Unit III: Finding the Epicenter of an Earthquake

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LAB 5: (20 pts) The purpose of this lab is to help you understand how how the epicenter of an earthquake is determined. Because P and S waves travel at different velocities, The P wave arrives at a seismic station ahead of the S wave. The lag time between them is directly related to the distance that the waves travel. Although the lag time can be used to determine the distance to the epicenter of an earthquake, it does not provide information on the direction from which the waves traveled. Thus, one need information from at least three seismic stations to determine the location of the epicenter.

Instructions for Doing This Lab: Print out this lab so you can work off line. If you want, attend the chat session on the lab after completing as much of the lab as possible. The date of the chat session is posted in the calendar in the Communications Center. To save time online, I suggest that you write your answers out on the printout of the lab.

P Wave
S Wave
Lag Time


  1. The data above is that for an earthquake that occurred in the morning of October 13, 1995. The arrival times P and S waves are provided. To determine the distance to the earthquake epicenter, you first have to calculate the lag time between the arrival of the P and S waves for the three stations. Be sure to express calculated answer in seconds. Enter your calculated lag times into the data table.
  2. Knowing the lag time, you can use the time-travel curve to determine the distance from the earthquake to each station. Print out a copy of the time-travel curve. After completing this tutorial, use the time travel curve to determine the distance of each station from the epicenter of the earthquake.
  3. Print out a copy of a map of the Pacific region. Use a compass and the scale at the bottom of the map to draw arcs around the respective stations. The arcs should have a radius equal to the distance from each station to the earthquake. The three arcs you draw should come close to intersection. NOTE: There are reasons why your arcs may not intersect exactly. First, there is inherent error in any measurement. Second, since maps are projections of a spherical globe onto a flat surface, any map projection introduces distortions in the distances between different points on the Earth. Nonetheless, if they are not within 2 mm of intersecting, check your calculations and your reading of the time-travel curve.
  4. After completing the lab, write a report that addresses each of the questions below. Be sure that your report is in an essay format and that you address each question in your essay.

    a. What conclusion were you able to draw from this lab?

    b. What was the latitude and longitude of your earthquake?

    c. Why was it necessary to draw circles around the seismic stations?

    d. Why was one station insufficient for determining the epicenter of the earthquake?

    e. What was the minimum number of stations that you had to draw to locate the epicenter?

    f. Why didn't the three circles necessarily intersect.

Instructions for submitting this lab:

After answering the questions:

Pick up your graded lab and submit corrections.:

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Updated: August 12, 2003

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