Tsunamis only affect coastal regions. The best thing to do if you suspect a tsunami is about to occur is to get on higher ground. One sign of a tsunami is a strong earthquake that lasts twenty seconds or longer and occurs in or near the ocean. The sign that Tilly Smith noticed was the water rising. When a tsunami occurs, the water does not rise in the same way as tidal waters do, with closely spaced waves that one could surf on; instead, the water rises rapidly and is often very turbulent and contains a lot of debris. Sometimes the first sign observed is a rapid receding of the ocean followed by a wall of rising water. When this happens, it is time to run!