N.Z. Version, 1998
Class time required: 2 class sessions, if all student groups have access to computers.
In a major earthquake, people at the earthquake's center (epicenter) can feel, and sometimes see, the earth move strongly. People further away may feel the motion too, although less strongly. The movement travels away from the epicenter, spreading out in waves and becoming smaller and weaker as they travel. Scientists have constructed devices called seismometers that can detect and record these motions even when they are far too small for humans to feel. You will start by thinking and reading about ways in which the earth might move and by looking at records of earthquakes (seismograms). By the end of this project, you will be able to locate an earthquake's epicenter by interpreting seismograms from around the world, and you will be able to choose a research question to investigate on your own.
Objectives:
To view a variety of seismograms to become familiar with the shape and sequence of typical wave traces; to relate wave traces to existing knowledge, beliefs, and experiences of earthquakes.
You will need:
Look at images of the motion of the surface of the earth during an earthquake.
Also, if you have experienced an earthquake first-hand (or if you've heard
stories from someone who has), describe the kinds of motion that you felt.
These motions at the earth's surface are caused by motions deep in the
earth. Brainstorm the kinds of motion that may take place in the earth
during an earthquake: In what direction(s) do you think the earth moves?
Do you think the motion is contained in one place? Finally, what do you
know about how scientists record earthquakes?
Note
1 to Teachers
For background, read the Seismic Waves page in the Learning Library. The motions caused by earthquakes travel away from the earthquake's center and get smaller and weaker as they go. While the size of the motion at the epicenter may be on the scale of centimeters or even meters, the motion recorded by distant seismographs is on the scale of micrometers, or millionths of meters!
Now look over the set of seismograms recorded during several earthquakes, keeping an eye out for any patterns. Read the descriptions of the earthquakes that caused the motion recorded in the seismogram. Also take note of the quantities and units that are displayed on each axis. One is vertical ground displacement or the distance the ground moved up and down, measured in microns, which are the same as micrometers or millionths of a meter. (Figure 7 also shows ground displacement in additional directions.) Later you will see seismograms that display ground velocity instead, in microns/sec. (Do not reveal the arrival times until you have completed activity 3).
Click
here to see the same seismogram with arrival times shown.
This seismogram shows the recording of vertical motion of the earth in Kevo, Finland, beginning 500 seconds (8.3 minutes) after an earthquake occurred in Loma Prieta, California, near San Francisco. This 1989 quake, with a magnitude of 7.1, caused the collapse of a highway, crushing many people, and also postponed the World Series because of damage to Candlestick Park.
Seismologists say that the distance from the epicenter in California to the seismometer in Finland is 71° . It means that if you use the center of the earth as the vertex, the angle between the epicenter and the seismograph is 71°. A point on the opposite side of the globe from the epicenter would be at a distance of 180° . Seismologists often use degrees as a measure of distance between two points on the earth's surface. There are about 111 km per degree.
Click
here to see the same seismogram with arrival times shown.
This seismogram was recorded after an earthquake in Northridge, California on January 17, 1994. This magnitude 6.7 quake seriously damaged the freeways around Los Angeles, disrupting the heavy commuter traffic. It was recorded 35° away, in Fairbanks, Alaska.
How is this seismogram similar to and different from Figure 1? How do the shapes compare? Look at the scales on the axes in each seismogram. Which shows larger displacements? Which shows a greater period of time?
Click
here to see the same seismogram with arrival times shown.
These three seismograms were recorded following a magnitude 6.3 quake in Columbia on January 19, 1994 at 15:05:03 UT. The top seismogram was recorded in Albuquerque, New Mexico, 43° from the epicenter. The middle seismogram was recorded in Ankara, Turkey, 99° from the epicenter. The bottom seismogram was recorded on Ascension Island, part of the British colony of St. Helena in the South Atlantic, 60° away from the epicenter.
Note that the ground-displacement scales are different for the three seismograms. Which one shows the largest displacements? The smallest? Are these differences related to the distance from the epicenter? Why or why not?
All three seismograms are shown on the same time axis, so you can compare when the motions began in the different locations. Can you propose any reasons for why the motions begin at different times?
Click
here to see the same seismogram with arrival times shown.
These two seismograms were recorded after the same Northridge, California quake as shown in Figure 2. Both these seismograms were recorded much closer to the epicenter than those in Figures 1 through 3. For greater precision, seismologists usually use kilometers rather than degrees to measure distances less than 10° (or 1110 km). The top seismogram, recorded in Columbia, California, was recorded 455 km from Northridge, while the bottom seismogram was recorded in Albuquerque, NM, 1110 km from Northridge.
Note the units on the ground-displacement scale. While Figures 1, 2 and 3 showed displacement in microns, these use millimeters. How big are these displacements compared with those in Figure 2, from the same quake? Why?
Click
here to see the same seismogram with arrival times shown.
Here are more seismograms of the same Northridge quake, recorded far away in Spain (85° away) and China (90° away). Note that these seismograms show time in thousands of seconds, rather than hundreds of seconds. What does that tell you about the motion that reaches these distant seismometers, compared to those in California and New Mexico? Also note that the displacement scales are in microns.
Here, one seismogram recorded far from Northridge and one recorded close to Northridge are shown on the same axes for comparison. The Albuquerque seismogram from Figure 4 and the Spain seismogram from Figure 5 are placed together, with time shown in thousands of seconds and displacement shown in microns. Compare the timing, sizes, and shapes of the two seismograms.
Click
here to see the same seismogram with arrival times shown.
This figure contains the same seismograms as the previous one, but the Spain seismogram's displacement has been magnified by a factor of ten, so that you can see the details of its shape. What similarities and differences between the two seismograms are apparent now?
Click
here to see the same seismogram with arrival times shown.
These seismograms are from the same quake as in Figure 3. All the seismograms were recorded in San Pablo, Spain, 70° away. Rather than measuring only the vertical ground displacement, the San Pablo seismic station also measured the forward-and-backward and side-to-side ground displacement. Here, the top seismogram shows vertical displacement. The middle seismogram shows radial displacement, along an imaginary line drawn on the earth's surface, from the epicenter to the seismic station. The bottom seismogram shows transverse displacement, side to side, perpendicular to the vertical and radial displacement.
How do you think these seismograms were recorded?
What similarities do you see in the shapes of the various seismograms?
What do you think causes these shapes in the seismogram?
In Figure 3, are the shapes of the three seismograms of the same quake, recorded at different locations, identical? If not, what do you think causes the differences?
Do differences among the seismograms give any indication of the size (magnitude) or distance of the quakes that caused them?
What other observations can you make about the seismograms? What questions do you have?
Use the Learning Library to investigate how seismograms are recorded (on the Seismograph pages) and what kinds of motion create the shapes in the seismograms (on the pages: More about Seismic Waves, Body Waves, P Waves, S Waves, Surface Waves, Rayleigh Waves, and Love Waves). If you have experienced an earthquake, did the kinds of motion you felt seem similar to the kinds of motions in P, S, and surface waves?
Also, check the Learning Library's pages on Mechanical Waves and Wave Parameters to find out how scientists describe the shapes of waves, including seismic waves. If you have studied waves in other contexts, you will notice that the same terminology applies to seismic waves. Using a common terminology can help you communicate clearly with your classmates, other members of the PEPP network, and seismologists.
Ask us...
Choose one or two questions that you would like to ask a seismologist.
Check the Question and Answer Discussion Forum on the PEPP Web site to
see whether these questions have already been answered for other students.
If not, post your questions and check later to see whether they have been
answered.
Objectives:
To gain practice in identifying P-wave arrival times; to consider how the times at which P waves arrive at seismic stations can be used to determine the distance between the stations and the epicenter of the earthquake
You will need:
Go back to a few of the figures from Activity One and click on the button beneath the figure, to see where seismologists marked the arrivals of P, S, and surface waves. Try to identify characteristics of each kind of wave. Note that the seismogram isn't always a flat line before the P wave arrival. Why do you think that might be?
Seismologists have discovered that seismometers can sometimes detect movement of the earth caused by nearby events such as the motion of the ocean, wind, and even nearby construction work or heavy traffic. These non-earthquake-related movements are called noise. Seismologists themselves sometimes have trouble determining whether a given blip in a seismogram is noise or a P-wave arrival.
Now, on the rest of the figures from Activity 1, try to identify the
arrivals of these waves on the unmarked seismograms before clicking to
see how well your arrival times match the seismologists' choices. Which
waves are the easiest to identify? Also, why do you think the P waves,
S waves, and surface waves all arrive separately at the seismic station?
Note
2 to Teachers
Look at Figure 7. Notice which seismograms show the P wave. Can you explain why the P wave is completely absent on one of the seismograms? Remember which directions of motion are shown on each seismogram.
When seismologists want to determine the location of a particular earthquake, they are rarely lucky enough to have a seismograph right at the center of the quake. (See the Learning Library's Hypocenter and Epicenter pages to distinguish the different kinds of "centers" to which seismologists refer.) Instead, they must use the data recorded by seismographs at distant seismic stations to figure out where the source of the seismic waves is likely to be.
Did you agree with the seismologists more often as you gained practice?
What techniques did you develop for identifying P-wave arrival times?
Are some students more likely to agree with the seismologists than others? Are these students doing something differently, or do they just seem to have a feel for the waves?
Why do you think the arrival times of P waves might be important?
Do you think knowing the waves' arrival times could help you locate the earthquake's epicenter? How? Is it important to know how fast P waves travel through the earth?
Use the Learning Library's Ray Tracer page to find out how seismic waves travel through the earth to reach seismic stations, and why P waves arrive before S and surface waves. Also read about how arrival times can help seismologists locate a quake's epicenter. (not currently available in Learning Library)
Objective:
To arrange the seismograms used with the Earthquake Locator on axes representing time and distance from the epicenter, and to discuss the usefulness of such a record section
You will need:
Use the Learning Library's Record Section page to find out how seismologists arrange seismograms according to when they were recorded and how far the seismic station was from the epicenter. This arrangement of data is called a record section. Figure 8a is an example of a record section (tilted 90 degrees compared to the one in the Learning Library), and Figure 8b shows the same record section with wave arrivals marked.
This record section shows seismograms recorded after the magnitude 6.7 quake in Northridge, CA on January 17, 1994 at 12:30:55 UT. Each seismogram shows vertical ground displacement. The placement of each seismogram along the horizontal axis shows the distance of its station from the epicenter, measured in degrees around the Earth. The vertical axis shows the time in hundreds of seconds, measured from the beginning of the quake, 12:30:55 Universal Time.
Before you look at the next figure, try to identify the P wave arrivals on the various seismograms. If you were to connect them, what shape would they make? Do you think you can identify the arrival of the S wave or any other waves?
This is the same record section, but a seismologist has labeled arrival times for the P and S waves, as well as some other waves you will learn about later: PP, PKIKP, and PKP. These waves have traveled more complicated paths through the Earth from the epicenter to the seismic station (as shown on the Learning Library's Ray Tracer page). As this record section shows, seismograms contain complex and subtle information.
What do the shapes defined by the P and S-wave arrival times tell you about the speeds at which these waves travel through the earth?
Now, cut or print-out each of the seismograms from your set.
Determine latitude and longitude at which the recording station is located. Then, determine how far each seismic station is from the epicenter, as an angular distance in degrees. You may wish to use the following method: Place the piece of string on the globe from the epicenter to the station and mark the distance between them. Now hold the string along a meridian (where latitude and longitude lines intersect) and determine the number of degrees that represents that distance. Record the angular distance in the margin of the seismogram print-out.
Also, record the P-wave arrival time on each seismogram.
Next, get some graph paper and draw a vertical axis for time. The axis may begin with the starting time of the earliest-arriving seismogram and must extend to the ending time of the latest-arriving seismogram. (You may need to tape together two pieces of graph paper for all the seismograms to fit.)
Draw a horizontal axis for the angular distance from the epicenter. The axis need not start with 0, but the range must include the locations of all of the seismic stations. (Again, you may need to tape on additional pieces of graph paper.)
For each seismogram, line up the P- wave arrival with the correct time on the time axis. Line up the midline of the seismogram horizontally with the correct distance from the epicenter, and tape down the seismogram. It is possible that the seismograms may overlap. If so, try to think of a method for displaying them so that every seismogram can be seen.
How does the record section tell the story of the seismic waves that traveled away from the epicenter?
If two seismograms were recorded at similar distances from the epicenter, will their P-wave arrivals be close together in time?
What does it mean if two seismograms are close together horizontally (along the distance axis)? Are the stations at which they were recorded necessarily close to one another?
Gather sample seismograms from both deep and shallow earthquakes. Are there differences in the shapes of the deep and shallow seismograms that could be used to tell them apart? Can you explain these differences? Construct record sections of deep and shallow earthquakes. Are there differences in the shapes of the record sections that could be used to tell them apart? Can you explain these differences?
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