An earthquake is the result of a release of stored energy in the Earth's crust that creates seismic waves. Seismographs are machines that detect and measure seismic waves caused by earthquakes. P waves, or primary seismic waves, are the fastest moving waves. They travel through solids, liquids and gases. S waves or secondary waves are slower and can travel through solids, but not liquids. The slowest and most destructive seismic waves are called surface waves.