Compression waves travel faster and arrive at the surface first. From this, they came to be known as primary waves, or P waves. Shear waves do not travel as fast and are called secondary, or S waves. This is followed by the arrival and stronger shaking of the S waves. The visual record produced is called a seismogram.
Tsunamis are huge ocean waves caused by earthquakes, or volcanoes that erupt under water.
The waves of the tsunami spread in a circular manner from the point of disturbance and can move great distances. Their speed can reach more than 800 kilometres per hour. In deep waters, tsunami are about less than a metre high. However, when they reach shallow waters or narrow bays, the waves pile up into a tall wall of water which causes devastation to things on shore. When gas and molten rock flow from the earth's interior to its surface, a volcano is created.