Electromagnetism, Properties Of Waves, Motion, Gases & The Kinetic Theory Worksheet With Answers Page 15
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36Seismogram.
Seismograms can be used to locate the epicentre of an earthquake.
P-waves arrive first then S-waves
followed by the surface wave. The
greater the distance from the
earthquake to the monitoring
station the greater the time
lag/gap between the waves.
Remember not all monitoring
stations will receive the seismic
waves due to the shadow zones.
Example question.The diagram shows the first seismic signals received from an
earthquake at two monitoring stations A and B.
1.
What evidence is shown by the seismic data that suggests A is nearer the epicentre than B?
Answer: The seismic waves arrive at A before they arrive at B.
2. What evidence suggests P and S waves have travelled with different speeds from the earthquake?
Answer: P and S waves do not arrive at the same time.
3. The time lag between the arrival of the P and S waves for a seismic station which is 100km from the
epicentre of an earthquake is 12s. Calculate the distance of the monitoring station A from the epicentre
of this earthquake.
st
Answer :
1
step is to work out the time gap between P and S waves for station A. Between 12:21 :30 and
12:22:41 there is a 71s gap/delay.
nd
2
step is to realise that there is a 12s delay for each 100km (as stated). How many times
more is 12s than 71s ?
592km
So, 71 ÷ 12 = 5.92
and then
5.92 x 100 =
15
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