Locating an Earthquake Epicenter Lab

Background: Examining Seismograms
The three basic types of seismic wave generated by an earthquake at its focus are P‐ waves, S‐waves, and Surface waves. P and S‐waves are body waves and travel through the interior of the earth. P‐waves have the greatest velocity and reach the seismic station first. S‐waves arrive at the seismic station after the P‐waves. The amount of time that passes between the P‐wave arrival and the S‐wave arrival is important in helping seismologists determine the epicenter of the earthquake.
Today we will be using a travel‐time graph, where the vertical separation between the P and S curves is equal to the difference in the arrival times between the P‐wave and S‐wave. To accurately locate an earthquake epicenter, records from three seismograms from 3 different stations are necessary. Seismograms are the graphs produced by seismographs.

Instructions:
Here is a visual guide of how to solve the first question for each section. Visit your assignment information page for a video on how to complete this assignment:
Visual Guide of Determining Epicenter Lab Worksheet

Background: Using a travel ­time graph
(5 points)

1. Use Figure 2 (below) to determine the difference in arrival times (in minutes) between the first P‐wave and first S‐wave for stations that are the following distances from an epicenter.

700 miles: _ minutes difference

450 miles: _ minutes difference

2500 miles: _ minutes difference

2. From the seismogram in Figure 1, the differences in arrival times between the first P‐wave and the first S‐waves equal (3, 7, or 10 minutes): _
3. Refer to the travel‐time graph. What is the distance from the epicenter to the station that recorded the earthquake in Figure 1? _ miles

Figure 2: Travel Time Graph

Observations: Measuring P & S Waves
(15 points)
Figure 3 illustrates seismograms from the same earthquake recorded at Los Angeles, CA, St. Louis, MO and Houston, TX. Use this information to answer the following questions.

1. Use the travel‐time graph, Figure 2, to determine the distance that each station in Figure 3 is from the epicenter. Write your answers in the epicenter data table, Table 1. Describe the process you took to determine the distance. (4 points):
2. After you have determined the distance that each station is from the epicenter, use the drawing compass provided to draw a circle around each of the three stations with a radius, in miles, equal to its distance from the epicenter. Use the map of the United States.
   (Note: Use the distance scale provided on the map to set the distance on the drawing compass for each station.
   In Google Docs: Double-click the image of the map below for editing. Then select “Actions” > “Guides” > “Show guides” to bring up guidelines. Drag the guidelines to location of the city on the map. Then draw a circle the correct distance from the epicenter. Remember to click “Save and Close” when you are finished).
   ● Table: 6 points;
   ● Circles on Map: 5 points = 1 point per city (total 3 points) and 2 points overall for accuracy in following distance scale

Table 1. Epicenter Data Table
Los Angeles, CA St. Louis, MO Houston, TX
Elapsed time between first P and S waves (minutes)
Distance from epicenter in miles

Analysis: Determining the Epicenter
(5 points)

1. Did all three of the circles drawn with the compass overlap in one spot? (1 point)
2. This earthquake occurred closest to which city, in which state? (1 point)
3. This earthquake occurred in a well‐known seismic zone. Do some research on your own. What is the name of this seismic zone?
   (1 point)
4. To the best you of your ability, use a pencil, or colored pencil to draw the approximate location of the seismic zone named in question 3. Be sure to label the seismic zone on the map. In Google Docs: use the scribble line tool to shade in the seismic zone. (2 points)

Evaluate
(5 points)

Assume that the same earthquake occurred, but the seismic station in Los Angeles malfunctioned, and did not record the earthquake. With only the information from seismic stations in St. Louis and Houston, brainstorm how a geologist might determine the location of the earthquake epicenter? Explain your reasoning.