Seismic waves radiate outward from the focus of an
earthquake throughout the Earth. These waves can cause damage in some
areas, but not in others. Many factors can affect the amount of damage.
Distance from an earthquake focus is important. Seismic energy is lost
as waves travel through the Earth, so the further you are from the
epicenter of an earthquake, the less shaking you will feel. The type of
ground through which the waves travel is another factor. For instance,
if the waves shake sand particles, the energy will tend to make the
particles "settle." This may cause the Earth’s surface to
sink with large movements, damaging human-made structures. In contrast,
if the waves pass through hard, solid rock, no settling occurs, and the
movement is less.
Collapse of two layered freeway in a 1989 earthquake in the San
Francisco Bay area.