Last week I discussed faults and the different fault types that can be found. Now, let’s take a look at what happens when movement occurs along a fault. As the earth attempts to move along the fault, the rocks stick, so that they build up energy. When the rocks finally do move, this energy is released in waves and the result is an
earthquake. These waves are referred to as
seismic waves. When you feel the ground move during an earthquake, you are feeling the seismic waves given off during the release of energy. Earthquakes have also been known to occur where there is no associated fault. Most of these earthquakes occur at great depths in the earth’s interior, generally deeper than 100 km. Most geologists still do not know the causes for these earthquakes.
Let us take a closer look at an earthquake, and some of the terminology used to describe them. When a fault breaks and causes an earthquake, the point in the earth’s interior where the movement initially occurred is called the
focus. The focus is where most of the movement along the fault happens during the earthquake, decreasing as the seismic waves travel away from the focus. The area on the earth’s surface directly above the focus is called the
epicenter. As the energy is released, two separate types of seismic waves emanate from the focus.
Body waves are the seismic waves that travel through the earth’s interior, radiating from the focus in all directions.
Surface waves are seismic waves that travel along the surface moving away from the epicenter.
The body waves are further described as P waves and S waves. P waves are compressional waves that vibrate in the same direction that the waves are moving. S waves are transverse waves that vibrate perpendicular to the direction the wave is traveling. A good demonstration of P and S waves is two objects attached to a wall, a spring and a rope. When motion is applied to the spring, the motion of the spring travels forward and back, just like a P wave. On a rope, when motion is applied by shaking the rope, the motion occurs as an “S” shape as it travels down the rope, similar to how S waves travel.
It is important to know the difference between P and S waves. P waves are called the primary wave; they travel at about 4 to 7 kilometers per second through the rock (that’s 9,000 to 15,000 miles per hour!). P waves travel through both solids and fluids. An S wave, or secondary wave, travels much slower at about 2 to 5 kilometers per second. S waves can also pass through solids, but they are unable to pass through fluids, like the fluid magma at the earths core. Finally, surface waves travel the slowest of all. Their speeds vary, but are always slower than the P and S waves. Surface waves also cause the most damage from earthquakes because of their slow movement and larger ground motion.
