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Locating an Epicenter

Contributed by Brad Ridgley, Princeton H.S., Princeton, Indiana


This is a lab I use to help familiarize students with calculating distances to epicenters and using triangulation to locate an epicenter. It is preceded by lecture and discussion of a chapter on earthquakes.

PURPOSE: to find the location of an earthquake’s epicenter

MATERIALS: drawing compass, ruler, calculator


1. Calculate the following problems, which apply to information in the procedure. The average speed of S waves is 4.1 km/s. The average speed of P waves is 6.1 km/s. To calculate the time it takes seismic waves to travel a given distance, divide that distance by the average speed of each wave.
A. How long would it take P waves to travel 100 km? How long would it take them to travel 200km?

B. How long would it take S waves to travel 100 km? How long would it take them to travel 200 km?

C. What is the time between the arrival of P waves and S waves over a distance of 100 km? What is the lag time for a distance of 200 km?


1. Students are given illustrations of seismograph records in three cities following an earthquake. These illustrations begin at the left with the arrival of the P waves indicated. Use the time scale provided to find the lag time between the P waves and the S waves for each city. Be sure to measure the time from the arrival of the P wave to the arrival of the S wave. Record this information in a table of your own with columns for city, lag time, and distance from epicenter.

2. Find the distance from each city to the epicenter of the earthquake. To calculate these distances, use the lag times you found in Step 1, information from the prelab preparation, and the following formula. Distance = measured lag times x  (100 km / calculated lag time)

3. From a copied map which shows the location of the three cities and a scale in kilometers, adjust the compass so that the radius of the circle with Austin at the center is equal to the distance from the epicenter of the earthquake to Austin as calculated in Step 2.  Put the point of the compass on Austin. Draw a circle on your copy of the map.

4. Repeat Step 3 for Bismarck and then for Portland. The epicenter of the earthquake is located near the point at which the three circles intersect.

1. The location of the earthquake epicenter is closest to what city?

2. Why must there be measurements from three different locations to find the epicenter of an earthquake?

3. What is the probability of a major earthquake happening here?

4. If an earthquake did occur in this area what would be its probable cause?

1. Using data from real earthquakes recorded by our seismometer and other PEPP sites students can investigate real world earth science events.

2. There are several operating coal mines in the area. With cooperation of other PEPP sites in our area we may be able to identify mine blasts.

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