Shadow Zone of Earthquakes

Earthquake waves get recorded in seismographs located at far off locations. However there exist some specific areas where the waves are not reported. Such a zone is called the shadow zone. The study of different events reveals that for each earthquake, there exists an altogether different shadow zone. It was observed that seismographs located at any distance within 1050 from the epicentre, recorded the arrival of both Pand S-waves. However, the seismograph located beyond 1450 from epicentre; record the arrival of P-waves, but not that of S-waves. Thus, a zone between 1050 and 1450 from epicentre was identified as the shadow zone for both the types of waves. The entire zone beyond 1050 does not receive S-waves. 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 1050 and 1450 away from the epicentre. The shadow zone of S-waves is not only larger in extent but it is also a little over 40 per cent of the earth surface. If epicentre of an earthquake is known, one can easily draw its shadow zone.

The shadow zone for ‘P’ waves:

The shadow zone for ‘P’ waves is an area that corresponds to an angle between 103 and 142.P waves are refracted at the mantle-outer core boundary due to liquid nature of outer core.

 The shadow zone for S waves:

S waves radiate spherically away from an earthquake in all directions however they do not appear beyond an angular distance of ~103°. They are stopped by the liquid outer core. It must be noted that shadow zone of S waves is bigger than shadow zone of P waves because S waves do not appear after 103 degrees.
Moreover, shadow zone of each earthquake is different, and it depends on focus of the earthquake.