An earthquake is the shaking or trembling of the crust of the earth, caused by underground volcanic forces or by the sudden slipping or displacement or a portion of the earth's crust beneath the surface. The actual movement of the earth, frightening as it is, seldom is the direct cause of death and injury. The earth's movement can cause buildings and other structures to shake or collapse.
Most casualties result from falling or shifting objects and debris, splintering glass and fires.
Everything shakes and rattles - there is a lot of noise. The motion may be severe - if you are standing, you may be thrown to the ground. Some exterior windows will probably break, causing shattered glass and strong drafts. There will be a mess. The shaking may last only a minute or two, but there may be a number of aftershocks over several days, weeks or months.