Body waves travel through the interior of the earth. The P in P-waves stands for primary, because these are the fastest seismic waves and are the first to be detected once an earthquake has occurred. P-waves are predominantly compressional waves. As a P-wave passes, material compresses in the same direction the wave is moving, and then extends back to its original thickness once the wave has passed. P-waves travel through liquids and gases as well as through solids. Although liquids and gases have zero rigidity, they have compressibility, which enables them to transmit P-waves. Because the earth’s mantle becomes more rigid and compressible as the depth below the asthenosphere increases, P-waves travel faster as they go deeper in the mantle. The density of the mantle also increases with depth below the asthenosphere. The higher density reduces the speed of seismic waves.
The S in S-waves stands for secondary, because they are the second-fastest seismic waves and the second type to be detected once an earthquake has occurred. S-waves are shear waves (though that is not what the S stands for). They move by material flexing or deforming sideways (shearing) from the direction of wave travel, and then returning to the original shape once the wave passes. S-waves can travel only through solids, because only solids have rigidity. Because the earth’s mantle becomes more rigid as its depth below the asthenosphere increases, S-waves travel faster as they go deeper in the mantle.
Key Properties and Travel Behavior
In P or compressional waves, the vibration of the rock is in the direction of propagation. P waves travel fastest and are the first to arrive from the earthquake. A P wave travels through a medium by means of compression and dilatation. P waves can move through any type of material, solid or liquid, and do not weaken with distance (“attenuate”) as rapidly as S waves, so they retain relatively more high frequency energy when they arrive at distant seismic stations. In the ground, P waves travel quite quickly - even 5 or 6 kilometers per second (km/s) depending on the type of rock. Through water, they move at about 1.5 km/s. In the air, P waves take the form of sound waves and therefore move at the speed of sound: 330 meters per second (m/s) at sea level. Some people report hearing an earthquake before they feel shaking; what they are hearing is the P wave, vibrating either the ground or nearby objects. Eventually, they will feel the more obvious shaking from the slower-traveling, later-arriving S waves.
S waves are shear waves that deform the ground perpendicular to their direction of travel. Think of our slinky example but wiggling it side to side. S waves travel more slowly than P waves at about 3 km/s, but they produce more damaging shaking. When S waves deform the ground, they cause lateral or shear (back and forth) forces on structures. Older buildings were constructed primarily to withstand the vertical forces of gravity rather than shear forces, and therefore are more prone to failure during big earthquakes due to the strong lateral loading experienced. Also, S wave amplitudes are bigger than P wave amplitudes. Unlike P waves, S waves are unable to pass through air and liquids such as water or magma. In fact, this is how we know Earth’s outer core is molten - S waves cannot pass through it.
Surface Waves and Their Interaction
When body waves reach the surface of the Earth, some of their energy is combined into complex waves that are trapped near and propagate along the surface. These surface waves generally produce lower frequency ground motions and travel more slowly than body waves, but they can have very large amplitudes and cause damaging ground motions. The two types of surface waves are called Rayleigh waves and Love waves. Rayleigh waves produce a long rolling motion along the Earth's surface, while Love waves have horizontal motion perpendicular to the direction the wave is traveling. Surface waves from large earthquakes can circumnavigate the earth and excite long periods of motion.
Visual Aids and Examples
In P waves, the vibration is in the direction of propagation, and they compress and expand as they move. In S waves, the rock oscillates perpendicular to the direction of propagation, imparting strong shear stresses. A 1.7 times slower relationship between S and P waves is a typical rule of thumb in many rocks, with S waves generally traveling about 60% the speed of P waves. A demonstration of how P waves travel through a medium and a demonstration of how S waves travel through a medium illustrate these differences clearly.
Practical Implications
- P waves are the first to arrive at a seismic station and help locate earthquakes quickly.
- S waves reveal information about the rigidity of materials and indicate whether the core is liquid.
- Attenuation and geometric spreading cause energy to diminish with distance, affecting perceived shaking.
In summary, P waves are the fastest, compressional body waves that travel through solids, liquids, and gases, while S waves are slower, shear body waves that travel only through solids and provide crucial information about the Earth’s internal structure. Primary waves set the initial arrival pattern, and secondary waves outline the rigidity of the medium and the presence of fluids in the interior. The interaction between body waves and surface waves shapes the overall ground motion observed during earthquakes.
- Seismic waves originate from the rupture of faults and radiate as seismic energy.
- Body waves include P waves and S waves, with P waves arriving first.
- Surface waves develop when body waves reach the Earth's surface and can cause significant damage.
