How can a tsunami cross an ocean and grow near shore?

A tsunami can begin when an earthquake lifts or lowers a broad part of the seafloor. The water above the moving bottom is displaced too. Gravity acts to restore the disturbed surface, launching long waves away from the source. An earthquake that does not produce enough of the right water displacement may produce no significant tsunami.

A tsunami's wavelength is much longer than that of ordinary short wind waves, and its motion involves the water column. In deep ocean water, crests can be widely separated and relatively low. The traveling pattern carries energy across the ocean; one parcel of water does not have to ride with it from the source to every distant coast.

The speed of these long waves depends strongly on depth. As a tsunami enters shallower water, it slows, and successive crests become closer together. If much of the incoming wave energy continues toward shore while its transport slows, energy can become more concentrated and wave height can increase. This shoaling process helps explain why a modest-looking deep-ocean wave can become dangerous at a coast.

The local result also depends on seafloor shape, coastline geometry, breaking and other energy losses. A tsunami usually arrives as a series of waves, and landslides or other rapid disturbances can generate one too. The drawing shows the connected roles of displacement, gravity and depth; it cannot determine the timing or height at a particular beach.