Seismic waves travel outward from the epicenter of an earthquake. A single earth
ID: 1542931 • Letter: S
Question
Seismic waves travel outward from the epicenter of an earthquake. A single earthquake produces both longitudinal seismic waves known as P waves and transverse waves known as S waves. Both transverse and longitudinal waves can travel through solids such as rock. Longitudinal waves can travel through fluids, whereas transverse waves can only be sustained near the surface of a fluid, not inside the fluid. When seismic waves encounter a fluid medium such as the liquid outer core of the Earth, only the longitudinal P wave can propagate through. Geophysicists can model the interior of the Earth by knowing where and when S and P waves were detected by seismographs after an earthquake (see figure below). Assume the average speed of an S wave through the Earth's mantle is 5.2 km/s and the average speed of a P wave is 9.8 km/s. After an earthquake, a seismograph finds that the P wave arrives 2.0 min before the S wave. How far is the epicenter from the detector?
Explanation / Answer
let distance be d KM.
then time taken for the P wave to arrive =distance/speed=d/9.8 seconds
time taken for S wave=d/5.2 seconds
as per the question, time taken by S wave - time taken by P wave=2 minutes=120 seconds
==>(d/5.2)-(d/9.8)=120
==> 0.090267*d=120
==>d=1329.4 km
hence the epicenter is 1329.4 km from the detector.