Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Using scissors or a paper cutter, cut apart the Time Delay Scales.
2. Obtain a transparency of the Finding the Epicenter Seismic Waves Graph and cut out a transparent Time Delay Scale found above the graph.
3. Photocopy enough United States maps for each group of students and make one transparency.

Safety Precautions

The compasses included in this kit are considered safe. Please follow all classroom safety guidelines.

Lab Hints

• Enough materials are provided in this kit for 30 students working in groups of three, or for 10 groups of students. This laboratory activity can reasonably be completed in one 45- to 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the questions may be completed the day after the lab.
• The graph transparency may be used to guide students in reading the Seismic Waves Graph and using the Time Delay Scale. The U.S. map transparency may be used to guide students in measuring and drawing with a compass as well as to allow students to check their results.

Teacher Tips

• This is a great activity to use during a study of plate tectonics and earthquakes or a study of waves.
• The epicenter in this activity is in the area of the New Madrid Fault Zone that extends from northeast Arkansas into southern Illinois. This is an active fault, averaging over 200 measurable earthquakes annually. The New Madrid Earthquakes (a series of more than 2000 quakes) occurred during the winter of 1811–1812 and are believed to be the greatest earthquakes ever experienced in the contiguous United States. Based on eyewitness accounts and geologic disturbances, seismologists estimate at least five of the quakes had a magnitude of 8.0 on the Richter scale. One of the quakes caused the Mississippi River to change its course, and the vibrations caused church bells to ring as far away as Boston on the East Coast.
• Often a loud rumbling sound is heard by people when experiencing a major earthquake. This rumbling sound is produced when P-waves reach the Earth’s surface and the vibrations continue into the air, creating low-frequency sound waves. S-waves do not create this sound, because they cannot travel through fluids such as air.
• Visit the U.S. Geological Survey website http://earthquake.usgs.gov/learning/kids (accessed March 2008) to see where earthquakes have occurred recently around the globe.
• A Slinky^®, available from Flinn Scientific (Catalog No. AP1957), may be used to demonstrate compression and transverse waves.
• The “Mapping Earthquakes and Volcanoes” Student Laboratory Kit available from Flinn Scientific (Catalog No. AP6881) may be used to explore the concept of earthquakes and their relationship to volcanic activity and plate tectonics.

Answers to Prelab Questions

1. Compare and contrast primary and secondary waves.

   Primary waves are compression waves in which the rocks move back and forth in the same direction the wave travels. Secondary waves are transverse waves in which the rocks move perpendicular to the direction of the wave. P-waves travel faster than S-waves. Both originate at the focus of an earthquake and travel through the body of the Earth.

2. Why must data be obtained from at least three seismograph stations to locate the epicenter of an earthquake? Hint: Draw two intersecting circles (like a Venn diagram) and see how many points of intersection are formed.

   Knowing the distance from one station to the epicenter is not enough because direction is not known—the epicenter could be at any point on a circle with the station at the center. Two overlapping circles have two points of intersection. Data must be obtained from at least three stations since three overlapping circles have only one common point of intersection.

   {12709_PreLabAnswers_Figure_7}
3. Vibrations from a primary wave were detected on a seismograph at 2:04:54 p.m. The secondary wave arrived at 2:09:13 p.m. How much time in seconds elapsed between the two arrival times?

   259 seconds.

Sample Data

Answers to Questions

1. Near what major city is the epicenter located? (Look at a more detailed map of the U.S. if necessary.)

   St. Louis, MO

Use the Seismic Waves Graph to answer Questions 2 and 3.

2. A seismograph station is 3000 kilometers away from the epicenter of an earthquake. How many seconds after the arrival of the P-wave would the S-wave arrive?

   260 seconds

3. What happens to the distance between the P-wave line and the S-wave line as the distance from the epicenter increases? Why is this so?

   As the distance from the epicenter increases, the distance between the P-wave and S-wave lines also increases. This happens because P-waves travel faster than S-waves. The greater the distance the waves travel, the greater the difference between their arrival times will be.

4. Describe the difference between the focus and the epicenter of an earthquake.

   The focus is the point within the Earth where rocks shift or break along a fault, causing an earthquake. The epicenter is the point on the Earth’s surface directly above the focus.

5. Why is useful to know the location of the epicenter once an earthquake has occurred?

   Knowing the location of an earthquake’s epicenter helps seismologists track patterns and trends of rock movement along various faults. This knowledge may eventually lead to better earthquake predictions. If an earthquake occurs under the ocean, tsunami warnings may be issued.

Teacher Handouts

12709_Teacher1.pdf

References

National Atlas of the United States^®, http://nationalatlas.gov/index.html (Accessed February 2008).

Introduction

On October 17, 1989, as the Oakland Athletics and San Francisco Giants were warming up in Candlestick Park for the third game of the World Series, a major earthquake struck the area. Seismologists, individuals who study earthquakes, determined the origin of the quake was located near Loma Prieta Peak in the Santa Cruz Mountains. Discover how the “starting point” of an earthquake is determined.

Concepts

• Earthquakes
• P-waves versus S-waves
• Epicenter
• Seismic waves

Background

The rocky plates that make up the Earth’s crust are in constant motion. The interactions of these plates create faults, or cracks, that offset the Earth’s crust. Continuous movement of the plates builds up pressure until the rocks along a fault shift or break, releasing energy that causes an earthquake. This is similar to what happens when you snap your fingers. The force between your fingers increases until the fingers suddenly slide past each other. The “snap” is caused by the release of energy in the form of sound waves. Energy from an earthquake is transmitted through the Earth in the form of vibrations known as seismic waves (from the Greek word seismos, to shake or quake).

Two types of seismic waves travel outward from the focus (origin within the Earth) of an earthquake. The primary wave, or P-wave, is a compression wave that forces rock to compress and expand in the same direction the wave travels (see Figure 1). P-waves travel through the Earth at an average speed of about 5 kilometers per second. Secondary waves travel at a slower rate, averaging about 3 kilometers per second.

Secondary or S-waves are transverse waves in which the vibrations displace matter perpendicular to the direction the wave is moving (see Figure 2). Primary and secondary waves are called body waves since they travel through the body of the Earth. Once these vibrations reach the Earth’s surface, the energy is transmitted as surface waves. These waves travel more slowly than body waves and cause the most destruction as the earth moves up and down, like an ocean wave, and also from side to side. The epicenter of an earthquake is the point on the Earth’s surface directly above the focus (see Figure 3). Knowing how seismic waves travel enables seismologists to determine the location of the epicenter of an earthquake. Vibrations from seismic waves are detected by instruments called seismographs and recorded on seismograms all over the world. The faster P-waves are detected first, followed by the S-waves (see Figure 4). The greater the delay between the arrival times of the two waves, the farther the waves have traveled. Think of two runners on a track, where one is running consistently faster than the other. The distance between the two runners will gradually increase as the race continues. Once the delay time between the P-wave and S-wave is known, the distance the waves have traveled can be determined using a graph. The direction from which the waves traveled, however, is unknown until data is collected from at least three seismograph stations. Circles are drawn around each station on a map, with the radius of the circle representing the distance the waves have traveled from the epicenter. The intersection of the three circles marks the epicenter of the earthquake.

Experiment Overview

The purpose of this activity is to locate the epicenter of an earthquake that occurred somewhere in the United States at 5:48 a.m. The time delay between the arrivals of the P- and S-waves at various seismograph stations around the country will be calculated and the distance from the epicenter will be determined for each station.

Materials

Prelab Questions

1. Compare and contrast primary and secondary waves.
2. Why must data be obtained from at least three seismograph stations to locate the epicenter of an earthquake? Hint: Draw two intersecting circles (like a Venn diagram) to see how many points of intersection are formed.
3. Vibrations from a primary wave were detected on a seismograph at 2:04:54 p.m. The secondary wave arrived at 2:09:13 p.m. Determine how much time in seconds elapsed between the arrivals of the two waves.

Safety Precautions

The materials used in this activity are considered safe. Please follow all classroom safety guidelines.

Procedure

Part A. Calculating the Time Delay and Distance

1. Using the data from the Find the Epicenter Worksheet, calculate the difference in time (TS-P) between the arrival of the P-wave and the S-wave for each city. Record the time delay in seconds in the data table.
2. Obtain a card with the time delay scale. Carefully fold the card back on the dotted line. Crease the fold.
3. Note the delay time for the first seismograph station, New York.
4. Using a pencil, make a small mark on the time delay scale corresponding to the delay time for New York (see Figure 5).
   {12709_Procedure_Figure_5}
5. Obtain a Seismic Waves Graph. Place the time delay scale along the y-axis (Time) of the graph, matching the zero point on the time delay scale with the zero on the graph.
6. Slowly move the time delay scale along the curved line representing the P-wave data, keeping the scale vertical and the zero point of the scale on the P-wave line.
7. Stop when the pencil mark on the time delay scale reaches the curved line representing the S-wave data. Make sure the time delay scale is straight vertically, the pencil mark is on the S-wave line and the zero point is on the P-wave line (see Figure 6).
   {12709_Procedure_Figure_6}
8. Follow the vertical edge of the time delay scale down to the x-axis (Distance) on the graph. This point represents the distance from the epicenter to the seismograph station for that particular time delay. In Figure 6, the distance is approximately 780 km.
9. Record the distance to the nearest hundred kilometers in the data table on the worksheet.
10. Repeat steps 3–9 using the data for each seismograph station.

Part B. Locating the Epicenter

11. Obtain a map of the United States. Note the map scale (1 cm = 200 km). Convert each distance recorded on the data table in kilometers to centimeters by dividing the distance in kilometers by 200. Record each value as “map distance” in the data table.
12. Choose one seismograph city on the map and obtain a drawing compass.
13. Using the scale on the map or a metric ruler, set the compass to the proper radius in centimeters. Note: The radius is the distance from the epicenter to the seismograph station.
14. Placing the point of the compass on the selected city, lightly draw a circle in pencil around the city, being careful to keep the compass set at the proper distance. Hint: Depending on the type of compass, it may be easier to hold the compass still and rotate the map.
15. As a check, measure the radius of the drawn circle to see if it is the same as the distance recorded in the table. If not, erase the circle, adjust the compass, and draw the circle again.
16. Repeat steps 12–15 for at least two more stations.
17. Circle the area on the map where the circles from the three cities intersect. Label this area “Epicenter.” Note: Three or more circles may not intersect at precisely one point; however, they should cluster together in a small area.
18. Answer the Post-Lab Questions.

Student Worksheet PDF

12709_Student1.pdf