The energy released during an earthquake causes the
ground to shake. If you are close to the epicenter of an earthquake, the
shaking is usually more severe than if you were farther away. This
change occurs for many reasons. Most important, seismic energy
dissipates as it travels through rocks, due to friction and other
effects. In addition, the different types of seismic waves travel at
different speeds. This means that close to the epicenter of an
earthquake, all the waves arrive at about the same time, and the ground
shakes very hard for a short period of time. Further from the epicenter,
the fastest waves arrive ahead of the slower waves. This spreading out
of energy makes the shaking less intense.
The seismic waves generated by an earthquake can
be recorded and measured on a seismograph. The record produced by a
seismograph is called a seismogram.