Basics of Seismic Waves: P Waves – S Waves Difference

The waves generated by the earthquake are called Seismic waves.  The study of earthquake and seismic waves is called Seismology and the researchers are called Seismologists. Seismic waves are divided into two broad categories viz. Body Waves and Surface Waves.

Body waves

In Body waves the speed decreases with increasing density of rock and increases with increasing rock elasticity. Rock elasticity increases faster than density with depth. There are two kinds of body waves viz. P-waves and S-waves.

Primary Waves or P-waves

The Primary waves or Push waves are longitudinal / compression waves that vibrate parallel to the direction of wave movement. They have shortest wavelength, fastest speed {5-7 km/s} and can travel through solid, liquid and gas. They travel fast in denser, solid materials.

Secondary waves or S-waves

Secondary waves or Sheer waves or shock waves are transverse waves which create vibrations perpendicular to the direction of wave movement. The S waves only travel through solids because liquids and gases have no sheer strength.

They have a medium wavelength and cause vibrations at right angles to the direction of propagation of waves. Their velocity is 3 to 4 km per second.

Surface Waves

Surface waves are of two types viz. Rayleigh Waves and Love waves

Rayleigh Waves or L-waves

L Waves or Surface Waves travel near the earth’s surface and within a depth of 30-32 kilometers from the surface.  These are also called Rayleigh waves after Lord Rayleigh who first described these waves. Behave like water waves with elliptical motion of material in wave. Generally slower than Love waves.

Love waves

Love waves make the ground vibrate at right angles to the direction of waves . They are a variety of S-waves where the particles of an elastic medium vibrate transversely to the direction of wave propagation, with no vertical components.  Involve shear motion in a horizontal plane. Most destructive kind of seismic wave.

How Seismic waves help in defining Earth’s interior?

The speed of the seismic waves varies with the composition of the medium. In earth crust their speed is around 2-8 kilometers per second, while in mantle the speed is up to 13 kilometer per second, because mantle is denser. In his observations, Mohorovičić found that when the focus of the Earthquake is not too deep, some waves are propagated along the surface and remains in the crust, while other set enters the mantle, speeds up and reaches the seismometer first. This means that for a seismograph stations located at about 150 Kilometers from a shallow focus earthquake epicentre received those waves first which came from beneath the ground via mantle. This was enough to conclude that there is something below earth crust which has a greater density and varied composition. It was later called Mohorovičić discontinuity or simply Moho.

The above finding led to determine that mantle is denser than crust and is viscous, semi-molten material. P-wave velocities are much slower in the outer core than in the deep mantle while S-waves do not travel at all in the liquid portion of the outer core.

Role of Seismic waves in determination of Epicentre

To determine the location of an earthquake, the following two things of info are required:

• Recorded seismograph of the earthquake from at least three seismographic stations at different distances from the epicentre of the quake.
• Time it takes for P-waves and S-waves to travel through the Earth and arrive at a seismographic station.

As we know that the P waves reach the to the seismographs first at a station, the difference between the time of P waves and S waves is called S-P Interval. The S-P interval increases with increasing distance from the epicentre. At each station a circle on a map can be drawn which has a radius equal to the distance from the epicenter.

Earthquake Shadow Zone

Seismic waves recorded at increasing distances from an earthquake indicate that seismic velocities gradually increase with depth in the mantle. However, at arc distances of between about 105° and 140° no P waves are recorded. Furthermore, no S waves are record beyond about 105°. This is called Shadow zone.