Living in stable Ohio, we don't give much thought or worry to earthquakes. However, many people do and have lived in areas that are prone to numerous earthquakes. Earthquakes have greatly influenced the history of mankind throughout the centuries (i.e., perhaps causing the fall of Jericho) and have been responsible for incredible destruction of property and life.
Fault movement: Most, although not all, earthquakes are associated with faulting. When faulting occurs (see diagram below) it begins at a point beneath the surface, called the focus, and spreads rapidly outward. When newspapers report about earthquakes, the location they refer to is the point on the earth's surface directly above the focus (called the epicenter). Fault motion often causes the ground surface to rupture, leaving a fault scarp.
Seismic Waves: Faulting generates seismic waves, which pass through the rocks and cause the ground to shake. This is similar to the way a stone thrown into a pond causes ripples on the pond's surface. In rocks, however, three different types of seismic waves are generated. These are surface waves, which travel along the ground surface, and two different types of body waves (P and S), which travel through the Earth. P (or Primary) waves cause the rock particles to vibrate parallel to the direction the wave is moving. S (or Secondary) waves cause the rock particles to vibrate perpendicular to the direction of wave propagation. You can image/see these waves using a slinky. We have one on hand in the lab room.
Detecting Seismic Waves: Seismic waves are detected on a seismograph. When all is quiet the pen draws a straight line, but when seismic shaking occurs, the pen creates a graphical image. Note how the horizontal axis represents time (in seconds) and the vertical axis represents ground motion. This information provides a basis for locating earthquakes precisely as well as for determining the magnitude of the earthquake. The Richter magnitude (i.e., energy release) of an earthquake is measured by the amplitude (height) of the ground shaking indicated by the seismogram.
Locating Earthquakes: Locating where earthquakes occur, whether in a populated area or in the deep ocean, depends on the behavior of seismic waves. An important aspect of seismic waves is that they travel at different speeds. Using the slinky, it is easy to see that P waves travel faster than S waves. Do this and see for yourself! This fact is the KEY to locating epicenters. To see this, consider two cars that start from the same spot at the same time but travel at different speeds. Early on, the gap between the cars is not great, indicating they are not far from where they started. Over time, though, the gap between the two cars increases, indicating that they are both further from where they started. In the same way, the difference in time between the first arrival of the P-wave and of the S-wave represents how far the seismic station is from the earthquake's epicenter.
