What’s an earthquake?
Earthquakes occur because of a sudden release of stored energy. This energy has built up over long periods of time as a result of tectonic forces within the earth. Most earthquakes take place along faults in the upper 25 miles of the earth’s surface when one side rapidly moves relative to the other side of the fault. This sudden motion causes shock waves (seismic waves) to radiate from their point of origin called the focus and travel through the earth. It is these seismic waves that can produce ground motion which people call an earthquake. Each year there are thousands of earthquakes that can be felt by people and over one million that are strong enough to be recorded by instruments. Strong seismic waves can cause great local damage and they can travel large distances. But even weaker seismic waves can travel far and can be detected by sensitive scientific instruments called seismographs.
What are earthquake (Seismic) Waves?
A seismic wave is simply a means of transferring energy from one spot to another within the earth. Although seismologists recognize different types of waves, we are interested in only two types: P (primary) waves, which are similar to sound waves,
and S (secondary) waves, which are a kind of shear wave. Within the earth, P waves can travel through solids and liquids, whereas S waves can only travel through solids. The speed of an earthquake wave is not constant but varies with many factors.
Speed changes mostly with depth and rock type. P waves travel between 6 and 13 km/sec. S waves are slower and travel between 3.5 and 7.5 km/sec.