The severity of an earthquake depends upon:
■ The amount of potential energy that had been stored.
■ The distance the rock mass moved when the energy was released.
■ How far below the surface the movement occurred.
■ The makeup of the rock material and the soil type. With all other factors
being equal, hard igneous rock produces the least amplification and results
in the least damage to structures while structures built on soft soil or
artificial fill receive the most damage.Body and Surface Waves
There are two classes of seismic waves: body waves and surface waves. Body
waves travel through Earth’s interior. There are two types of body waves: P
waves and S waves. P waves travel through Earth and are caused by the
expansion and contraction of bedrock. S waves are produced when material
moves either vertically or horizontally and travel only within the uppermost
layers of Earth (along its surface). Surface waves produce rolling and/or swaying
motion and are slower than P or S waves. Surface waves cause ground motion
and damage.