UPSC Prelims 2023 · Question 75 of 97
UPSC Prelims 2023 question on Seismic Waves P and S
- ExamUPSC CSE
- Year2023
- PaperGeneral Studies Paper I
- SubjectGeography
- TopicGeomorphology
- DifficultyMedium
- TypeStatement
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 fro 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?
Show answer
Answer: C. Both 1 and 2
Verdict
Both statements are correct → Option (c) Both 1 and 2.
Statement by statement
Statement 1 – CORRECT: P-waves move faster and are the first to arrive at the surface. These are also called 'primary waves'. The P-waves are similar to sound waves. They travel through gaseous, liquid and solid materials. S-waves arrive at the surface with some time lag. These are called secondary waves and they can travel only through solid materials. Hence in a seismograph, P waves are recorded earlier than S waves.
Statement 2 – CORRECT: P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation. 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.