Earthquake starts tsunami
An earthquake along a subduction zone happens when the leading edge of the overriding plate breaks free and springs seaward, raising the sea floor and the water above it. This uplift starts a tsunami. Meanwhile, the bulge behind the leading edge collapses, thinning the plate and lowering coastal areas.
Surviving a tsunami: lessons from Chile, Hawaii, and Japan; USGS Circular 1187
Actions that save lives, and actions that cost lives, as recounted by eyewitnesses to the tsunami from the largest earthquake ever measured--the magnitude 9.5 earthquake in Chile on May 22, 1960.