A tsunami begins when an earthquake shifts the ocean bottom and creates a small step in the height of the water surface. This step is usually only a few tens of centimeters high. This small disturbance in the surface spreads out in circular waves from the point above the earthquake, but the amplitude remains small as long as the water depth does not change. However, when the wave reaches more shallow water, the wave speed drops, and the back of the wave overruns the front, increasing the amplitude just as if the waves were piling up on top of each other. This is exactly the same effect, but on a much larger scale, that creates breaking waves at ocean beaches.