes, and glaciated mountain coastlines are becoming more tsunami-prone as the climate warms. Many of these locations have small communities, tourist infrastructure, or shipping traffic that developed during a period when catastrophic landslides were rare.
Early Warning Systems and the Detection Challenge
For earthquake-generated tsunamis, detection systems are reasonably mature. The standard approach uses pressure sensors placed on the deep seafloor, which can detect the subtle pressure change of a passing tsunami wave even in thousands of meters of water. The seafloor instruments transmit data acoustically to a surface buoy, which then relays it to a warning center via satellite.18Marine Technology Society Journal. Development and Sea Test Results of a Deep-Sea Tsunami Warning Buoy System This gives coastal communities anywhere from minutes to hours of warning, depending on distance from the source.
A newer and more surprising detection method uses the ionosphere. A tsunami’s surface wave pushes the atmosphere upward as it passes, and that displacement propagates as a gravity wave up through the atmosphere. When it reaches the ionosphere, it disturbs the electron density in ways that GPS satellites can detect as subtle signal distortions.19Scientific Reports. Real-Time Detection of Tsunami Ionospheric Disturbances with a Stand-Alone GNSS Receiver: A Preliminary Feasibility Demonstration Researchers have successfully detected ionospheric signatures from several recent tsunamis, including the 2014 Chile event, where the disturbances traveled at the same speed and in the same direction as the ocean waves.20Journal of Geophysical Research: Space Physics. Detection of ionospheric disturbances driven by the 2014 Chile tsunami using GPS total electron content in New Zealand The technique is still being refined, but it offers the tantalizing possibility of using the global GPS satellite constellation as a supplementary tsunami-detection network.
The harder challenge is warning for landslide-generated tsunamis, where there’s almost no lead time. The wave arrives within minutes of the landslide, sometimes seconds if the slope is directly adjacent to a populated area. For these events, the most effective mitigation isn’t detection but hazard mapping: identifying unstable slopes near water and restricting development in the potential inundation zone.
Tsunamis on Mars
Perhaps the most striking expansion of the tsunami story extends beyond Earth entirely. Geological evidence increasingly suggests that early Mars, when it hosted a large ocean in its northern lowlands roughly 3.4 billion years ago, experienced enormous impact-generated tsunamis. Geomorphic mapping in the Arabia Terra region has revealed what appear to be tsunami run-up deposits extending tens to hundreds of kilometers inland from the ancient shoreline, along with backwash channels carved by the retreating water.21Scientific Reports. Tsunami waves extensively resurfaced the shorelines of an early Martian ocean
Researchers have identified at least two separate megatsunami events, possibly triggered by asteroid impacts that created craters around 30 to 50 kilometers in diameter.22Journal of Geophysical Research: Planets. Modeling tsunami propagation and the emplacement of thumbprint terrain in an early Mars ocean A strong candidate for one of these source craters is Pohl, a 110-kilometer-diameter impact structure located about 900 kilometers from the Viking 1 landing site. Its stratigraphic position, sitting above flood-eroded surfaces from the oceanic period and below one of the mapped megatsunami deposits, is consistent with a marine impact origin.23Scientific Reports. Evidence of an oceanic impact and megatsunami sedimentation in Chryse Planitia, Mars If confirmed, these Martian megatsunamis would have been among the largest wave events ever to occur on a rocky planet in our solar system, reshaping hundreds of thousands of square kilometers of coastline in a single event.
The Martian tsunami research also matters for understanding Earth’s deep past. Mars preserves ancient shoreline features that have long since been erased on our own geologically active planet, offering a kind of comparative laboratory for studying how enormous wave events interact with coastal terrain. The same physics applies on both worlds, just scaled by different gravity and different ocean sizes.