The Wave That Hides in Deep Water

Wind Stirs Only the Surface

The ordinary waves at sea are raised by the wind. However rough they look, their energy stays near the top: a few dozen meters down, the water barely moves.

Watch the dots in the cross-section. Only a thin layer just under the surface turns in circles; the depths are still.

Model coast · sea surface and land heights exaggerated about 1,300 times, ocean depth about 28 times

The Seafloor Lurches

A tsunami is different. Its most common cause is a great earthquake under the sea: a fault slips, and within minutes a vast stretch of seabed is thrust up or dragged down.

The water above is lifted with it — all of it, from the seafloor to the surface. A long mound rises on the sea, then slumps under gravity and spreads outward both ways. This time, every dot in the cross-section moves.

As Fast as a Jet, and Nearly Invisible

In deep water, a tsunami’s speed depends only on the depth: the deeper the ocean, the faster it runs. Over water four kilometers deep it covers more than 700 kilometers an hour, about the speed of a jetliner.

Yet it is rarely more than a meter high, while a single wave can stretch for hundreds of kilometers from front to back. The slope is so gentle that people on a passing ship usually notice nothing. We have colored the crest red and the trough blue; otherwise you would not see it either.

Tap anywhere on the sea to see how deep the water is there, and how fast the tsunami travels.

Red marks the crest, blue the trough

When the Sea Pulls Back

The first sign of a tsunami is sometimes not a wave but a retreat. If a trough reaches the coast first, the sea drains away suddenly, like a very low tide, exposing the seafloor, the reefs and stranded fish.

Do not go down to look. Head for high ground or inland at once, without waiting for a warning. A strong or long earthquake felt at the coast, or a roar from the ocean, is the same signal.

In this model, the seafloor on the landward side of the fault sank in the earthquake, so a trough reaches the shore first. At the red marker, in water 20 meters deep, the sea falls more than two meters before the first crest arrives.

Shallow Water Makes It Stand Up

Near the coast the seafloor rises sharply, and the tsunami slows. At the shore it is down to 30 to 50 kilometers an hour — about the speed of a car in town.

But the back of the wave is still in deep water, moving faster than the front. The back catches up, the same water is squeezed into less and less distance, and it has nowhere to go but up. Off this model coast, a wave under two meters high in the open ocean grows to about six in the shallows.

Not One Wave but Many

A tsunami does not break on the beach and slide back like an ordinary wave. It comes ashore more like a fast-rising flood, pouring over seawalls, streets and fields, then draining back to sea with everything it has picked up.

And it does not come alone. A tsunami is a series of waves; the first may not be the last or the largest, and the danger can last for hours or even days. Stay on high ground until officials say it is safe.

Pale ground has been flooded

March 11, 2011

At 2:46 p.m. local time, a magnitude 9.0–9.1 earthquake struck about 70 kilometers off northeastern Japan, the largest ever recorded there. Within an hour the tsunami had flooded much of Japan’s eastern coast. In Miyako it ran up a hillside to 40.5 meters above sea level; near Sendai the water reached 10 kilometers inland. Nearly 16,000 people were killed, and some 2,500 were still missing ten years later.

The tsunami kept spreading across the Pacific. About 25 minutes after the earthquake it passed the nearest deep-ocean buoy. Later that day it reached Hawaii, where the water along the Big Island’s coast rose about 1.8 meters on average, and more than 3 in places.

The wave shown here is computed from the real shape of the seafloor and a simplified earthquake. Each line marks where the tsunami had reached after another hour.

Pacific Ocean · half-degree grid · real seafloor · simplified source