Answer:
Seismic waves produced during an earthquake are one of the important sources of information about the interior of the earth. Earthquakes radiate seismic energy as both body waves (P wave and S wave) and surface waves. P wave or primary wave is the fastest kind of seismic wave, and, consequently, the first to ‘arrive’ at the surface/seismic station. They can travel through gaseous, liquid and solid materials.
S wave or secondary wave is slower than a P wave and unlike the P waves, it can only move through solid rock.
During an earthquake, the seismic waves (P and S waves) spread out in all directions through the Earth’s interior. Their velocities through the earth’s interior depend on the material properties such as composition, mineral phase and packing structure, temperature, and pressure of the media. For instance:
• They travel more quickly through denser materials and therefore generally travel more quickly with depth.
• Anomalously, hot areas slow down seismic waves.
• They move more slowly through a liquid than a solid
• Molten areas within the Earth slow down P waves and stop S waves because their shearing motion
cannot be transmitted through a liquid.
• Partially molten areas may slow down the P waves and attenuate or weaken S waves.
• Reflection causes waves to rebound; refraction makes waves move in different directions causing
the seismic waves to travel in curved path through the Earth.
Appearance of shadow zones
The shadow zone results from P-waves being refracted by the liquid core and S-waves being stopped completely by the liquid core. Seismographs located at any distance within 105° from the epicentre, records the arrival of both P and S-waves. Thus, a zone between 105° and 145° from epicentre is the shadow zone for both the types of waves. However, the seismographs located beyond 145° from epicentre, records the arrival of P-waves, but not that of S-waves. The entire zone beyond 105° does not receive S-waves. The shadow zone of S-waves is much larger than that of the P-waves.
The geometric distribution and extent of these shadows as measured for a given earthquake allows us to calculate the position of major boundaries in the Earth’s interior, as well as give us information about the solid vs liquid character of the various layers, and even about some of their physical properties.
