1. The waves which are caused by earthquake are called seismic waves. There are three types of waves: P-waves, S-waves and longitudinal waves.
2. P-waves can pass through all the mediums while S-wave can pass only through solid medium.
3. Even though P-waves pass through all mediums, they experience refraction as they pass from one medium to the other.
4. However, there exist some specific areas where the waves of that earthquake are not reported. Such zone is called ‘shadow zone’.
5. Generally, seismographs located at any distance within 105° from the epicentre, record both P-waves and Swave. However, beyond 140° P-wave are recorded but not S-waves.
6. Thus, the zone between 105° and 140° is identified as the shadow zone for both types of waves.
7. The entire zone beyond 105° does not receive S-waves. The shadow zone of S- waves is much larger than that of Pwaves.
8. The shadow zone of P-waves appears as a band round the earth between 105° and 140° away from the epicentre.
