- In a seismograph, P waves are recorded earlier than S waves.
- In P waves, the individual particles vibrate to and fro in the direction of waves 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
- Answer: (c)
Statement 1 is correct: P waves, also known as primary waves or compressional waves, are the fastest seismic waves and travel through the Earth’s interior in a push-pull motion. They can travel through solid rock, liquids, and gases, and therefore arrive at a seismic station before other types of waves. S waves, also known as secondary waves or shear waves, travel more slowly than P waves. Unlike P waves, S waves cannot travel through liquids, so they are only observed in solid materials. Due to their faster speed, P waves reach a seismograph station before the arrival of S waves. The time interval between the arrival of P waves and S waves can be used to determine the distance between the seismic station and the earthquake’s epicentre.
Statement 2 is correct:In P waves (primary waves), the individual particles vibrate in the same direction as the wave propagation. The motion of the particles is in a push-pull or compressional motion, similar to how a slinky compresses and expands. This means that the particles move back and forth parallel to the direction in which the wave is travelling. On the other hand, in S waves (secondary waves), the particles vibrate perpendicular to the direction of wave propagation. The motion of the particles is in a shearing or transverse motion, similar to the movement of a rope when you shake it side to side. This means that the particles move up and down or side to side, but their motion is perpendicular to the direction of wave travel.
