Question 84 from UPSC CSE 2023 - Geography

Question

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?

IASCODEX · Accepted Answer

Correct Option: C. Both 1 and 2

Strategy: Recognize that P (primary) waves travel faster than S (secondary) waves, so are always recorded first on a seismograph--this affirms statement 1. For statement 2, recall that P waves are longitudinal with particle motion parallel to wave propagation, while S waves are transverse with motion at right angles--making statement 2 correct as well. Both statements align with standard seismology concepts.

Key Info: Seismic waves generated by earthquakes are categorized as body waves (P and S waves) and surface waves.

## P waves (Primary waves)
- These are longitudinal or compressional waves; particle motion is parallel to the direction of wave propagation.
- They are the fastest seismic waves and thus are the first to be recorded on a seismograph following an earthquake.
- P waves can travel through solids, liquids, and gases.
- Their velocity depends on material density; typically about 6-13 km/s in Earth's crust.

## S waves (Secondary waves)
- These are transversal or shear waves; particle motion is perpendicular to the direction of wave propagation (up-and-down or side-to-side relative to motion).
- S waves are slower than P waves and arrive later on the seismograph.
- They can travel only through solids and are absorbed or reflected by liquids and gases, providing crucial data for Earth's internal structure (e.g., liquid outer core).

Seismographs detect and record both P and S waves, helping to determine the earthquake's focus and properties of the Earth's interior. The difference in arrival times between P and S waves is vital for calculating epicentral distance. Surface waves, distinct from body waves, cause most of the damage during earthquakes.

IASCODEX · Answer

Option A: 1 only

IASCODEX · Answer

Option B: 2 only

IASCODEX · Answer

Option D: Neither 1 nor 2