20
How to locate an earthquake
WORK AND EXPERIMENTATION TECHNIQUES
300 400 500 600 700
70
60
50
40
30
Time (s)
Distance
United States
Tucson
El Paso
Houston
Chihuahua
Rosarito
La Paz
Mazatlán
Acapulco
Mexico
Monterrey
0 100 200 300 400 500 600 700
Km
Materials
❚❚
Three seismograms from the same earthquake from three
different seismic stations
❚❚
Map of the location of the three seismic stations with a
scale
❚❚
Graph of the calculated distance of the delay between
the arrival of P and S waves
❚❚
Ruler and compass
❚❚
Calculator
1.
Use a seismogram to work out the delay, in seconds, between the
arrival of the P waves and S waves.
2.
Put this value on the vertical axis that represents the time (at the
Mazatlán station, 43) and extend it until the S-P line.
3.
Go down vertically to the horizontal axis to obtain the distance
corresponding to the delay (420 km in the example).
4.
Repeat the process with the other two seismograms. In this way you
will know the distance of the earthquake from each station.
5.
Use thegraphic scaleon themap to convert thedistances to centimetres.
6.
Copy the map on a sheet of paper. Draw a circumference around
each station with the same radius as the distance to the epicentre.
For this, use the graphic scale. The epicentre is located where the
circumferences overlap.
Procedure
Using what you have studied in the unit, you can use a
seismogram to calculate the distance of an earthquake
and locate the epicentre.
0 10 20 30 40 50 60 70 80 90 100 110
Time delay between waves P and S (s)
Mazatlán
Rosarito
Chihuahua
P
P
P
S
S
S
1.
A cyclist and a motorcyclist leave
the same point at the same time.
Calculate the delay between their
arrival times.
2.
What relation is there between the
previous example and P waves and S
waves recorded on the seismogram?
3.
What relation can you find between
the S-P delay and the amplitude of the
vibrations on the seismogram?
4.
What city is the epicentre of this
earthquake in? Explain in your own
words the process you followed.
5.
What would the seismogram of a
station very near the earthquake
be like?
