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Earthquake Epicenter

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The purpose of this activity is to use real earthquake data to find the epicenter of the aftershock from the El Mayor earthquake.


K-12 Earthquake Activity Teaching Modules


In the spring of 2011, NEES at the University of California Santa Barbara (NEES@UCSB) embarked on a project to develop a comprehensive set of teaching modules for K-12 students that would cover the basics of plate tectonics and earthquake dynamics. The idea for the project grew from the success of the "Make Your Own Earthquake"; outreach activity developed by NEES@UCSB, which recently has included the use of the Quake Catcher Network MEMS accelerometer.


Over the course of the summer of 2011, the students met weekly with their mentor and the science teacher. In August, a group of local 4th &ndash; 6th grade students came to the UCSB campus and tested several of the earthquake activities. The summer interns presented their work at the NEES REU Young Researchers Symposium at UCSB in August and at the annual SCEC meeting in Palm Springs in September.

The 12 earthquake activity modules are summarized below:


Jamison Steidl, Ph.D., Principal Investigator, NEES@UCSB

Summer Undergraduate Interns:

Sean Allen, Civil Engineering, University of Nevada, Reno
Heidi Pence, Civil Engineering, University of Michigan
Joseph Trudeau, Geology, University of Wisconsin
Hanna Vincent, Mechanical Engineering and Materials, MIT

Earthquake Activity Modules:

6th - 8th Grade: Earthquake Epicenter, Sean Allen


Earthquake Engineering Component

Learning Objectives and Standards



California Science Standard for the 6th Grade

1. g. Students know how to determine the epicenter of an earthquake and know that the effects of an earthquake on any region vary, depending on the size of the earthquake, the distance of the region from the epicenter, the local geology, and the type of construction in the region.

Material List

Materials Needed:

  1. Seismograms and location map
  2. Drawing Compass
  3. Ruler
  4. Calculator



  1. Choose a station. On the included seismograms, use the Vertical graph to mark the arrival of the P-wave and the East or North graph to mark the arrival of the S wave.
  2. Use the scale below the graphs to count the time between, ∆t (sec), the start of the P-wave and S-wave for each seismogram.
  3. Multiply the difference in the arrival time, ∆t (sec), by a velocity, v of 7 km/s. This will give you a distance, d (km).
  4. Repeat this step for each station.
  5. Using the distance scale at the bottom of the map, open the compass to the distance, d, obtained in Step #3.
  6. Using the appropriate station location as the center, draw a circle with the compass.
  7. The location where the three circles intersect, or come close to intersecting, is the location of the epicenter.
  8. Mark the epicenter on the location map.


  1. P-wave: type of seismic body wave. The P stands for primary, because it has the highest velocity and is therefore the first to be recorded by a seismogram.
  2. S-Wave: a type of seismic body wave. Its name, S for secondary, comes from the fact that it is the second arrival on an earthquake seismogram after the P-wave, because S-waves travel slower.
  3. Epicenter: The epicenter is the point on the Earth's surface that is directly above the point where an earthquake originates.

The Math:

The difference between the arrival time of the P-wave, tP, and the arrival time of the S-wave, tS, can be expressed as ∆t = tS - tP

The difference between the velocity of the P-wave, vP, and the velocity of the S-wave, vS, can be expressed as ∆v = vS – vP

Note: a typical approximation for ∆v = 7 km/s.

To find the distance we will use the formula distance = rate x time. Substituting the variables from above gives us the formula: d = ∆v x ∆t. Using this formula will produce the distance away from the sensor in km.



Triangulation uses the three seismometers as centers of three different circles. Each circle has a radius that matches the distance measurement from the seismometer to the epicenter. By drawing three circles, each around a seismometer, the point where the circles intersect is the epicenter of the earthquake. In general, using this method to determine an earthquake's epicenter may not result in a precise point.

Links and Resources


The 2010 El Mayor was an earthquake of magnitude 7.2 on the moment magnitude scale. It started 26 kilometers (16 mi) south of Guadalupe Victoria, Baja California, Mexico, on Easter Sunday, April 4, 2010, and it is said to have lasted about a minute and a half. The quake probably occurred on the Laguna Salada Fault, which is about 60 kilometers (37 mi) to 80 km (50 mi) long and straddles the California–Baja California border. The active Laguna Salada Fault ruptured in February 1892 with an estimated Mw 7.2 earthquake. The quake was widely felt throughout the Western United States, and some Southern zones, and Northwest Mexico. The earthquake was the strongest to rock Southern California in at least 18 years.

A strong aftershock with a magnitude of 5.7 occurred on June 14, 2010, with an epicenter near Ocotillo in southwestern Imperial County. On the shake map below, the approximate location of the aftershock is marked by the red star (Figure 1). Because of the magnitude of this aftershock, it produced its own series aftershocks.


Figure 1: Location of 5.7 aftershock

The University of California Santa Barbara, maintains several different earthquake monitoring locations. Three of those locations, or stations, are Borrego Valley Downhole Array (BVDA), Garner Valley Downhole Array (GVDA), and Wildlife Liquefaction Array (WLA). Each of these stations has a many seismometers located in the area, and each of these seismometers has a north-facing, an east-facing, and a vertical component. These three different components make it easier to distinguish the P & S wave arrival times.

Web Resources:

Earthquake History



Original Activity

Station Information

Learning modules in this series:


Why is it necessary for three stations to be used in order to find the epicenter of an earthquake?

What would happen if fewer than three stations were used? Or if more than three stations were used?



Cite this work

Researchers should cite this work as follows:

  • Sandra Seale; Sean Allen; NEES EOT (2011), "Earthquake Epicenter,"

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