A tidal wave and a tsunami are two completely different phenomena, despite decades of people using the terms interchangeably. A tsunami is caused by a sudden, violent displacement of ocean water, usually from an earthquake. A tidal wave is the regular, predictable rise and fall of the ocean driven by the gravitational pull of the moon and sun. The confusion exists because tsunamis were historically called “tidal waves” in English, even though they have nothing to do with tides.
What Causes a Tsunami
A tsunami is a series of waves set off when something large and sudden displaces a massive volume of ocean water. About 80% of all known tsunamis are triggered by earthquakes beneath or near the ocean floor. These seismic events shift the Earth’s surface, pushing the water column above it upward and outward in waves that radiate in all directions.
Earthquakes aren’t the only trigger. Underwater landslides, volcanic eruptions, and even asteroid impacts have caused catastrophic tsunamis. A landslide can generate a tsunami either by sliding into the water from above or by collapsing beneath the surface, shoving water ahead of it. The key ingredient in every case is a large, abrupt displacement of water, not a gradual force like gravity or wind.
What Causes a Tidal Wave
True tidal waves are the ocean’s response to gravity. The moon and sun pull on Earth’s water, creating the predictable cycle of high and low tides you see at any coastline. The moon is the dominant force here, not because of its size (the sun is 27 million times more massive) but because of its proximity. Tidal forces weaken with the cube of the distance, and since the sun is 390 times farther away than the moon, its tide-generating force ends up being only about half that of the moon.
This gravitational tug creates a wave so enormous you’d never recognize it as one. A tidal “wave” has a wavelength of roughly 20,000 kilometers, spanning half the circumference of the Earth. Where two high tides occur each day, the wave period is about 12.5 hours. It’s an incredibly slow, gentle oscillation compared to anything you’d normally think of as a wave.
How They Behave in the Ocean
The physical differences between these two phenomena are dramatic. A tsunami in the open ocean moves as fast as a jet plane, over 500 mph, with a wavelength between 500 and 1,000 kilometers. Its wave period ranges from 5 minutes to 2 hours. In deep water, a tsunami’s height might be less than a meter, making it nearly invisible to ships at sea. The energy is spread across the entire water column, from surface to seafloor.
Tides, by contrast, move on a schedule measured in hours. The water rises and falls gradually, twice a day in most locations, and you can predict the timing years in advance using astronomical tables. There’s no speed to speak of in the way you’d describe a tsunami. The water simply rises slowly, reaches its peak, and recedes.
What They Look Like at the Coast
When a tsunami reaches shallow water near shore, it slows down and compresses. All that energy piles up, and the wave height increases dramatically. Most tsunamis don’t arrive as a towering, curling wave like you’d see in a movie. Instead, they typically look more like a rapidly rising flood or a powerful wall of water surging inland. The ocean may first pull back unusually far from shore, exposing the seafloor, before the wave arrives. Multiple waves follow, sometimes over hours, and the later waves can be larger than the first.
A high tide, on the other hand, is a slow, familiar rise. The water creeps up the beach over several hours, fills tidal pools, and covers rocks that were exposed at low tide. There’s no surge, no sudden arrival, and no destructive force beyond the normal erosion that coastlines experience over time.
Where the Confusion Comes From
English speakers called tsunamis “tidal waves” for centuries simply because the surging water at the coast looked, to untrained eyes, like an impossibly fast and violent tide coming in. The word “tsunami” itself is Japanese, meaning “harbor wave,” and it entered common English usage largely after the devastating 2004 Indian Ocean tsunami brought global attention to the science. Scientists and agencies like NOAA actively discourage using “tidal wave” to describe a tsunami because it implies a connection to tides that doesn’t exist.
Tidal Bores: A Third Phenomenon
Adding to the confusion, there’s a real phenomenon where tides do produce something that looks wave-like: a tidal bore. This happens when an incoming tide rushes into a funnel-shaped river mouth, creating a visible wall of water that travels upstream. Tidal bores can move dozens of kilometers inland and produce a rumble you can hear from far away. They occur in places like the Amazon River, the Severn Estuary in England, and the Qiantang River in China.
A tidal bore passes in just a few minutes and is driven entirely by tidal forces. It’s visually dramatic but far less destructive than a tsunami. Interestingly, after a tsunami breaks in shallow water, it can produce a surge that behaves similarly to a tidal bore, traveling upstream into river mouths. This was observed during the 2004 Indian Ocean tsunami in rivers across Malaysia, Thailand, and Sri Lanka. But the energy source is completely different: one comes from gravitational rhythms, the other from a catastrophic displacement of ocean water.