GEOLOGY

EARTH SCIENCE

EARTHQUAKES

Question [CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
To tell how far an earthquake’s epicenter is from a seismograph, scientists measure the difference between the
A
arrival times of the P waves and S waves.
B
strengths of the P waves and S waves.
C
speeds of the P waves and S waves.
D
sizes of the P waves and S waves.
Explanation: 

Detailed explanation-1: -Use the time difference between the arrival of the P and S waves to estimate the distance from the earthquake to the station. (From Bolt, 1978.) Measure the distance between the first P wave and the first S wave. In this case, the first P and S waves are 24 seconds apart.

Detailed explanation-2: -The primary, or P, waves travel most quickly and are the first to be registered by the seismograph. Secondary, or S, waves travel more slowly. As S waves have a greater amplitude than P waves the two groups are easily distinguishable on the seismogram.

Detailed explanation-3: -By looking at the amount of time between the P and S wave on a seismogram recorded on a seismograph, scientists can tell how far away the earthquake was from that location. However, they can’t tell in what direction from the seismograph the earthquake was, only how far away it was.

Detailed explanation-4: -Determining Richter Magnitudes The distance of the seismic recording station from the earthquake epicenter is determined from the time difference between the first arrival of the P-wave and the S-wave. This is known as the S-P interval.

Detailed explanation-5: -S waves are indicated by an abrupt change in wave amplitude. In the seismogram below, we see that the S wave arrived at about 34 sec after the P wave arrived. This time difference is called the S-P interval and is the lag time between the P and S wave.

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