Place three or more seismographs in three different locations far apart, but encompassed in your map in the eventuality of an earthquake. Take all the seismograms from the seismographs and study them carefully. Look for a series of small waves that will suddenly build up into bigger ones and eventually fade away. The first small waves are the P waves, while the large waves on the graphs are S waves.
Use a ruler to measure the distance from the first peak at the beginning of the P waves to the first peak at the point where the S waves start in centimeters for each seismogram. The magnitude of the distance is similar to that of the time in seconds. For example, if the distance between the two waves is 24 cm, the two waves are 24 seconds apart.
Pinpoint the time in seconds on the scale marked "S-P (seconds)" of the travel time chart for converting time difference to distance. Find a scale for distance in kilometers on the opposite side of the scale. Note the corresponding distance in kilometers from each seismogram reading.
Place the map on the table and mark with a visible dot all three locations of the seismic readings. Convert the distance from kilometers to centimeters or any other units used for scaling, depending on the type of map used. Part the compass and pencil to these lengths and draw circles on the map using dots on the map as the centers and the radii, or distances from the chart, derived from each respective seismogram.
Observe the circles drawn on the map and find the point at which all of them touch on the map, which is the location of the epicenter. This point may vary slightly from the true point, due to reading errors from the seismogram.