If there were a factor to make sea surface change vertically, tsunami would possibly happen. There are several reasons to cause tsunami. For example, seaquakes, sea bed landslip, submarine volcanic eruption, and meteorite impact. And, the most common factor is seaquake.

The reason for tsunami causing as below: If there were a big shallow earthquake happened, terrain of sea bed would change and make sea level rising up. After these actions, tsunami will convey all around with circle water wave. When tsunami convey in the deep sea at a speed of 700 ~ 800 kilometers an hour, and at amplitude of tsunami is only from 10 to 200 centimeters, the ship navigate the sea will not sway apparently. But, when tsunami arrives at shallow sea, the speed of tsunami will slow down, and the amplitude will grow multiple than before. So, it will damage coast very much.