Tsunamis can range from barely perceptible ripples in the open ocean to walls of water that climb hundreds of meters up a hillside. In deep water, a tsunami’s wave height is often less than a meter, but by the time it reaches shore, that same wave can amplify into a surge ten, twenty, or even forty meters tall. The most extreme event ever recorded, a landslide-triggered wave in Alaska’s Lituya Bay in 1958, produced a run-up of 524 meters. Understanding how a wave measured in centimeters at sea can become a catastrophe onshore requires grasping the difference between wave height and run-up, and why that distinction matters more than any single number.
Wave Height Versus Run-Up
When people ask how “high” a tsunami gets, they usually picture the wall of water hitting a coast. But scientists track several different measurements, and confusing them leads to wildly misleading comparisons. Wave height, sometimes called wave amplitude, refers to the vertical distance between the crest and trough of the wave itself. In the deep ocean, this is small. A tsunami generated by a major earthquake might cross the Pacific with a wave height of only 30 to 60 centimeters. In open water, tsunamis travel at speeds between roughly 500 and 1,000 kilometers per hour, but their surface expression is so subtle that ships at sea often do not even notice them passing underneath.1ScienceDirect (Journal of Ocean Engineering and Science). Measurement and validation of tsunami Eigen values for the various water wave conditions
Run-up is the measurement that captures destruction. It describes the maximum vertical elevation the water reaches above sea level as it rushes inland. A tsunami might arrive at a coastline with a nearshore wave height of five meters, but if the slope is gradual or a valley funnels the water, the run-up can be several times that. In the 2010 Mentawai tsunami, for instance, the offshore wave amplitude detected by a deep-ocean buoy was modest, yet run-up heights in the Mentawai Islands were devastating, driven by localized seafloor uplift of eight to ten meters at the source.2Earth and Planetary Science Letters. Tsunami earthquakes: Vertical pop-up expulsion at the forefront of subduction megathrust So the “height” of a tsunami depends entirely on where and how you measure it.
Why Tsunamis Grow As They Approach Shore
The dramatic increase from a barely visible open-ocean ripple to a towering coastal surge comes down to basic physics. As a tsunami enters shallower water, it slows down. The front of the wave decelerates while the trailing water, still in deeper ocean, continues pushing forward at higher speed. This compression squeezes the wave’s energy into a smaller volume, forcing the wave height upward. Researchers studying the 2011 Japan Tohoku tsunami found that the leading waves in both the near-field and far-field were small-amplitude long waves, quite different from the steep, solitary-wave shape many people imagine.3CrossRef / Journal of Geophysical Research: Oceans. On the runup of long waves on a plane beach Those long, low swells transformed into devastating surges only as they met the Japanese coastline.
This shoaling process is not uniform. Two stretches of coast hit by the same tsunami can experience drastically different run-up heights because the shape of the seafloor, the width of the continental shelf, and the geometry of bays and headlands all influence how energy is concentrated. Research has shown that wave-height amplification is tied to both small-scale resonance controlled by the local coastal shape and large-scale resonance governed by the broader continental shelf bathymetry.4Journal of Geophysical Research: Oceans. The combined role of bay and shelf modes in tsunami amplification along the coast A narrow, V-shaped bay can funnel incoming energy like a megaphone, multiplying run-up far beyond what a straight, open beach would experience. This is why neighboring towns sometimes report wildly different damage from the same event.
Earthquake-Generated Tsunamis and Typical Heights
Most tsunamis that make the news are triggered by undersea earthquakes along subduction zones, where one tectonic plate dives beneath another. The vertical displacement of the seafloor during an earthquake lifts or drops the entire water column above it, launching a wave. The height of that wave depends on the magnitude of the earthquake, the geometry of the fault, and how efficiently the rupture displaces the ocean floor. For a large subduction earthquake in the magnitude 8 to 9 range, coastal run-up heights of five to fifteen meters are common, with localized peaks reaching thirty meters or more where the coastline amplifies the wave.
Some earthquakes are especially sneaky. So-called “tsunami earthquakes” release their energy slowly, producing relatively modest seismic shaking but disproportionately large waves. The 2010 Mentawai event off Sumatra was a magnitude 7.8 earthquake that generated a far more destructive tsunami than its shaking alone would suggest. High-resolution seafloor imaging revealed that the fault rupture activated steeply dipping thrust faults that bounded triangular pop-up structures near the seafloor surface, producing localized uplift of eight to ten meters across a narrow belt only six to nine kilometers wide.2Earth and Planetary Science Letters. Tsunami earthquakes: Vertical pop-up expulsion at the forefront of subduction megathrust That concentrated vertical throw launched a wave much taller than what a smooth, gradual fault slip would have created.
Conventional tsunami simulations have historically simplified how the seafloor moves during an earthquake, treating the displacement as essentially static and instantaneous. But research comparing those models to actual observations has found considerable discrepancies in both arrival time and wave height, particularly close to the fault. When simulations account for the dynamic, time-varying motion of the seafloor and the acoustic effects in the water column, the resulting waves are substantially larger in the near-fault area.5Bulletin of the Seismological Society of America. Simulation of Tsunami Induced by Dynamic Displacement of Seabed due to Seismic Faulting In other words, even our models have historically underestimated how high earthquake tsunamis can be close to their source.
Landslide Megatsunamis and the Record at Lituya Bay
The tallest tsunami run-up ever documented did not come from an earthquake rupturing the seafloor. On July 10, 1958, a magnitude 8.3 earthquake along the Fairweather Fault in southeastern Alaska shook loose an enormous rockslide into the head of Lituya Bay, a narrow fjord. The rockslide plunged into the water and generated a wave that surged 524 meters up the opposite slope, stripping trees and soil down to bare rock.6Geophysical Research Letters. Hybrid modeling of the mega‐tsunami runup in Lituya Bay after half a century That run-up height is taller than the Empire State Building and more than ten times the run-up of the devastating 2004 Indian Ocean tsunami at its worst.
How is such a height possible? Lituya Bay is a confined inlet with steep walls, and the rockslide dropped roughly 30 million cubic meters of material into the water from a considerable height. The splash, essentially a massive displacement wave, had nowhere to go but up the opposite slope. Numerical models using advanced fluid-simulation techniques have been able to reproduce both the entry of the landslide and the resulting 524-meter run-up, confirming that the geometry of the bay was critical to the outcome.7PubMed Central. Landslides and tsunamis predicted by incompressible smoothed particle hydrodynamics (SPH) with application to the 1958 Lituya Bay event and idealized experiment A landslide of the same volume falling into open ocean would have produced a far smaller wave because the energy would have spread out in all directions. Confinement is what turns a landslide tsunami into a megatsunami.
Landslide-generated tsunamis behave differently from earthquake tsunamis in other ways, too. Laboratory-scale experiments with granular landslides show that the maximum run-up occurs in the immediate impact zone, with secondary peaks forming laterally once the waves fully develop.8Journal of Geophysical Research: Oceans. Runup of granular landslide‐generated tsunamis on planar coasts and conical islands On a conical island, run-up can actually amplify on the far side from the landslide, a counterintuitive effect caused by waves wrapping around the island and converging. These patterns matter for hazard planning because they mean the most dangerous spot is not always the one directly facing the source.
Volcanic Flank Collapses and Ancient Megatsunamis
Volcanic islands pose their own category of tsunami threat. When a steep volcanic flank gives way in a massive gravitational collapse, the resulting landslide can displace enough ocean to generate waves far larger than any earthquake-driven event. Evidence from the Cape Verde Islands shows that about 73,000 years ago, a catastrophic flank collapse of Fogo volcano triggered a megatsunami with run-up heights exceeding 270 meters on nearby Santiago Island.9PubMed Central. Hazard potential of volcanic flank collapses raised by new megatsunami evidence Researchers identified and dated tsunamigenic boulder deposits on Santiago to reconstruct the event, which involved at least one fast, voluminous collapse rather than the gradual slumping some earlier models had suggested.
A more recent volcanic tsunami occurred on December 22, 2018, when Anak Krakatau in the Sunda Strait of Indonesia experienced a major lateral collapse during an ongoing eruption. The collapsed material entered the roughly 250-meter-deep caldera, generating a tsunami with run-up of up to 13 meters on the coasts of Sumatra and Java.10Scientific Reports. Modelling of the tsunami from the December 22, 2018 lateral collapse of Anak Krakatau volcano in the Sunda Straits, Indonesia Thirteen meters is well within the range of a severe earthquake tsunami, but this event was particularly dangerous because it struck at night with almost no warning. Earthquake tsunamis at least give seismic networks something to detect. A volcanic collapse can happen without a detectable earthquake precursor, leaving coastal residents with only minutes between the collapse and the wave’s arrival.
Atmospheric Coupling and the 2022 Hunga Tonga Surprise
Until January 2022, most tsunami scientists focused on seafloor displacement as the primary wave-generation mechanism. Then the Hunga Tonga–Hunga Ha’apai submarine volcano in the South Pacific erupted with explosive force, and the resulting tsunami rewrote some assumptions. The eruption generated waves through multiple mechanisms simultaneously: the initial shock wave coupling with the sea surface near the eruption, the collapse of the water cavity created by the underwater blast, and a pressure pulse in the atmosphere that circled the Earth several times, pushing on the ocean surface as it traveled.11PubMed Central. Diverse tsunamigenesis triggered by the Hunga Tonga-Hunga Ha’apai eruption
The atmospheric Lamb wave mechanism was the real surprise. Atmospheric Lamb waves travel at roughly the speed of sound in air, about 310 meters per second, which is faster than a conventional tsunami crosses the deep ocean. This meant some coastlines thousands of kilometers from Tonga recorded tsunami-like wave arrivals more than two hours before a conventional ocean-traveling tsunami could have reached them.12PubMed. Global fast-traveling tsunamis driven by atmospheric Lamb waves on the 2022 Tonga eruption In the near field close to Tonga, the water-cavity collapse dominated. In the far field, the air-pressure pulse was the main driver, producing an unusually persistent tsunami that oscillated for hours at tide gauges worldwide. The wave heights from this mechanism were generally modest (often under a meter at far-field coasts), but the event demonstrated that atmospheric coupling can produce a truly global tsunami from a volcanic eruption, something warning systems had not been designed around.
Paleotsunami Evidence and What the Geological Record Preserves
Much of what we know about extreme tsunami heights comes not from instruments but from geological detective work. Paleotsunamis leave behind distinctive deposits: boulders transported to clifftops, layers of marine sand and gravel deposited well above normal storm limits, and displaced coral rubble. On the Penghu Islands off Taiwan, researchers identified a seventeenth-century tsunami deposit requiring a minimum wave height of about 3.2 meters to move boulders to their observed positions, with run-up reaching 2.4 to 4.0 meters above sea level at multiple outcrops.13Natural Hazards and Earth System Sciences. Boulder transport and wave height of a seventeenth-century South China Sea tsunami on Penghu Islands, Taiwan
On the coast of Chile’s Atacama Desert, a cliff-top deposit of boulders, including one weighing over 40 tonnes, points to a prehistoric tsunami with a flood height exceeding 18.5 meters above high tide and inland penetration greater than 284 meters from the cliff edge.14Sedimentology. Coastal boulder deposit as evidence of an ocean‐wide prehistoric tsunami originated on the Atacama Desert coast (northern Chile) These geological fingerprints help fill in the gaps of the instrumental record, which only goes back about a century for tsunamis. The paleorecord consistently shows that the largest events in any given region are bigger than anything witnessed in modern times, which is a sobering reminder for coastal planning.
How Coastal Defenses Change the Equation
Given that run-up heights of five to fifteen meters are common for major earthquake tsunamis, and that shoaling and coastal geometry can amplify waves further, coastal communities have tried various strategies to reduce the impact. Japan, which faces more tsunami risk than almost any other country, has invested heavily in both seawalls and coastal forest belts. Studies of the 2011 Tohoku tsunami found that large seawalls were effective at reducing both mortality and structural damage, but smaller seawalls around five meters high showed no measurable effectiveness in reducing impact.15PubMed Central. The Role of Seawalls and Coastal Forests in Mitigating Tsunami Impacts in Japan The 2011 tsunami produced run-up exceeding 15 meters in many locations and topped nearly 40 meters in some spots, so a five-meter wall was simply too short to matter.
Coastal forests showed a measurable benefit in the same analysis, reducing both mortality and damage rates, though the researchers noted it was difficult to separate the physical wave-breaking effect of trees from the indirect effect of forests preventing residential development in hazard-prone areas. Either way, the practical lesson is that defenses work only when scaled to realistic wave heights. Underestimating the potential run-up, which many communities have done historically, leads to walls that provide a false sense of security.
Asteroid Impacts and Theoretical Extremes
At the far end of the probability spectrum sit ocean impacts from asteroids and comets. A large impactor striking the ocean would displace an enormous volume of water, and early models predicted catastrophic tsunami heights reaching coastlines worldwide. More recent computational work has tempered those estimates somewhat. Detailed simulations find that the efficiency of converting impact energy into wave energy is much lower than older models assumed, roughly 3 to 5 percent for impacts up to about one gigatonne of energy, compared to the 15 percent some earlier estimates used.16Acta Astronautica. Near and far-field hazards of asteroid impacts in oceans The reason is that unlike an underwater explosion, an asteroid impact into water vaporizes much of the displaced water and ejects it into the atmosphere rather than pushing it outward as a coherent lip of liquid.
Even with lower efficiencies, an impact from a body a few hundred meters across would still generate run-up heights of many meters at continental coastlines thousands of kilometers away. But the risk profile is quite different from earthquake tsunamis: the probability is extremely low for any given century, and early-warning lead times would be measured in hours rather than minutes, given the distances involved. The more immediate concern for hazard planners remains the earthquake and landslide sources that produce damaging tsunamis every decade or two.
Sea Level Rise and Shifting Baselines
One factor that rarely enters public discussions of tsunami height is the starting water level. As sea levels rise due to climate change, the baseline from which a tsunami’s run-up begins creeps higher. Research developing non-stationary probabilistic tsunami hazard assessments that incorporate projected sea-level rise has found that the effect becomes significant when the magnitude of sea-level rise is comparable to the amplitude of tsunamis with moderate probability of occurrence.17Earth’s Future. Non‐Stationary Probabilistic Tsunami Hazard Assessments Incorporating Climate‐Change‐Driven Sea Level Rise In practical terms, a region where the hazard-relevant tsunami might produce a two-meter surge is significantly affected by half a meter of sea-level rise. A region bracing for a twenty-meter tsunami cares less about that half meter, at least in terms of wave height alone.
The subtlety is that sea-level rise does not just add a constant offset. It changes the depth profile of coastal waters and can alter where and how shoaling occurs, potentially shifting which areas experience the highest run-up. For low-lying coasts that already sit close to sea level, even a modest increase in baseline water depth means the wave reaches further inland before it dissipates. This is a slow-moving shift in risk that compounds every other factor driving tsunami height, and it means that historical records of past tsunami impacts may underestimate the inundation the same-sized event would cause in coming decades.
Putting the Numbers in Perspective
If you want a rough mental framework: in the open ocean, a tsunami from a major earthquake is typically well under a meter tall and essentially invisible. By the time it reaches a coastline, shoaling and coastal geometry can amplify it to anywhere from a few meters to over thirty meters. Most damaging earthquake tsunamis fall in the five to fifteen meter run-up range at the coast. Landslide tsunamis in confined settings can reach run-up heights of hundreds of meters, as Lituya Bay’s 524-meter record demonstrates, but those events affect very limited areas. Volcanic flank collapses occupy a middle ground, capable of 270-plus meters locally while still sending dangerous waves across an ocean basin. Atmospherically coupled waves, as demonstrated by the Hunga Tonga eruption, add a mechanism that can produce a global but generally smaller tsunami faster than the ocean wave itself travels.
What makes tsunami height so hard to pin down with a single number is that it is never just about the wave. It is about the wave interacting with a specific stretch of seafloor, a specific bay shape, and a specific coastal slope. Two towns ten kilometers apart can experience the same tsunami very differently. The geological record shows events far larger than anything in the instrumental era, and rising sea levels are quietly raising the floor on which future tsunamis will build. For anyone living on a coast that faces tsunami risk, the question worth asking is not just how high they get on average, but how high they can get at your particular piece of shoreline.