Earthquake is a shaking of the planet’s surface caused by sudden movements deep below the outer layer, or crust, of the Earth. Earth looks like a solid place from the outside, but it’s a very active place just below the surface, with a solid crust, a hot, nearly liquid mantle and a molten inner core. When stress in the crust becomes too great, rock breaks along lines of weakness, either pre-existing faults or new ones that form during an earthquake. The point below the surface where this happens is called the focus or hypocenter and the spot directly above it on the surface is known as the epicenter.
Earthquakes usually start at the boundary between two tectonic plates. The plates are like puzzle pieces covering the surface of the Earth and they keep moving around, sliding past each other and bumping into each other. Sometimes the edges of the plates get stuck on each other and that causes an earthquake. The most powerful earthquakes are on the faults where the plates stick together.
The size of an earthquake depends on the amount of slip on the fault and how far that slip travels from the fault. Scientists determine the size of an earthquake by studying the P and S waves recorded on seismographs. The length of time the P and S waves take to reach an observatory tells scientists how far away the earthquake is. By comparing the difference between the P and S wave durations, scientists can calculate the size of the earthquake (see Figure below).
Earthquakes often occur with little or no warning. However, there are some things you can do to be safer during and after an earthquake. Find a safe place inside, preferably under a table or desk and away from windows, tall furniture, and power lines. Practice “drop, cover and hold on” so that it will be a natural reaction when the next earthquake hits.