How Do We Know Where an Earthquake Originated?

Jeffrey Barker (Binghamton University) & Michael Hubenthal (IRIS)


Students use real seismograms to determine the arrival times for P and S waves and use these times to determine the distance of the seismic station from the earthquake. Seismograms from three stations are provided to determine the epicenter using the S – P (S minus P) method. Because real seismograms contain some "noise" with resultant uncertainty in locating arrival times of P and S waves, this activity promotes appreciation for uncertainties in interpretation of real scientific data.

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This is most appropriate for a secondary school classroom and possibly introductory college level.

Skills and concepts that students must have mastered

Learners should be familiar with plate tectonics and seismic P and S waves. Activities that will help learners with the background knowledge for this one are Human Wave: Modeling P and S Waves and Seismic Slinky. Learners should also have basic map reading skills and know latitude and longitude. A helpful preparatory exercise to gain the needed map skills and also learn about earthquake magnitude is Plotting Earthquake Epicenters Activity (Acrobat (PDF) 3.6MB Sep10 22).

How the activity is situated in the course

This exercise would probably be best midway or late in a unit on plate tectonics and earthquakes.
It could also be done as part of a physical science unit as an applied example of wave types and characteristics.


Content/concepts goals for this activity

Learners will be able to locate the epicenter of an earthquake by triangulation.

Higher order thinking skills goals for this activity

Analyze real seismograms to determine P and S waves arrival times, calculate the distance of the epicenter from each seismic station, and determine a latitude and longitude of the earthquake epicenter.

Other skills goals for this activity

Practice with latitude and longitude on a map.

Description and Teaching Materials

See attached file for instructor notes, NGSS alignment, links to supporting resources, and student exercise.
How Do We Know Where an Earthquake Originated? Activity (Acrobat (PDF) 5.7MB Sep10 22) 

Teaching Notes and Tips

  • The activity requires mathematical compasses.
  • The terms "triangulation" and "trilateration" can be confused. Technically, triangulation is using angles or compass bearings from three different locations to determine the point of interest. Trilateration is using the distance from three different locations to determine the point of interest. Technically than, the process of locating an earthquake epicenter is trilateration although certainly you will see it called triangulation too. The exercise uses the term loosely.
  • You may need to spend a little time coaching the students on how to read a seismogram. Some students may find it a bit challenging to handle the uncertainty of making estimations from real data.
  • In reality, determining earthquake location and rupture characteristics is much more complicated than is depicted here. Nonetheless, this exercise gives students an introduction the basic premise for how it is done.


Formative assessment of student understanding can be gathered from classroom observation and discussions with individuals or small groups.
The student exercise serves as the summative assessment for the activity. The questions have clearly correct answers.

References and Resources