Seismic Waves

When you look at a seismogram the wiggles you see are an indication that the ground is being, or was, vibrated by seismic waves. Seismic waves are propagating vibrations that carry energy from the source of the shaking outward in all directions. You can picture this concept by recalling the circular waves that spread over the surface of a pond when a stone is thrown into the water. An earthquake is a more complicated process than a stone splashing into water, and the seismic waves that are set up during an earthquake are more varied than those on the pond.

The are many different seismic waves, but all of basically of four types:

  • Compressional or P (for primary)
  • Transverse or S (for secondary)
  • Love
  • Rayleigh

An earthquake radiates P and S waves in all directions and the interaction of the P and S waves with Earth's surface and shallow structure produces surface waves.

Near an earthquake the shaking is large and dominated by shear-waves and short-period surface waves. These are the waves that do the most damage to our buildings, highways, etc. Even in large earthquakes the intense shaking generally lasts only a few tens of seconds, but it can last for minutes in the greatest earthquakes. At farther distances the amplitude of the seismic waves decreases as the energy released by the earthquake spreads throughout a larger volume of Earth. Also with increasing distance from the earthquake, the waves are separated apart in time and dispersed because P, S, and surface waves travel at different speeds.

Seismic waves can be distinguished by a number of properties including the speed the waves travel, the direction that the waves move particles as they pass by, and where they don't propagate (in fluids). 

The first two wave types, P and S, are called body waves because they travel or propagate through the body of Earth. The latter two are called surface waves they the travel along Earth's surface and their amplitude decreases with depth into Earth.

Compressional or P-Waves

P-waves are the first waves to arrive on a complete record of ground shaking because they travel the fastest (their name derives from this fact - P is an abbreviation for primary, first wave to arrive). They typically travel at speeds between ~1 and ~14 km/sec. The slower values corresponds to a P-wave traveling in water, the higher number represents the P-wave speed near the base of Earth's mantle.

P-waves are sound waves, it's just that in seismology we are interested in frequencies that are lower than humans' range of hearing (the speed of sound in air is about 0.3 km/sec). The vibration caused by P waves is a volume change, alternating from compression to expansion in the direction that the wave is traveling. P-waves travel through all types of media - solid, liquid, or gas.

As a P-wave passes the ground is vibrated in the direction that the wave is propagating.

The animation below shows the propgation of a P wave through a 50-km long elastic block. The red rectangle identifies the wavefront (the onset of the wave's deformation). The wave travels through the block at a speed of 6.0 km/s so the wave takes about 8.33 seconds to cover the 50-km distance. The frequency of the vibrations is 0.5 Hz so the wavelength of the signals is 12 km. The motions are greatly exaggerated so that you can see the style of deformation. 

 

 

Shear or S-Waves

 

Secondary , or S waves, travel slower than P waves and are also called "shear" waves because they don't change the volume of the material through which they propagate, they shear it. S-waves are transverse waves because they vibrate the ground in a the direction "transverse", or perpendicular, to the direction that the wave is traveling. 

As a transverse wave passes the ground perpendicular to the direction that thewave is propagating. S-waves are transverse waves.

The animation below shows the propgation of a S wave through a 50-km long elastic block. The red rectangle identifies the wavefront (the onset of the wave's deformation). The wave travels through the block at a speed of 3.5 km/s so the wave takes about 14 seconds to cover the 50-km distance. The frequency of the vibrations is 0.5 Hz so the wavelength of the signals is 7 km. Note that the motion is perpendicular to the direction the wave propagates, but both vertical and horionzontal motions occur.  The motions are greatly exaggerated so that you can see the style of deformation. 

 

An important distinguishing characteristic of an S-wave is its inability to propagate through a fluid or a gas because a fluids and gasses cannot transmit a shear stress and S-waves are waves that shear the material.

Even though they travel more slowly than P-waves, the S-waves move quickly. Typical S-wave propagation speeds are on the order of 1 to 8 km/sec. The lower value corresponds to the wave speed in loose, unconsolidated sediment, the higher value is near the base of Earth's mantle.

In general, earthquakes generate larger shear waves than compressional waves and much of the damage close to an earthquake is the result of strong shaking caused by shear waves.

Love Waves

Love waves are transverse waves that vibrate the ground in the horizontal direction perpendicular to the direction that the waves are traveling. 

Love waves are transverse and restricted to horizontal movement - they are recorded only on seismometers that measure the horizontal ground motion.

Love waves are formed by the interaction of S waves with Earth's surface and shallow structure and are dispersive waves. The speed at which a dispersive wave travels depends on the wave's period. In general, earthquakes generate Love waves over a range of periods from 1000 to a fraction of a second, and each period travels at a different velocity but the typical range of velocities is between 2 and 6 km/second.

The animation below shows the propgation of a monochromatic Love wave wave through a 50-km long elastic block. The red rectangle identifies the wavefront (the onset of the wave's deformation). The wave travels through the block at a speed of about 3.4 km/s so the wave takes about 15 seconds to cover the 50-km distance. Note that the motions change with depth; the vertical variations shown are exaggerated so that some depth variations are visible (this frequency Love wave would not deform very deep below the surface of the block).  The motions are greatly exaggerated so that you can see the style of deformation. 

Rayleigh Waves

Rayleigh waves are the slowest of all the seismic wave types and in some ways the most complicated. Like Love waves they are dispersive so the particular speed at which they travel depends on the wave period and the near-surface geologic structure, and they also decrease in amplitude with depth. Typical speeds for Rayleigh waves are on the order of 1 to 5 km/s.

Rayleigh waves are similar to water waves in the ocean (before they "break" at the surf line). As a Rayleigh wave passes, a particle moves in an elliptical trajectory that is counterclockwise (if the wave is traveling to your right). The amplitude of Rayleigh-wave shaking decreases with depth.

The animation below shows the propgation of a monochromatic Rayleigh wave wave through a 50-km long elastic block. The red rectangle identifies the wavefront (the onset of the wave's deformation). The wave travels through the block at a speed of about 3.2 km/s so the wave takes about 15 seconds to cover the 50-km distance. Note that the motion is retrograde elliptical as shown in the cartoon above, both vertical and horionzontal motions occur. You can see the retrograde motion most easily if you watch the left side of the block. Note also that the motions change with depth; the variations shown are exaggerated so that the depth variations are visible  (this frequency Rayleigh wave would not deform very deep below the surface of the block).  The motions are greatly exaggerated so that you can see the style of deformation. 

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.