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 Swave is about 4.0 km s^–1, and that of P wave is 8.0 km s^–1. 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 straight line, at what distance does the earthquake occur?

Answer

Given here,
Speed of P waves, v_P = 8.0 km/s
Speed of S waves, vs = 4.0 km/s
Let, t be the time taken by the first wave P to reach from point where earth quake occurs.
Time taken,
t = 4 min = 4 x 60 s = 240 s
And, let x be the distance upto which the earthquake occurs is given by,
x = v_pt
= 8 km/s x 240 s
= 1920 km