How to locate the epicentre of an Earthquake?

The first thing you need to find the location of an earthquake is the recorded seismograms of three differently located seismographs. You will also need a map of the world, a ruler, a pencil, and a compass for drawing the lines for locating the epicenter. Now you can easily find the location of the epicenter of an earthquake by following steps:

1. First of all the time interval between the first P-wave and the first S-wave should be measured. Suppose the time interval between the two types of waves is 30 seconds.
2. Find the point at 30 seconds on the left column of the following chart and mark that point.
3. The second thing you need to measure is the amplitude of the earthquake. The amplitude of an earthquake is the height of the strongest wave among all the recorded waves. Measure the height of the strongest (longest) wave. According to the readings of this seismogram, the amplitude of the earthquake is 40 millimeters.
4. Now again go to the chart and in the right-most column, mark the amplitude at 40 mm.
5. Now you need to connect the marked points by drawing a straight line with the help of a ruler. This straight line will connect the amplitude mark with the P-S time interval mark while crossing over the magnitude column. In the magnitude column, the degree where the line crosses over will be the accurate magnitude of the earthquake. According to the chart, the earthquake of 40 mm amplitude with 30-sec P-S time interval, the magnitude of this earthquake will be 5.4. Meanwhile, this line will touch the distance-graduations on the left side of the chart, which will be 260 km, meaning that the epicenter is 260 km away from the station of the seismograph.
6. On the left side of the chart, you can see the distance graduations alongside the P-S lag time. In the distance column, the line intersects the distance line at about 260 kilometers. This means the epicenter is 260 km away from the station of the seismograph.

Finding the location of the Epicenter:

You have merely figured out the distance of the epicentre, but still, you need to identify the exact location of the epicenter. For this purpose, you need three recordings of three differently stationed seismographs. Once you got the recordings of the three seismographs, you can, now, find the exact location of the epicenter with the help of a world map and compass. If you follow the following procedural steps one by one, you will be able to figure out the location of the epicenter.

1. Check the scale on your map. It should look something like a piece of a ruler. The sizes of all maps are different. But on the map you have, one centimeter should be equal to 100 km.
2. Figure out how long the distance to the epicenter (in centimeters) is on your map. For example, say your map has a scale where one centimeter is equal to 100 kilometers. If the epicenter of the earthquake is 260 kilometers away, that equals 2.60 centimeters on the map.
3. The third thing you need to figure out is the stations of your seismographs (all three) on the map.
4. After having identified the location of all the three seismographs on the map, you will utilize the compass now.
5. Using your compass, draw a circle with a radius equal to the distance you had figured out, i.e 2.60 cm (based on 1 cm = 100km assumption). The center of the circle will be the location of your seismograph. The epicenter of the earthquake is somewhere on the circumference of that circle.
6. Do the same thing for the rest of the seismograms. When you had gotten three circles; one for each seismograph station, you can see that all the three circles will intersect one another at a point, which will be the exact location of the epicenter.