Ncert Exercises & Solutions

Earthquakes generate sound waves inside the earth. Unlike a gas, the earth can experience both transverse (S) and longitudinal (P) sound waves. Typically the speed of S wave is about 4.0 km/s, and that of the P waves is 8.0 km/s. A seismograph records P and S waves from an earthquake. The first P wave arrives 4 min before the first S wave. Assuming the waves travel in a straight line, at what distance does the earthquake occur?

Let v_{s} and v_{p} be the velocities of S and P waves respectively .

Let L be the distance between the epicentre and the seismograph .

Let the time taken by the P waves take time t \min . to reach the surface .

Then, the time taken by the S waves take time (t + 4) \min . to reach the surface .

So the distance covered by the P waves and the S waves is equal .

\Rightarrow 8 t = 4 (t + 4)

\Rightarrow t = 4 \min

The distance covered by the P waves = 8 \times 240 \sec = 1920 km

Hence, the earthquake occurs at a distance of 1920 km from the seismograph .

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