Correct Option
The correct option is They create troughs and crests in the material..
Explanation
Earthquake waves are primarily classified into Body Waves and Surface Waves. Body waves, which travel through the Earth's interior, are further divided into Primary waves (P-waves) and Secondary waves (S-waves). These waves are distinguished by their mode of propagation and the materials they can traverse.
Option Analysis
- They are similar to sound waves. is incorrect: P-waves are analogous to sound waves because they are longitudinal waves. S-waves are not similar to sound waves; they behave more like light waves or ripples on water (transverse waves).
- They vibrate parallel to the direction of the wave. is incorrect: P-waves vibrate parallel to the direction of wave propagation. S-waves vibrate perpendicular (at right angles) to the direction of wave propagation.
- They create troughs and crests in the material. is correct: Because S-waves are transverse waves with perpendicular vibration, they create crests and troughs in the material. In contrast, P-waves create density differences leading to compression and rarefaction (squeezing and stretching) of the material.
- They travel through gaseous, liquid and solid materials. is incorrect: P-waves can travel through gaseous, liquid, and solid materials. S-waves can travel only through solid materials. This characteristic helps seismologists identify the state of the Earth's interior, such as the liquid outer core.
Key Takeaway: S-waves are transverse waves that generate crests and troughs by vibrating perpendicular to the direction of propagation and are incapable of passing through liquids or gases.