Chapter 4 · Try it yourself
Tsunami Racer
A tsunami is not a big surfing wave. It is the whole ocean, top to bottom, sloshing after the seafloor jumps. Because it is so long, it “feels” the bottom even in the deepest ocean, and its speed depends only on the depth: speed = √(g × depth), with g = 9.8 m/s².
1. How deep, how fast?
The deep Pacific averages about 4,000 m. The slider is stretched so shallow water gets more room.
Depth 4,000 m
713 km/h
2. Race it across the Pacific
Where does the earthquake happen?
Where is the tsunami going?
On a small screen, swipe the map sideways.
Travel time
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3. Coming ashore
Depth 4,000 m
Height ≈ 0.5 m
Why it grows: the front of the wave slows down in shallow water while the back is still coming fast, so the same energy piles up into a shorter, taller wave. In the open ocean a tsunami is rarely more than about 1 m tall; ships don’t notice it pass. At the coast it can be deadly: the 2011 tsunami in Japan surged up to almost 40 m above sea level in places.
If you are ever at the coast
If the ground shakes hard or for a long time, or the sea suddenly pulls back and exposes the seafloor, don’t wait for an official warning. Go to high ground or far inland right away, and stay there. A tsunami is a series of waves, and the first one is often not the biggest.
Simplified model: speed = √(9.8 × depth). Race times assume the whole trip is over water with an average depth of 4,000 m (about 713 km/h) along the shortest great-circle route; real tsunamis bend around islands and ridges. The shoaling panel uses Green’s law (height grows as depth−1/4) starting from a 0.5 m wave and a 20-minute wave period; real coastlines can make waves much higher.