An earthquake generates a series of waves that penetrate the Earth and, depending on magnitude, can be detected by sensitive instruments thousands of miles from the epicenter. The motion of the ground tells us what type of seismic wave arrived. Though we animate individual wave paths, remember that like sound waves, seismic waves travel in all directions away from the source. We also greatly exaggerate the motion to illustrate the behavior.
Seismic waves travel either through the Earth’s interior or near Earth’s surface with a characteristic speed and style of motion. There are four basic types of seismic waves; two preliminary body waves (P & S) that travel through the Earth’s interior and two slower surface waves (Love and Rayleigh) that travel along the surface of the Earth. Their speeds vary depending on the density and the elastic properties of the material they pass through, and they are ampli ed as they reach the surface. At great distances, the motion is detected by sensitive seismograph stations.
Depending on proximity to the focus of the earthquake the seismic waves move the ground, thus any structure according to their wave motion. The different types of seismic-energy waves shake the ground in different ways. Combinations of waves,
as well as re ections and refractions off boundaries within the earth produce many other types of seismic waves, but they can be left to seismologists
The P wave, or primary wave, is the fastest of the three waves and the rst detected by seismographs. They are able to move through both solid rock as
well as through liquids. These are compressional or longitudinal waves that oscillate the ground back and forth along the direction of wave travel, in much the same way that sound waves (which are also compressional) move air back and forth as the waves travel from the sound source to a sound receiver. Compressional waves compress and expand matter as they move through it . When a seismic wave comes from below, it bumps the house upward.
P waves, or compressional waves, are seismic body waves that shake the ground back and forth in the direction that the wave is moving. P waves travel fastest and are generally felt first. They usually cause very little damage.
S, or secondary waves or shear waves, are seismic body waves that shake the ground back and forth perpendicular to the direction the wave is moving.
Surface waves roll the ground in a back-and-forth, up- and-down motion.
S waves, or secondary waves, are the waves directly following the P waves. S waves travel
in the same direction, but instead of being a compressive wave, they oscillate with a shearing behavior at right angles to the direction of motion. Figure 1B shows that though the wave direction is the same as the P wave, the ground motion moves the house side to side. They travel about 1.7 times slower than P waves. Because liquids will not sustain shear stresses, S waves will not travel through liquids like water, molten rock, or the Earth’s outer core. S waves are more dangerous than P waves because they have greater amplitude and produce vertical and horizontal motion of the ground surface.
For many classroom applications simply addressing P and S waves is enough, but it is also good to know about the destructive and convoluted surface waves.
This last type of wave, and the slowest, is the surface wave which moves close to or on the outside surface of the ground. There are two types of surface waves:
1) Love waves move like S waves in that they have a shearing motion in the direction of travel, but the movement is back and forth horizontally.
2) Rayleigh waves move both horizontally and vertically in a vertical plane pointed in the direction of travel.
Love and Rayleigh waves both produce ground shaking at the Earth’s surface but very little motion deep in the Earth. Because the amplitude of surface waves diminishes less rapidly with distance than the amplitude of P
or S waves, surface waves are often the most important component of ground shaking far from the earthquake source, thus can be the most destructive.