How to Locate an Earthquake's Epicenter

We have all heard about earthquakes in the news. And, always, the epicenter of the earthquake is given. However, if you have ever wondered how scientists figure out the epicenter of an earthquake, there are some simple steps anyone can follow to figure it out.

Things You'll Need

• Map
• Pencil
• Ruler
• Compass
• Graph of travel-time curves
• Seismograms from three different seismic stations

Instructions

1. Look at a seismogram that depicts an earthquake as recorded at one seismic station. Read the x-axis of the seismogram to determine the arrival times of the earthquake's P-waves and S-waves at that seismic station. The P-waves travel faster than the S-waves so they will always arrive first. Use these arrival times to calculate the difference (in minutes) of arrival times of the two waves.

2. Look at your travel time curves graph that shows two curves, the S-curve and the P-curve. Find the place on the graph where the P-curve and the S-curve are separated by the difference in arrival times found in step one. The difference in arrival times is read on the y-axis of the travel time curves graph.

3. Draw a vertical line using your ruler and pencil between the two travel time curves where the difference in arrival times from step one occurs. Continue to draw this vertical line all the way down until it intersects the x-axis of the travel time curves graph. The point where this vertical line intersects the x-axis is read off as the distance to the earthquake's epicenter from this particular seismic station.

4. Check the scale on your map. Take the distance to the earthquake's epicenter that you found in the previous step and figure out how long that distance is on your map using the map scale. Find the location of this seismic station on your map, and using your compass draw a circle around this seismic station that has a radius equal to the map-scaled distance to the earthquake's epicenter. The center of this circle will be the particular seismic station, and the epicenter of the earthquake is somewhere on this circle.

5. Repeat Steps 1 through 4 for the seismograms at two other seismic stations.

6. Determine the location of the earthquake's epicenter. You will have drawn three circles on your map, one for each of your three seismic stations. The point where these three circles intersect is the epicenter of the earthquake.

Tips & Warnings

• Try to be as accurate as possible when measuring arrival times and drawing circles. This way you can be sure that the three circles all intersect at a single point.

Resources

• An example of locating an earthquake's epicenter
• Information on earthquakes and locating an epicenter