Earthquakes cause tsunamis by abruptly shoving a large section of the seafloor upward or downward, displacing the water column above it and sending energy radiating outward as long-period waves. The process is most efficient at subduction zones, where one tectonic plate dives beneath another and a sudden slip can lift or drop hundreds of kilometers of ocean floor in seconds. But the connection between quakes and tsunamis is less straightforward than many people assume, and some of the most destructive recent tsunamis came from earthquake types that were not supposed to produce them at all.
How Seafloor Displacement Launches a Wave
Picture the ocean as a column of water sitting on a solid floor. When an earthquake ruptures a fault beneath that floor, the rock on one side lurches vertically. Because water is nearly incompressible, it faithfully mirrors the shape of that deformation at the surface: a bulge where the seafloor rose, a depression where it dropped. That initial disturbance then spreads outward under gravity. In the open ocean the wave is barely noticeable, often less than a meter high, but it can travel at the speed of a jet airliner because the wave speed depends on ocean depth.
The 2011 Tohoku earthquake off Japan’s Pacific coast illustrates this at extreme scale. More than 50 meters of slip occurred on a fault that ruptured all the way to the seafloor in the Japan Trench, producing roughly 50 meters of vertical seafloor displacement in places.1Annual Review of Earth and Planetary Sciences. Large Coseismic Slip to the Trench During the 2011 Tohoku-Oki Earthquake That massive vertical motion pushed an enormous volume of water upward, and the resulting tsunami reached run-up heights exceeding 30 meters along parts of the Japanese coast. The clay-rich sediments lining the shallow fault zone turned out to be a key factor: they were weak and slippery enough to allow the rupture to propagate all the way to the trench, maximizing the seafloor deformation and, consequently, the tsunami.
Why Earthquake Depth Matters So Much
Not every powerful earthquake produces a dangerous tsunami. One of the clearest examples is a magnitude 8.3 earthquake that struck beneath the Sea of Okhotsk in May 2013. Despite its enormous energy release, the resulting tsunami was tiny, barely detectable even on the nearest deep-ocean sensors. The reason was depth: the earthquake’s focus was about 609 kilometers below the surface.2Turkish Journal of Earth Sciences. The depth effect of earthquakes on tsunami heights in the Sea of Okhotsk At that depth, the rupture is so far from the ocean floor that the vertical displacement at the seabed is negligible. Had the same earthquake occurred at a shallow depth, beneath the ocean floor rather than deep in the mantle, it could have produced a devastating wave.
This is why shallow earthquakes, typically those with a focal depth of less than about 70 kilometers, are the ones that matter for tsunami generation. Subduction-zone megathrust earthquakes are the prime culprits because they involve enormous fault areas at shallow depth, directly beneath the ocean. The 2004 Indian Ocean earthquake ruptured about 1,200 kilometers of the plate boundary along the Andaman Trough, with the largest slip reaching 23 meters off the northwest coast of Aceh province in Sumatra.3Earth, Planets and Space. Rupture process of the 2004 great Sumatra-Andaman earthquake estimated from tsunami waveforms That rupture propagated at an average speed of roughly 2 to 2.7 kilometers per second and lasted around eight minutes, deforming the seafloor over such a vast area that waves radiated across the entire Indian Ocean basin.4Nature. Tracking the rupture of the Mw = 9.3 Sumatra earthquake over 1,150 km at teleseismic distance
When the “Wrong” Kind of Earthquake Makes a Tsunami
Textbooks traditionally taught that only thrust and normal faults, the ones that move rock up or down, could generate tsunamis. Strike-slip faults, where the two sides slide horizontally past each other, were considered incapable of producing major waves because they do not directly lift or lower the seafloor. The 2018 Palu earthquake in Sulawesi, Indonesia, threw that assumption into serious question.
The magnitude 7.5 event occurred along the Palu-Koro fault, a strike-slip system. Yet a devastating tsunami struck Palu Bay within minutes, with waves reaching several meters. Post-event surveys and video evidence showed that the waves arrived almost instantaneously in parts of the bay, suggesting the source was very close.5Geophysical Research Letters. Nearly Instantaneous Tsunamis Following the Mw 7.5 2018 Palu Earthquake Several mechanisms likely combined to produce this unexpected tsunami. Geodetic data revealed that a restraining bend in the fault, where two parallel segments meet at an angle, produced over 2 meters of vertical seafloor uplift beneath the bay despite the overall strike-slip motion.6Journal of Geophysical Research: Solid Earth. A Tsunami Generated by a Strike‐Slip Event: Constraints From GPS and SAR Data on the 2018 Palu Earthquake On top of that, the long, narrow, funnel-shaped geometry of Palu Bay trapped and amplified the waves through a focusing effect, and submarine landslides along the bay’s steep underwater slopes added additional displacement.7Natural Hazards Research. A comprehensive report on the 28th September 2018 Indonesian Tsunami along with its causes
Research since Palu has shown more broadly that ground motions from strike-slip earthquakes can contribute to tsunamis larger than one meter under fairly common conditions, without requiring landslides at all.8PubMed Central. Anatomy of strike-slip fault tsunami genesis This finding matters because many coastlines sit near active strike-slip faults, and tsunami hazard assessments have historically given those faults little attention.
Submarine Landslides and Other Secondary Triggers
An earthquake does not have to displace the seafloor directly to cause a tsunami. Shaking can destabilize steep underwater slopes, triggering submarine landslides that rapidly displace the water above them. This mechanism works differently from fault-driven tsunamis. Because a landslide is essentially a point source compared to a long fault rupture, it produces shorter-wavelength, more rapidly dispersing waves that lose height faster with distance. That makes landslide-generated tsunamis primarily a local hazard, potentially catastrophic near the source but diminishing quickly farther away.
The Palu event is a case where both fault deformation and submarine landslides appear to have contributed simultaneously. Other historical events, like the 1998 Papua New Guinea tsunami, are believed to have been driven primarily by earthquake-triggered landslides rather than the earthquake’s own seafloor displacement. The practical problem is that landslide-generated tsunamis are harder to predict and model than fault-driven ones, because the location, volume, and speed of a submarine landslide are not readily estimated in real time.
Gravitational Energy and the Puzzle of Tsunami Earthquakes
Some earthquakes produce tsunamis wildly out of proportion to what you would expect from their magnitude. Seismologists call these “tsunami earthquakes,” and they have been a puzzle for decades. They tend to rupture slowly, involve huge displacements in the wedge of loose sediment that sits at the front of a subduction zone, and produce many aftershocks with unusual faulting patterns. The 2010 Mentawai earthquake off Sumatra was one such event: a moderate magnitude that generated a deadly, outsized tsunami.
One explanation involves gravitational potential energy. In standard earthquake models, all the energy comes from elastic strain stored in compressed rock. But in tsunami earthquakes, the enormous displacements of poorly consolidated sediment in the accretionary wedge suggest that gravitational energy, the weight of material that has been pushed up over geologic time and collapses during the earthquake, contributes substantially to the total energy budget. This additional energy source helps explain the unusually large seafloor displacements, sometimes 10 meters or more, that characterize these events.9Earth and Planetary Science Letters. Tsunami earthquake generation by the release of gravitational potential energy
The rupture velocity itself also matters. Research on very slow earthquakes, those with rupture speeds below about 0.5 kilometers per second, shows that slow rupture can amplify tsunami energy dramatically. For the largest slow-rupture megathrust events, tsunami energy can be amplified 10 to 60 times compared to faster-rupturing earthquakes of similar size.10Seismological Research Letters. Tsunami Efficiency Due to Very Slow Earthquakes This is deeply counterintuitive: you might expect a more violent, fast-rupturing earthquake to produce a bigger wave, but the opposite can be true. A slow rupture displaces the seafloor over a longer time in a way that more efficiently couples energy into the ocean.
The Mentawai earthquake also revealed something unexpected about the fault zone itself. The shallow portion of a subduction zone’s plate interface was widely assumed to resist sudden slip, behaving more like a slowly creeping surface than a locked fault. But analysis showed that the Mentawai rupture area contained patches of both stable and unstable frictional behavior. Slow creep deeper on the fault progressively stressed the shallow unstable patch until it broke, producing the tsunami earthquake.11Communications Earth & Environment. Tsunamigenic earthquake at the Sunda trench promoted by aseismic slip after a previous megathrust event This finding complicates the picture further, because it means regions considered unlikely to produce sudden large earthquakes can still generate devastating tsunamis under the right conditions.
How the Wave Transforms on Its Way to Shore
In the deep ocean, a tsunami wave might be only half a meter tall but hundreds of kilometers long from crest to crest. It moves fast, roughly 700 to 800 kilometers per hour in water 4,000 meters deep, and passes beneath ships without anyone aboard noticing. The wave preserves its shape well in open water because there is no slope to disrupt it.12Journal of Ocean Engineering and Science. Mathematical modeling of tsunami wave propagation at mid ocean and its amplification and run-up on shore
Everything changes as the seafloor rises toward the coast. The wave slows down in shallower water, but the energy it carries does not disappear. Instead, the wave compresses: it gets shorter and taller. By the time it reaches the coast, a wave that was invisible in deep water may have grown to many meters in height. The steepness of the nearshore slope, the shape of the coastline, and the presence of offshore features all influence how much the wave amplifies.
Fringing coral reefs are a good example of how local bathymetry complicates predictions. Modeling of the 2009 American Samoa tsunami showed that narrow reefs, less than about 200 meters wide, could actually increase wave heights and inland inundation rather than protect the shore. The shallow water over the reef causes the wave to shoal more aggressively, and a narrow reef does not provide enough friction to compensate.13Earth-Science Reviews. Effects of fringing reefs on tsunami inundation: American Samoa Wider reefs offer more protection, but the effect depends strongly on tide level, reef condition, and wave period.
Coastal Features That Amplify or Reduce Damage
Once a tsunami reaches the coastline, the damage it causes depends heavily on the local geography and built environment. Simulations of tsunami inundation along Japan’s Sea of Japan coastline found that natural sand dunes and port breakwaters significantly reduced the total volume of water that flooded inland in the areas directly behind them. But breakwaters also redirected energy in unexpected ways, increasing wave heights just beyond the protected port areas.14Earth, Planets and Space. Tsunami inundation characteristics along the Japan Sea coastline: effect of dunes, breakwaters, and rivers Rivers proved to be another vulnerability: even areas shielded by dunes could flood when a tsunami funneled up a river channel and overtopped the levees.
The Palu Bay case highlights the danger of funnel-shaped coastal geometry. The bay’s long, narrow shape acted like a horn, concentrating wave energy as the tsunami moved inland and producing wave heights far larger than would have occurred along a straight, open coast.7Natural Hazards Research. A comprehensive report on the 28th September 2018 Indonesian Tsunami along with its causes Enclosed bays and harbors can also experience sloshing, where the wave bounces back and forth between the bay walls, sustaining dangerous water levels for an extended period. These effects mean two communities the same distance from an earthquake can experience radically different tsunami impacts based solely on their local topography.
Volcanic Eruptions and Atmosphere-Driven Tsunamis
Earthquakes are the most common cause of tsunamis, but they are not the only one, and comparing the two types helps clarify what makes earthquake-generated tsunamis distinctive. Volcanic eruptions can also displace water, either through flank collapse, pyroclastic flows entering the sea, or caldera formation. Volcano-generated tsunamis typically have shorter wave periods than earthquake-generated ones, which means they disperse more quickly during propagation and are usually dangerous only locally or regionally.
The January 2022 eruption of Hunga Tonga-Hunga Ha’apai introduced a third mechanism that startled researchers. The explosion launched pressure waves into the atmosphere, including atmospheric gravity waves that traveled at a wide range of speeds. Some of those gravity waves happened to travel at the same speed as shallow-water ocean waves, creating a resonance that pumped energy into the sea surface over enormous distances. The result was that measurable tsunami-like waves arrived at coastlines thousands of kilometers away, in some cases ahead of what any ocean-traveling wave could have produced. Modeling showed that these resonant atmospheric gravity waves, despite being smaller in amplitude than the faster-moving pressure pulses, were responsible for the large sea-level fluctuations observed at distant coastlines like Japan’s.15PubMed Central. Observation and simulation of atmospheric gravity waves exciting subsequent tsunami along the coastline of Japan after Tonga explosion event
Tsunamis also send energy upward. Numerical simulations have shown that a tsunami as small as 50 centimeters at the ocean surface can generate gravity waves that propagate upward through the atmosphere into the ionosphere, producing measurable disturbances in electron density at altitudes above 300 kilometers.16Journal of Geophysical Research: Space Physics. Propagation of tsunami‐driven gravity waves into the thermosphere and ionosphere This coupling between ocean and upper atmosphere has practical value: detecting ionospheric disturbances via GPS signals is being explored as one additional tool for tsunami detection, since those signals can travel faster than the ocean wave itself.
What Tsunamis Do to the Land They Hit
The immediate destruction from a tsunami is obvious, but the longer-term environmental damage is less well known and often severe. When seawater floods agricultural land, it saturates the soil with salt. After the 2004 Indian Ocean tsunami, coastal areas in India’s Nagapattinam District experienced salinization of both soil and groundwater, along with salt injuries to crops.17PubMed. Impact of the December 2004 tsunami on soil, groundwater and vegetation in the Nagapattinam District, India Recovering from this kind of damage takes years. Saltwater infiltration into freshwater aquifers can make well water undrinkable long after the floodwaters have receded, and soil salinity may remain elevated for multiple growing seasons, forcing communities to abandon farmland or invest in remediation.
Tsunamis also rearrange the physical coastline itself. They strip away beaches, destroy wetlands, and deposit thick layers of sediment inland. These deposits, which geologists study as evidence of past tsunamis, contain a mix of marine sand, shell fragments, and debris from the land surface. The study of ancient tsunami deposits has become an important tool for estimating how often tsunamis strike a given coast and how far inland they reach, filling gaps in the written historical record that extends back only a few centuries in most regions. In places like the Pacific Northwest of the United States, where no written records exist from before European contact, these geological records are the primary evidence that the Cascadia subduction zone has produced catastrophic tsunamis in the past and is expected to do so again.