The Faults in Our Earth…
Earthquakes occur because friction holds the rock masses on either side of the fault together, while plate motion slowly bends and stretches the rock, storing energy much like the energy stored in a stretched rubber band. An individual earthquake occurs when the stored energy becomes greater than the friction, and the rock masses suddenly slip along the fault. Much of the energy stored in the stretched rock is released in seismic waves, which travel out in all directions from the fault on which the earthquake occurred.
To illustrate how bending an object can generate waves, take a long flexible metal or plastic ruler that can bend, bend it over and then let go. The ruler should swing back and forth as you release it, much like how the earth’s crust swings back and forth as seismic waves (S-waves) are generated.
Types of waves…
Seismic waves come in two major types, body waves which can travel through the interior of the Earth, and surface waves which are confined to the Earth’s surface. Body waves travel the fastest but their strength diminishes quickly; surface waves travel slower but retain their strength over greater distances.
A number of other natural forces and even human activities can generate seismic waves. Landslides and volcanic eruptions shake the Earth and generate seismic waves; ocean waves hitting the beach and continental shelf create what is called microseismic noise. In coastal South Carolina, this microseismic noise greatly increases when there is a hurricane creating large waves in the Atlantic Ocean. Human activities that impact the ground also create seismic waves. Seismographs in the city will record seismic waves from cars and trucks driving on nearby roads, pumps and motors from buildings, and even the footsteps of people walking very close to the seismograph. Other activities, such as setting off explosions to break up rocks in mines, also create seismic waves. All of these non-earthquake sources of seismic waves have a distinct type of motion on a seismograph that is different from a natural earthquake. Seismologists are trained to distinguish these different types of signals and can usually quickly discriminate between a natural or man-made source of seismic waves.