An earthquake generates three kinds of waves: surface waves (L-waves), which are the slowest and weakest, shear (S) waves, which are transverse waves and carry most of the energy, and
Pressure (P) waves, which are longitudinal waves and are the fastest. The speed of P waves is
Approximately 7 km/s, and that of S waves is about 4 km/s. People do not generally feel the P
Waves, but animals seem to be sensitive to them. If a person reports that her dog started barking 20
Seconds "before the earthquake," then approximately how far was the origin of the earthquake?
A) 500 km
B) 100 km
C) 300 km
D) 200 km
E) 400 km

Question

Quizplus · Accepted Answer

The difference in arrival times of P and S waves is used to estimate the distance. The P waves travel at 7 km/s and S waves at 4 km/s. The difference in speed is 3 km/s. If the dog barked 20 seconds before the S waves were felt, the distance is 20 seconds * 3 km/s = 60 km. However, this is the difference in distance traveled by the two waves. To find the total distance, we use the formula: Distance = Speed of P wave * Time of P wave. Since the time difference is 20 seconds, the total time for P waves is 20 seconds + time for S waves to reach. Solving gives approximately 200 km.

An Earthquake Generates Three Kinds of Waves: Surface Waves (L-Waves)