An earthquake is ground shaking caused by a sudden movement of rock in the earth’s crust. Such movements occur along faults, which are thin zones of crushed rock separating blocks of crust. When one block suddenly slips and moves relative to the other along a fault, the energy released creates vibrations called seismic waves that radiate up through the crust to the earth’s surface, causing the ground to shake.Earthquakes may last only a few seconds or may continue for up to several minutes. They can occur at any time of the day or night and at any time of the year.
They are caused by stress that builds up over time as blocks of crust attempt to move but are held in place by friction along a fault. (The earth’s crust is divided into large plates that continually move over, under, alongside or apart from one another atop the partly molten outer layer of the earth’s core.) When the pressure to move becomes stronger than the friction holding them together, adjoining blocks of crust can suddenly slip, rupturing the fault and creating an earthquake.
