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Locating the epicenter of an earthquake is a relatively simple matter. The tools required to do this by hand are: three seismograms of the earthquake, a map of the world, a ruler, a pencil, and a drawing compass.

On seismograms, ticks mark one minute intervals. Measure the distance between the arrival of the first P wave and the S wave. Subtract the arrival time of the P wave from the S wave. This number will be used to find the distance from the seismograph to the epicenter on a special chart. You also need to measure the amplitude of the strongest wave. Do this by measuring from the baseline to the top of the highest line. Record this measurement in millimeters.

Now you need to consult a chart for the finding the Richter scale magnitude. It looks something like this -

     _                                  _
  500|                                  |100
     |50                |               |
     |                  |6              |50
     |	                |               |
     |                  |5              |
     |                  |               |5	
     |                  |4              |
     |	                |               |
  100|                  |3              |.5
     |                  |               |
   50|                  |2              |
     |                  |               |.1
     |                  |1              Amplitude
   20|2                 |               (millimeters)
    5|                  |0	
     |                  Magnitude
     |
     |
    0|
dist.  S-P
(km)   (seconds)

On the chart, mark the left-most graph at the point corresponding to the measured S-P time. This corresponds to a distance - the distance from the epicenter to that seismograph. On the right-most graph, mark the measured amplitude. Place a ruler or straight-edge on the two marked points, and mark where it crosses the middle line. This tells you the magnitude of the earthquake. Do this for all three seismograms.

Next, to locate the epicenter, check the scale on your map. Figure out how long the distance to the epicenter is on your map. Use your compass to draw a circle with a radius equal to that distance. Center the circle on the location of the seismograph. Do this for all three stations. All of the circles should overlap. The point were all the cirles intersect is the approximate epicenter of the earthquake.

Remember, the epicenter is merely the surface location of the earthquake's focus. This method does not tell you how deep the earthquake was. To determine that requires more information.

Of course, earthquake location is most often done on computer, not on paper. To see an example of this, try the Virtual Earthquake program located at http://vcourseware5.calstatela.edu/VirtualEarthquake/VQuakeIntro.html

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