How Tsunamis Form.

Tsunamis come up three separate times in Notable Historic Earthquakes (Valdivia, Sumatra, Tōhoku) without ever being explained — worth fixing, especially since the mechanism ties directly back to the plate boundary types already covered.

A tsunami isn't caused by an earthquake's shaking directly — it's caused by the sudden, physical displacement of an enormous volume of ocean water. At a subduction zone, one plate is normally locked against the other for decades or centuries, bending and storing strain the way a bent stick stores energy. When that strain finally releases in a megathrust earthquake, the seafloor itself can lurch upward or downward by several meters, nearly instantly, across an area sometimes hundreds of miles long. All the water sitting above that stretch of seafloor gets shoved along with it — and that sudden displacement is what radiates outward as a tsunami.

This is exactly why the mechanism matters: subduction-zone (convergent boundary) earthquakes are disproportionately tsunami-dangerous, while transform-boundary earthquakes like 1906 San Francisco generally aren't. A transform fault like the San Andreas moves the ground sideways, not vertically — no major vertical seafloor displacement, no major water displacement, no tsunami, even in a very large quake. It's specifically the vertical seafloor motion unique to subduction zones that does it.

Out in open ocean, a tsunami wave is barely noticeable — often just a foot or two of sea-surface rise, moving at the speed of a jet plane (500+ mph), completely unfelt by ships passing directly over it. The danger appears only as it approaches shallow coastal water: the wave slows down, and as it slows, all that energy compresses into a much taller, much slower-moving wall of water — the same reason ordinary wind waves grow taller as they approach a beach, just at a vastly larger scale.