Answer: Earthquake waves, also called seismic waves, are the vibrations generated due to shifting of rocks during an earthquake which travel within earth or along its surface. They are of various types depending upon the medium of travel, velocity etc.

Different types of earthquake waves: They’re of two types- body waves and surface waves. Body waves are generated due to the release of energy at the focus and move in all directions traveling through the body of the earth. 

Body waves are of two types:

• P-waves: They are the first to arrive at the surface. They travel through gaseous, liquid and solid materials. P-waves vibrate parallel to the direction of the wave. 
• S-waves: They arrive at the surface with some time lag. S-waves can travel only through solid materials. 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. 

When body waves interact with the surface rocks, they generate a new set of waves called            surface waves. These waves move along the surface. They cause displacement of rocks, and hence, the collapse of structures occurs. Surface waves are of two types:

• Love waves: They have a particle motion, which, like the S-wave, is transverse to the direction of propagation but with no vertical motion. Their side-to-side motion causes the ground to twist from side to side.
• Rayleigh waves: They create a rolling, up and down motion with an elliptical and forward and backward motion in the direction that the wave is moving. 

Emergence of shadow zone:

• Earthquake waves are recorded using seismographs located at far off locations. Shadow zones are specific regions where the waves are not reported. 
• It was observed that seismographs located at any distance within 105° from the epicenter, recorded the arrival of both P and S-waves. 
• However, those located beyond 145° from the epicenter, record the arrival of only p-waves. This is because p waves can travel through all mediums while S waves travel only via solids. 
• P waves are refracted by the liquid outer core and are not detected between 105° and 145°
• Since earth’s outer core is liquid, S waves don’t travel and hence have a larger shadow zone (not detected beyond 105°).
• Thus, a zone between 105° and 145° from epicenter was identified as the shadow zone for both the types of waves. 

Conclusive reason for the emergence of the shadow zone is the internal structure of the earth which is not homogeneous. Thus, the concept of the shadow zone has helped seismologists get information about the interior of the earth’s surface.