A tsunami is caused by an earthquake. When an earthquake occurs under the sea, it sometimes produces a circular motion of water, perhaps 1 km in diameter. This impulse can travel up to 800 km per hour, speeding through the open water virtually unnoticed. When conditions are right, this disturbance in the water forms a very definite wave form which has incredible power behind it. As it enters shallow water, it forms a large wave which we know as the destructive tsunami.