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Can you find an earthquake epicenter?
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The epicenter of the 2011 Japan earthquake was just offshore of Sendai where the Pacific Plate plunges into a subduction zone. The quake had a relatively shallow depth of 20 miles (32 km). Remember that shallow quakes typically cause the most damage. How do scientists find an earthquake epicenter?

### Finding the Epicenter

Here are the steps to finding an earthquake epicenter using three seismograms:

1. Determine the epicenter distance from three different seismographs. The longer the time between the arrival of the P-wave and S-wave, the farther away is the epicenter. So the difference in the P- and S-wave arrival times determines the distance between the epicenter and a seismometer.

This animation shows how to determine distance using P, S, and surface waves: http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/12/IRIStravelTime_Bounce_480.mov .

2. Draw a circle with a radius equal to the distance from the epicenter for that seismograph. The epicenter is somewhere along that circle. Do this for three locations. Using data from two seismographs, the two circles will intercept at two points. A third circle will intercept the other two circles at a single point. This point is the earthquake epicenter (
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Figure
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below
).

Three circles drawn from three seismic stations each equal to the radius from the station to the epicenter of the quake will intercept at the actual epicenter.

Seismic stations record ten earthquakes in this animation: http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/12/TravelTime_Sphere_10Stn_480.mov .

Of course, it's been a long time since scientists drew circles to locate an earthquake epicenter. This is all done digitally now. but it's a great way to learn the basics of how locating an epicenter works.

### Summary

- To find an earthquake epicenter you need at least three seismographs.
- Find the distance from each seismograph to the earthquake epicenter.
- The interception of the three circles is the epicenter.

### Practice

Use this resource to answer the questions that follow.

1. What is a seismogram?

2. What waves does a seismogram show?

3. What is the S-P interval?

4. How many data stations are required to determine the epicenter?

5. What is triangulation?

### Review

1. How do you determine the distance from the seismograph to the earthquake epicenter?

2. How do you find the epicenter from three seismographs? What if you have more seismographs involved?

3. In what circumstance would three seismographs not give you enough information to find an earthquake epicenter?