Spring 2007 Name_____________________

 

Physical Science

Earthquakes

Objectives:

To use replicas of the seismograms of the seismograph at Bellingham, Washington for March 27, 1964, date of a very large earthquake in Alaska, to discover how such seismograms are useful in obtaining information about earthquakes.

 

Please be very careful not to make marks of any type on the seismograms!

 

Answer the questions on the first two pages:

 

1. Describe primary (P) waves.

 

 

 

 

 

2. Describe secondary (s) waves.

 

 

 

 

3. Which of the above types of waves travels faster? _________________

 

4. Read the paragraph below and tell how electromagnetic induction is important in the operation of the seismograph.

 

 

 

 

 

 

 

 


The Geology Department at Western Washington State College in Bellingham, Washington was operating a seismograph station in 1964 when the Alaskan earthquake occurred. Three seismographs were operating and recorded the North-South and East-West as well as the vertical movements. (You will study seismograms taken on the North-South seismograph. The seismographic equipment at this station is on a cement table beneath the building. To prevent building vibrations (door-slamming, heavy footsteps, etc.) from being recorded, the table is not attached to the surrounding building, except for some electrical wires hanging from the ceiling of the room. Each seismograph consists of a mass (pendulum) which consists, in part, of a copper coil which is delicately suspended in a magnetic field. Any motion of the bedrock causes a similar motion of the table and everything resting on it, including the magnets. However, the important exception to this motion is the pendulum (heavy mass) suspended by a long spring. The inertia of the heavy mass means that it does not move as fast as the table, and this causes a relative motion between coils and the magnetic field, producing a current. This current is transmitted to an ammeter where the electrical energy is used to turn a mirror which directs a narrow beam of light on photographic paper (the seismogram) wound on a drum which makes one revolution every thirty minutes. The wiggles on the seismogram thus correspond to the movement of the table relative to the pendulum caused by movements in the bedrock.

 

To aid with the fixing of time, a signal is sent to the ammeter every minute, causing it to be lower for two seconds. At the beginning of each hour a four second signal is sent. The figure below shows how these signals appear on the seismogram.

Figure 1

The "tick mark" is caused by a two second signal which is sent at the beginning of each minute. At the beginning of an hour a four second signal is sent to the seismogram. You are to circle the "hour mark" on this figure.

 

5. Read the entire experiment then sketch below S and P waves from the same earthquakes as they will appear on the seismogram. On the figure you draw clearly mark the S waves, the P waves and the distance you will use to determine the time difference between the arrival of these two types of waves. During the experiment you will use this time difference to determine the distance between the place where the earthquake happened and the seismograph.

 

 

 

 

 

Carefully place your seismogram on the table and put weights on the four corners. Please be very careful not to make marks of any type on the seismograms!

 

6. What are you asked not to do to the copy of the seismograms on your table?

0

 

 

 

 

If you do not understand any of the instructions or if you need assistance, do not hesitate to ask the instructor.

 

The paper you have consists of two seismograms, one taken immediately after the other. The times that each seismogram started is written at the top of the left hand corner.

 

7. Look at the time the first seismogram finished and the time the second started. How long did it take the attendant to change to paper on the recording drum?_____________________

 

8. Look at figure 1 on the previous page of this report. Use the method explain in this figure to determine the following times and write 9:00, 9:30, 11:00 and 7:00 on the paper triangles and place them on the seismogram at the marks indicating these times.

9. Use the meter stick to measure the number of millimeters in 30 minutes (one complete line across the page) and calculate the number of minutes/mm. (Be sure to use millimeters instead of centimeters.)

 

Distance in millimeters that corresponds to 30 minutes

 

mm=_______________

 

Divide this by 30 min to obtain mm/min=_________________

 

10. Find the tic mark on the seismogram that corresponds to 7:30 pm, then count from there to determine the time of start of the first earthquake (to nearest second). To do this you will need to measure the distance between the last tic before the earthquake began and the start of the quake. To calculate the seconds, divide this distance by the conversion factor that you found in step 8, then multiply by 60 to change to seconds.

Start of earthquake = : :

 

11. Place paper arrows with the numbers 1 through 12 on the seismograms at the beginning of any twelve earthquakes that can be identified. (You should use both the top and bottom of your seismogram.) Leave these in place! Call the instructor to sign in the blank. ___________________

 

12. One of the most accurate ways to tell the distance an earthquake is from the seismograph is by timing the difference in the arrival of the S and P waves. The first step in this process is to identify the S and P waves in each quake. This figure will help you do this.

The P waves arrive first (in this case about 3 minutes before the S waves). Often the vibrations have somewhat diminished before the P waves begin.

Write S and P on each of 4 paper triangles and place them by the S and P waves of 4 earthquakes. You will need to use the earthquakes in the bottom seismogram where you can identify the S and P waves. If the distance you measure between the S and P waves is not between 100 and 150 mm, you have not found S and P waves. Leave these triangles in place also.

 

13. Now measure the difference between the arrival times of the P and S waves for 4 of the waves. Then use the answer you obtained in question 8 to calculate the time between the arrival of the S and P waves.

 

Earthquake No. Measured distance for Time difference for P and

S and P waves S waves

 

__________ ________________ ___________________

 

__________ ________________ ___________________

 

__________ ________________ ___________________

 

__________ ________________ ___________________

 

 

14. The instructor will give you a graph that will enable you to use the time difference on the last question to determine distance from the earthquake to Bellingham. The instructor will show you how to use a strip of paper with each of the above distances marked, to do this.

 

Write down the four distances you obtain from the graph.

_______________, ______________, ______________, ______________

 

 

15. When several seismographic stations determine the distance to the earthquakes the exact location of quakes can be determined. Your instructor will give you a list of locations and you are to place a letter on the map on the following page corresponding to the longitude and latitude of each earthquake as given on the sheet.

16. After the locations of the earthquakes are marked on the map, draw lines that you think might represent the position of plate boundaries in this region. If you have placed your earthquakes properly on the map, you will see that many of them tend to lie along certain lines. These are the lines you should sketch.

17. What are these plates named and what do they have to do with earthquakes?