Q. With respect to earthquake waves, which of the following statement is/are correct?
1. The Surface waves move along the surface and the velocity of these waves do not changes with change in the density of material they are travelling.
2. Body waves are generated due to the release of energy at the focus and move in all directions.
Select the correct answer using the codes given below:
Answer: B
Notes:
Explanation: Earthquake Waves:
- All natural earthquakes take place in the lithosphere. You will learn about different layers of the earth later in this chapter. It is sufficient to note here that the lithosphere refers to the portion of depth up to 200 km from the surface of the earth.
- An instrument called ‘seismograph’ records the waves reaching the surface. A curve of earthquake waves recorded on the seismograph is given in Figure 3.1. Note that the curve shows three distinct sections each representing different types of wave patterns.
- Earthquake waves are basically of two types — body waves and surface waves. Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth.
- Hence, the name body waves. The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface. The velocity of waves changes as they travel through materials with different densities.
- The denser the material, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities.
Source: NCERT Class 11
