Multiple Seismic Stations

To number out simply where the earthquake happened, you require recordings indigenous seismic station in various other places. If earthquake areas are generally done with a computer system that can quickly determine the courses of seismic waves with the planet to numerous seismic stations, you can obtain a an excellent estimate of an earthquake location using a map, a ruler, a pencil, and a compass for drawing circles on the map.

Here are instances of a seismograms from various places in phibìc America that taped the same earthquake. Distances from the stations to the earthquake are approximated from the difference in between P and also S wave arrival times:

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Illustrative example seismograms native IRIS.
Use the moment difference between the come of the P and also S waves to calculation the distance from the earthquake to the station. (From Bolt, 1978.)
measure the distance in between the first P wave and the very first S wave. In this case, the an initial P and also S waves room 24 secs apart. discover the point for 24 seconds on the left side of the chart of streamlined S and P take trip time curves and also mark that point. Follow to the chart, this earthquake"s epicenter to be 215 kilometers away. measure the amplitude that the strongest wave. The amplitude is the elevation (on paper) the the strongest wave. On this seismogram, the amplitude is 23 millimeters. Find 23 millimeters on the appropriate side that the chart and also mark that point. place a leader (or directly edge) on the chart between the points you marked for the distance to the epicenter and also the amplitude. The point where your ruler crosses the middle line top top the graph marks the magnitude (strength) of the earthquake. This earthquake had actually a magnitude of 5.0.

Finding the Epicenter

You have actually just determined how much your seismograph is native the epicenter and how strong the earthquake was, yet you still don"t know precisely where the earthquake occurred. This is whereby the compass, the map, and the other seismograph documents come in.

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The suggest where the three circles crossing is the epicenter the the earthquake. This an approach is dubbed "trilateration." (Image from IRIS.)
examine the range on her map. It need to look something like a piece of a ruler. All maps room different. On her map, one centimeter might be same to 100 kilometers or something prefer that. number out exactly how long the street to the epicenter (in centimeters) is on her map. Because that example, say her map has a scale where one centimeter is equal to 100 kilometers. If the epicenter the the earthquake is 215 kilometers away, that amounts to 2.15 centimeters ~ above the map. using your compass, attract a circle with a radius same to the number you come up through in step #2 (the radius is the street from the center of a circle to its edge). The center of the circle will certainly be the location of her seismograph. The epicenter of the earthquake is what on the sheet of that circle. perform the very same thing because that the street to the epicenter that the other seismograms taped (with the ar of those seismographs in ~ the facility of your circles). All of the circles must overlap. The suggest where all of the one overlap is the approximate epicenter the the earthquake.

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