Consider the following statements: 

1. In a seismograph, P waves are recorded earlier than S waves. 

2. In P waves, the individual particles vibrate to and from in the direction of wave propagation whereas in S waves, the particles vibrate up and down at right angles to the direction of wave propagation. 

Which of the statements given above is\are correct? 

(a) 1 only 

(b) 2 only 

(c) Both 1 and 2 

(d) Neither 1 nor 2

Ans: c

Explanation:

The variations in the direction of waves are inferred with the help of their record on seismography. There are two types of body waves. They are called P and S-waves. P-waves move faster and are the first to arrive at the surface. These are also called ‘primary waves’. They travel through gaseous, liquid and solid materials. S-waves arrive at the surface with some time lag. These are called secondary waves. They can travel only through solid materials. The surface waves are the last to report on seismography. 

Hence, statement 1 is correct.

Different types of earthquake waves travel in different manners. As they move or propagate, they cause vibration in the body of the rocks through which they pass. P-waves vibrate parallel to the direction of the wave. As a result, it creates density differences in the material leading to stretching and squeezing of the material. 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.

Hence, statement 2 is correct.

Read: Previous Year UPSC Geography Questions (PYQs) With Explanation 2023