Consider the following statements:

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 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?

Question

Consider the following statements:

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 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?

Accepted Answer

Both 1 and 2 Quick Verdict: Both statements are factually correct. Furthermore, the physical movement described in Statement 2 is the exact scientific reason why the phenomenon in Statement 1 occurs. Because P-waves push and pull particles longitudinally, they transfer energy much more efficiently and rapidly through rock than S-waves, which move particles in a transverse (up-and-down) shearing motion. Thematic Discussion: The Physics of Seismic Body Waves Earthquakes generate two main types of body waves (P-waves and S-waves). They differ significantly in how they physically displace material, and this specific style of vibration directly dictates their travel speed. P-waves (Primary Waves) - Compressional Motion: P-waves are longitudinal waves. As they travel, the individual particles of rock vibrate "to and fro" parallel to (in the exact same direction as) the wave's propagation. Why they are faster: This movement creates areas of compression and expansion. Think of a coiled spring (a slinky): if you push one end straight forward, that pulse of compression shoots to the other end almost instantly. Earth's rock strongly resists being compressed and snaps back quickly, propelling the wave forward at high velocity. S-waves (Secondary Waves) - Shear Motion: S-waves are transverse waves. As they travel, individual particles vibrate "up and down" at right angles (perpendicular) to the direction of wave propagation. Why they are slower: Going back to the slinky, if you shake one end up and down, that wavy ripple takes noticeably longer to travel down the line. Rocks resist this "shearing" or bending motion much less efficiently than direct compression, resulting in a significantly slower transfer of energy. The Conclusion: Because compressional (to-and-fro) motion transfers energy much more efficiently than shear (up-and-down) motion, P-waves inherently travel faster. This physical difference in particle vibration (Statement 2) is the precise reason why P-waves always outpace S-waves and are recorded first by the seismograph (Statement 1). (Reference: Class 11 NCERT, Fundamentals of Physical Geography, Chapter 3, Interior of the Earth, page 23)