The water in a tsunami swells because something suddenly shoves the ocean floor up or down, displacing an enormous volume of water that then moves outward as a series of long, powerful waves. Most often, that “something” is an undersea earthquake, but landslides and volcanic eruptions can do it too. The key factor isn’t wind or surface currents. It’s a rapid vertical shift in the seafloor itself, which forces the entire water column above it to rise or fall in response.
How the Seafloor Displaces the Water
Most tsunamis begin at subduction zones, where one tectonic plate dives beneath another. Stress builds along the boundary for decades or centuries until the plates suddenly slip. When they do, a section of seafloor may jolt upward (uplift) or drop downward (subside) by several meters in just seconds. The water sitting on top of that section of seafloor is pushed up or pulled down with it. As that displaced water tries to settle back to its normal level, it radiates outward in all directions. That radiating energy is the tsunami.
What makes this different from a splash or a ripple is scale. The displaced patch of seafloor can stretch hundreds of kilometers long. That means the resulting wave isn’t a narrow pulse; it’s an incredibly broad disturbance that carries energy across an entire ocean basin.
Why Tsunamis Behave Nothing Like Normal Waves
A wave you’d see at a beach, generated by wind, typically has a wavelength of about 150 meters and a period of around 10 seconds. A tsunami can have a wavelength exceeding 100 kilometers and a period of roughly an hour. That difference matters because a tsunami isn’t really a “wave” in the way most people picture one. It’s more like a sudden, sustained rise in the entire ocean surface, carrying energy from the seafloor to the surface and across vast distances.
In the deep ocean, a tsunami moves at roughly 500 mph, comparable to a commercial jet. At those depths, the wave might be less than a meter tall, spread across 100 kilometers of wavelength, making it virtually undetectable to ships at sea. The danger is hidden in the energy the wave carries, not in its visible height.
What Makes the Wave Grow Near Shore
The dramatic swelling people associate with tsunamis happens as the wave approaches land, through a process called shoaling. A tsunami’s speed depends on water depth. In deep water (around 4,600 meters), it travels at nearly 475 mph. As the seafloor rises toward the coast and depth decreases, the wave slows to roughly 20 to 30 mph. But the energy doesn’t disappear. It compresses. The wavelength shortens, and the wave height shoots upward.
This relationship follows a principle known as Green’s law: as water depth decreases, wave height increases proportionally. A wave barely noticeable in the open ocean can build to over 10 meters at the coast. During the 2004 Indian Ocean tsunami, surveyed areas along the northwest coast of Sumatra recorded wave run-up heights exceeding 20 meters, with a maximum of 51 meters in Aceh Province.
Coastal geography amplifies this further. Shallow, V-shaped bays focus the wave energy inward like a funnel, increasing the height even more. When a tsunami gets trapped over a continental shelf or inside a bay, the waves can bounce back and forth through resonance and reflection, sometimes making later waves in the series taller than the first one.
It’s a Series of Waves, Not Just One
A tsunami isn’t a single wall of water. It’s a wave train: a series of crests and troughs that can arrive over the course of hours. The time between successive crests ranges from about five minutes to two hours. This is why the danger persists long after the first wave hits. The second or third wave is often the largest.
Before the first crest arrives, many coastlines experience a dramatic drawback, where the ocean visibly recedes, exposing seafloor that’s normally underwater. This happens when the trough of the wave reaches shore before the crest does. Historically, this sudden disappearance of water has served as a natural warning sign, along with prolonged ground shaking and a deep roaring sound from the ocean.
Non-Earthquake Causes
Earthquakes cause the vast majority of tsunamis, but they aren’t the only trigger. Any event that suddenly displaces a large volume of ocean water can generate one.
- Underwater landslides push water aside as the sliding mass moves along the seafloor, or they displace water on impact when debris enters the ocean from above. Research in the Canary Islands has identified at least five massive volcanic landslides in the geologic record, each capable of generating ocean-crossing waves.
- Volcanic eruptions can trigger tsunamis through caldera collapse (when the roof of a magma chamber caves in), explosive blasts that push water outward, or by launching landslides down volcanic slopes into the sea.
- Atmospheric disturbances can produce tsunami-like waves called meteotsunamis, caused by rapid changes in air pressure pushing down on the ocean surface.
In every case, the core mechanism is the same: something forces a large volume of water to move suddenly, and the ocean’s attempt to rebalance itself sends that energy racing outward as long-period waves that grow as they reach shallow water.