All-natural earthquakes take place in the lithosphere. An instrument called ‘seismograph’ records the waves reaching the surface.
A curve of earthquake waves recorded on the seismograph. The curve shows three distinct sections, each representing different types of wave patterns.
Earthquake waves are basically of two types. They are body waves and surface waves.
Body waves move in all directions travelling through the body of the earth.
The body waves interact with the surface rocks and generate new set of waves that waves move along the surface, hence called surface waves.
The velocity of waves changes according to the density of material through which they travel. As the density increases the velocity of seismic waves decreases.
When seismic waves pass through materials with different densities, their direction will change as a result of their reflection or refraction.
Reflection causes waves to rebound whereas refraction makes waves move in different directions.
These variations in the direction of waves are studied with the help of their record on seismograph.

Body Waves

Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth.
There are two types of body waves. They are called P and S-waves.

P Wave

These are also called ‘primary waves’. The are short wave of high frequency.
P-waves move faster than S wave. It is the first wave to arrive at the surface.
The P-waves are Longitudinal similar to sound waves so it can travel through gaseous, liquid and solid materials.
P wave has its highest velocity in Solid. But its velocity decreases when it passes through liquid or gas.
P-waves vibrate parallel to the direction of the wave. When p waves pass through a material, it exerts pressure on the material in the direction of the propagation.
As a result, it creates density differences in the material leading to stretching and squeezing of the material.

S-waves are transverse wave which is also called secondary waves.
S-waves is that they can travel only through solid materials.
This characteristic of the S-waves helped scientists to understand the structure of the interior of the earth.
The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane.
Hence, they create troughs and crests in the material through which they pass.
They arrive at the surface after P wave.

The body waves interact with the surface rocks and generate new set of waves that waves move along the surface, hence called surface waves.
The surface waves are the last to report on seismograph.
These waves vibrate perpendicular to the direction of propagation.
They cause displacement of rocks, and hence, the collapse of structures occurs. Surface waves are considered to be the most damaging waves.
There are two type of Surface wave. They are Love Wave or L wave and Rayleigh Wave or R wave.
These are with of long wavelength and low frequency. They travel slower than Body Waves.

There exist some specific areas where the Earthquake waves are not reported. Such a zone is called the ‘shadow zone’.
The seismographs located at any distance within 105° from the epicentre, recorded the arrival of both P and S-waves.
The seismographs located beyond 145° from epicentre, record the arrival of P-waves, but not that of S-waves. A zone between 105° and 145° from epicentre was identified as the shadow zone for both P & S waves.
The entire zone beyond 105° does not receive S-waves. It is known as shadow zone of S-wave.
The shadow zone of S-wave is much larger than that of the P-waves.
The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicentre.