A magnitude of 7.0, for example, indicates measured amplitudes that are ten times greater than those of magnitude 6.0 and 100 times greater than those of magnitude 5.0. When an earthquake occurs, seismic waves travel from the focus through the earth and up to the surface. The speeds at which the waves travel depend on the waves themselves and the type of rock through which they pass, but may be one to ten kilometres per second. Some of the waves are of high enough frequency to be audible, while others are of very low frequency with seconds or minutes between them. Both waves pass through the earth's interior from the focus, but only compression waves pass through the molten outer core.
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.