You may confuse that many agencies locate different points for the epicentre of same earthquake, it may happens because of recording stations around the possible location.
Here is one example of one method to find out epicentre.
To figure out just where that earthquake
happened, you need to look at your seismogram and you need to know what at
least two other seismographs recorded for the same earthquake. You will also
need a map of the world, a ruler, a pencil, and a compass for drawing circles
on the map.
Here's an example of a seismogram:
FIGURE 1 - OUR TYPICAL SEISMOGRAM FROM BEFORE,
THIS TIME MARKED FOR THIS EXERCISE (FROM BOLT, 1978).
One minute intervals are marked by the small lines printed just above the squiggles made by the seismic waves (the time may be marked differently on some seismographs). The distance between the begining of the first P wave and First S wave tell you how many seconds the waves are apart. This number will be used to tell you how far your seismograph is from the epicenter of the earthquake.
Finding
the Distance to the Epicenter and the Earthquake's Magnitude
1.
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.
2.
Find the point for 24 seconds on the left
side of the chart below and mark that point. According to the chart, this
earthquake's epicenter was 215 kilometers away.
3.
Measure the amplitude of the strongest wave.
The amplitude is the height (on paper)
of the strongest wave. On this seismogram, the amplitude is 23 millimeters.
Find 23 millimeters on the right side of the chart and mark that point.
4.
Place a ruler (or straight edge) on the
chart between the points you marked for the distance to the epicenter and the
amplitude. The point where your ruler crosses the middle line on the chart
marks the magnitude (strength) of the
earthquake. This earthquake had a magnitude of 5.0.
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FIGURE 2 - USE THE AMPLITUDE TO DERIVE THE MAGNITUDE OF THE EARTHQUAKE,
AND THE DISTANCE FROM THE EARTHQUAKE TO THE STATION. (FROM BOLT, 1978) |
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Finding the Epicenter
You have just figured out how far your
seismograph is from the epicenter and how strong the earthquake was, but you
still don't know exactly where the earthquake occurred. This is where the
compass, the map, and the other seismograph records come in.