Locating Earthquake Epicenters: A Comparison of Two Different Methods

Locating Earthquake Epicenters: A Comparison of Two Different Methods

William Baker (Carmel H.S., Indianapolis, IN),

Ewa Shannon (Crown Point H.S., Crown Point, IN), and

Jan Weaver (Culver Academy, Culver, IN)

Background

Since adoption of the Indiana education Science Standards, students are required to comprehend the relationship between the location of earthquakes based on tectonic plate motion (Standard ES1.23). The Physics Standards include the behavior of waves. Specifically, Physics standard 1.25 states that students “use the concept of reflection, refraction, polarization, transmission, and absorption to predict the motion of waves through space and matter”.

Many students are exposed to the triangulation method for determining an earthquake epicenter by using selected seismograms for three seismograph stations. By calculating the time interval between the P and S waves, using a time-distance travel graph, and drawing three circles at the calculated distance from each station, the single point at which all three circles intersect on the map identifies the epicenter.

There are a number of computer-generated activities that guide students through both an understanding of earthquake waves and locating an earthquake epicenter. This lesson proposes to build on the time-honored, above-described manual triangulation method and comparing it to the determination of an epicenter using computer models.

This activity is intended to proceed through four class days (50 – 60 minute class periods). On the first day, students will be introduced to the “Virtual Earthquake” activity that is available on the internet. This program instructs students to pick the P and S wave and determine the time difference between the arrival times of the two waves. Using these time intervals, the student is instructed to determine the location of the epicenter using the triangulation method. On the second day, students will explore the method of triangulation using real data/seismograms from PEPP archives. On the third day, students will return to the computer lab. Using the same PEPP seismograms used on day two, the students will use a more realistic method of determining the epicenter by using the computer program SWAP.

The fourth day of this unit will be used to assess students’ understanding of the methods of epicenter location. A second earthquake data set will be selected from PEPP archives and students will be directed to determine the epicenter using either the triangulation method or SWAP. (Variations may be used here. For example, you could use a team approach and have 2 or more students work together and use both methods as a check and balance system to improve accuracy.)

Indiana Standards

ES.1.23 Explain motions, transformations, and locations of materials in the Earth’s lithosphere and interior. For example, describe the movement of the plates that make up the crust of the earth and the resulting formation of earthquakes, volcanoes, trenches, and mountains.

Note to Instructor: Data Set 1 is from the January 17, 1994, earthquake in Northridge, CA. Data Set 2 is from the February 28, 2001, earthquake in Seattle, WA. Data Set 2 uses PEPP data.

Activity 1: Virtual Seismology

Background

One method of locating an epicenter is through the time-honored method taught in most earth science classes called triangulation. Triangulation requires that you determine the exact arrival time of the P and S waves produced by an earthquake for at least three different seismograph stations. Since P waves travel roughly twice as fast as S waves, the further the earthquake’s epicenter the greater the time difference between the two arrival times. Plotting the difference between arrival times at a specific seismograph station on a travel-time graph will determine the distance to the earthquake. Drawing a circle from each of the three stations (the radius of each circle is equal to the distance to the epicenter from that station) will determine the location of the epicenter. The three circles should intersect at one point which is the epicenter.

Before actually using real data in Activity 2, you are going to do a similar activity on an internet site called Virtual Earthquake. This program will guide you through locating P and S waves, using a Travel-Time Graph, and locating the epicenter using triangulation. It will also guide you through determining the magnitude of an earthquake using one method that can be used to make a preliminary determination of an earthquake’s magnitude. Be very accurate in picking the arrival times and in making your calculations.

1. Launch Internet Explorer or Netscape Navigator and type in the following location –

http://www.indiana.edu/~pepp . This will take you to the Indiana PEPP website.

2. Scroll down through this page until you come to the link to The Virtual Earthquake. Click on this link.

3. Be sure to read everything because there is a lot of important information that you need to understand as you do the activity.

4. When you get to the website, you will have a choice of 4 earthquakes. Select the first one

(San Francisco Area) on which to practice. Take your time and work accurately.

5. When finished with the San Francisco earthquake, answer the questions below.

6. Upon finishing the questions, select one other earthquake (Southern California, Japan region, or Mexico). When finished with this earthquake, print out the certificate and hand it and the answers to the questions in to your teacher.

Activity 1 Questions for “Virtual Seismology”

Name ____________________________

1. From where do seismic waves originate?

Why?

What is the epicenter of an earthquake? (You may have to rely on prior knowledge to answer this question.)

2. P and S waves are the seismic two waves we are interested in for this activity. List some differences between these two waves.

* wave motion: P –

S –

*material it travels through: P –

S –

*velocity: P –

S –

3. Which seismic waves arrives first at a seismic recording station?

4. What are seismograms?

How are the seismograms depicted in this activity different from real seismograms?

5. In your own words, list the steps needed to locate the epicenter of an earthquake using triangulation?

6. Why should we not expect the method of triangulation to result in an exact point?

7. Approximately how far from San Francisco was the epicenter of the earthquake?

8. What is the magnitude of an earthquake?

9. What is needed from the seismogram of an earthquake to determine it’s magnitude?

10. How is the maximum amplitude of the seismic waves determined?

11. Using the nomogram, what is the magnitude of the earthquake?

Activity 2: Using Triangulation to Locate Earthquake Epicenters

INTRODUCTION

The study of earthquakes allows the geologist to study the interior of the Earth. Most drilling methods have only obtained samples from the upper ten kilometers of the Earth in an attempt to secure data relative to the composition of the Earth. The point on the Earth’s surface directly above the origin or focus of an earthquake is called the epicenter. Each earthquake produces two types of shock waves that travel through the Earth: Primary, or P-waves, and Secondary, or S-waves. The primary waves are the first waves to arrive at a seismic station and move approximately1.7 times faster than S-waves. The time interval can be measured between the P-waves and the S-waves. Triangulation can be used to determine the exact location of the epicenter when the distance between three different locations is identified.

OBJECTIVES

Seismograms are used to determine relative distances to earthquake epicenters.

Seismic travel time curves are used to determine the actual distance to epicenters.

Triangulation is used to locate epicenters

MATERIALS

Seismograms for three seismic stations; Seismic Travel Time Graph; drafting compass;

metric ruler; projection map and scale

PROCEDURE

1. Determine the location of the P-waves and S-waves using the seismograms for

the three seismic locations. Which seismic station is the closest to the earthquake

epicenter?

2. Determine the distance from each seismic station to the epicenter using the

following:

a. Examine the seismogram from Corvallis, OR, and determine in seconds the time interval between the P-waves and the S-waves.

b. Locate the time scale on the Travel Time Graph. Place the edge of a

piece of paper along this scale. On the paper mark off the distance obtained from the time interval.

c. Slide the paper to the right on the graph until the marked off distance fits

between the P- and S-wave curves and is parallel to the time axis. (Be precise in your measurements!)

d. The distance from the epicenter to the first location can be determined

by reading the value on the horizontal axis directly below the edge of

the paper.

e. Repeat the process for the other two locations.

Corvallis, OR _____________km Tucson, AZ ____________ km

Harvard, MA ____________ km

f. Using the map scale, take a drafting compass and set it to the appropriate length

for the distance from the first location to the epicenter. Place the compass point

at this location and draw an arc using the distance as the radius. Repeat for the

other two locations. The intersection of the three arcs identifies the epicenter

of the earthquake. Attempt to locate the latitude and longitude of the earthquake.

Latitude _______________ Longitude _______________

Activity 3:Using SWAP to Locate a Teleseismic Earthquake

When an earthquake is detected at your station you may be able to determine information such as distance and magnitude from your record, but not be able to determine the location of the epicenter. In order to do that you must have data from three stations, not too close together, and use a process called triangulation. You did this in Activity 1 with the Virtual Earthquake website and in Activity 2 with seismograms from an actual earthquake.

The purpose of this exercise is to show you how you can use SWAP to analyze a data set and determine an earthquake’s location. The method used in this activity is more like the actual method used by seismologists in locating the epicenter of an earthquake. In this method, you only need to determine the P-wave arrival time for several seismic stations. You will again be using data from only 3 seismic stations.

If the earthquake set is on a disk, insert the disk into the computer.
Open SWAP. When the screen appears, click . A window will appear in which you can select the location of the data set to be used: disk, hard drive, or desktop. Select the appropriate location. (The location will be determined by your instructor). The data set you will be using is called Data Set 1.
The top seismogram will be highlighted. Click the icon. This will open another screen with the selected seismogram across the main window. Depending on which version of SWAP you are using, the window should look like this:

There are 3 icon buttons to the left and below the top window. One is the letter P, another is a picture of calipers, and the third is a picture of a magnifying glass. The tool is used to pinpoint a point on the seismogram; this is used to mark where you think the P-wave arrival might be. The caliper tool is used to measure the wave amplitude and period (duration/time) of selected waves on the seismogram. The magnifying glass tool is used to "blow up" a portion of the seismogram by clicking and dragging the cursor across a selected area of the seismogram.

Directly above the 3 icons is a scroll menu. By pointing and clicking on the arrow, you can choose which previously-loaded seismogram you want to analyze.

The smaller window below the main window will display the area you have chosen with the magnifying glass tool.

4. Click on the magnifying glass icon. Move the cursor to an area above and to the left of where you suspect the P-wave arrival to be. Click and drag downward and to the right. A dotted-line box will show the area you have chosen. Release the mouse button; the area chosen will appear in the smaller box toward the bottom of the screen, magnified. This is your selected area. You may repeat the use of the magnifying glass symbol to further enlarge a portion of this selected area.

5. After you have magnified the area enough to select a P-wave arrival, click onto the P icon. This will generate a cursor of a P with an arrow below it. Move this cursor to the point of suspected P-wave arrival. As you move this cursor, note that the time (to a thousandth of a second) is readable in the box to the right of the selected area. Click on the selected point; a vertical line will appear on both the lower and upper seismograms. If not satisfied with this point, you may re-select and click on any other point. (Be accurate!)

6. Go to the small scroll menu below the top seismogram. Scroll to the next station's seismogram, release the mouse button, and that seismogram will appear in the upper box. Repeat the P-wave selection procedure described above for this seismogram.

7. Continue this procedure until you have chosen P-wave arrivals for each seismogram in the set.

Question: Which seismic station was closest to the epicenter?

Question: Which seismic station was farthest away from the epicenter?

8. Click on the Return bar at the bottom of the page and you will go back to your original screen. You should notice that vertical lines are now at the selected P-wave arrivals for the seismograms. If one or more arrivals are missing, highlight that seismogram, and press to go back to pick that arrival.

9. If all P arrivals are chosen, click the button. You will get a new screen with a globe and your selected seismograms in a table below it, like this:

In the Upper right are 4 buttons with icons. The 4 inward-pointing arrows are for centering your view of a location, the X with a ? is for selecting a potential epicenter, the magnifying glass with a "+" is for zooming out, and the magnifying glass with a "-" is for zooming in.

10. Under "map projection", click on local. Click the inward-arrow icon . Move the cursor to the globe and click onto the edge of the globe. It will rotate the globe. Do this until you can see your suspected location. Click on that location, which will center it into view.

11. Click the X? icon . Move the cursor to a suspected epicenter and click on that location. A square will appear; its color will tell you how close you are to the actual epicenter. If it is white, you are close; the farther away you click, the color of the square will change according to the scale at the lower left of the screen. After each estimated location the program will display your best choice as a green dot. When you have a green dot surrounded by white squares, that is your best location, based on your P-wave selections.

At this point, you may want to expand your view of the location by clicking on the Magnifying glass icon , then clicking on top of your green dot.

12. You will also notice in the window to the right above the CLEAR WINDOW button, a “Best Guess” number and a “Last RMS” number. The closer the “Best Guess” number is to “0”, the closer you are to the epicenter. The “Last RMS” number shows you if you are getting close to the epicenter or not as the number gets smaller or larger.

Question: What is your Best Guess number?

Question: What is the latitude and longitude of the earthquake?

Lat. ________________ Long. ___________________

Question: How close was this to the Triangulation method used in Activity 2?

Assessment

Locating Earthquake Epicenters: A Comparison of Two Different Methods

Now that you have had an opportunity to experience both methods of epicenter location, it is time to evaluate your proficiency. Your instructor will give you data for an earthquake that occurred on February 18, 2001. You will have the choice to use the triangulation method as in Activity 2 or the SWAP method as in

Activity 3.

Assessment A: Triangulation

Materials

Seismograms for three seismic stations; Seismic Travel Time Graph; drafting compass;

metric ruler; projection map and scale.

Data

Distance from Seismic Stations: Muncie _________________ km

Rehoboth _________________ km

Tulsa _________________ km

Epicenter Location

Latitude ___________________ Longitude ______________________