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The "Basketball Drop " Activity

An Idea On How to Use the PEPP Geophones and Vernier Software

David Burch

Eastern Greene Jr-Sr HS

The Data Logger software received in the PEPP workshop is an excellent way to demonstrate how seismographs work and some of the basics of how seismic waves travel. This can be either a qualitative activity for younger or lower-level students, or a quantitative activity for more advanced students, depending upon the sensitivity of your equipment. This activity is an adaptation of one presented at the EPIcenter wokshop for teachers at Purdue in 1996.

Materials needed: Sand-filled Basketball

Geophone set-up with Vernier Software

Printer attached to computer (if doing quantitatively)

Video from IU "How to Know if the Earth Moves", starring Gary Pavlis

Procedure

The tape from IU shows the principle of operation behind digital seismographs very well. You may want to show it before doing this activity.

Set up the geophone assembly. I have found that it is more sensitive if I put the geophone on a small table by itself (such as a computer table) away from the recording computer.

Set the data points at 50 (highest recording rate), and the time scale at about 30 seconds.

Demonstrate how it works by dropping a small rubber ball on the table, allowing it to bounce several times. The display should show several peaks and valleys, similar to the appearance of a seismogram.

Go to the Display choice on the pull-down menu bar. Select Data Points. The lines that made up the "waves" on the screen will be replaced with a series of dots. This is very much like the description on the IU tape, a good illustration of how a digital seismometer displays data.

Preparing the Basketball

Obtain a basketball that no longer will hold air. The PE department will probably have at least one, if not several. The outdoor-style rubber ones are really better than the game-type leather ones for this function.

Cut a small circle (less than 5cm) around the valve area of the ball with a utility knife.

Get a heavy-duty trash bag larger than the volume of the ball and poke the bag into the ball's interior, leaving the top of the bag out of the ball. Blow into the bag to spread out the bag inside the ball as much as possible.

Fill the ball with dry sand. I made a funnel for pouring the sand into the ball by cutting off a 2-liter plastic bottle. Try to fill the ball as completely as possible with sand. When full, tie off the trash bag with a knot to keep the sand from leaking out. The completed ball will weight about 15-20 pounds!

Using the Ball

It is sometimes very difficult to drop a weight consistently, especially if it is an irregular shape. The sand-filled basketball should give you some fairly consistent data, due to the shape of the ball and the consistency of the sand.

Mark several points on the floor of the room. One point should be directly in front of the table on which the geophone is resting. From that point , measure 1m away, 3m, 5m, 8m, 10m, etc. to the boundary of the room.

At each point, drop the ball from a fixed height, for example 2m. Do this quickly, so the drops from different points can be on the same display screen. You may have to change the time interval on the screen to 60 seconds or so. If a printer is attached to the computer, print the screen for further study. Ask students to answer questions such as:

Is there a pattern developing in the observed data?

If so, what is the pattern?

Can you suggest reasons that could explain any pattern?

How can you test your hypothesis?

Do you have any ideas about other experiments you might do using this equipment?

How can these experimental results be applied to seismic waves?

To apply this concept to real data, I used a data set from the California-Nevada Border Region quake of May 15, 1999. The quake was a 6.0 magnitude quake. From the PEPP website, I downloaded data from these stations:

Station

Distance (km)

Max Wave Height (estimated from SWAP)

PPCIB

656

9 X 105 , or 90 x 104

PPUHS

934

13 X 104

PPSAH

1001

9 X 104

PPEGH

2791

7 X 104

PPCDH

3614

3 X 104

 

This file set can be loaded into SWAP for analysis. Use the Display mode to put the seismograms in absolute time scale. This clearly shows the difference in travel times of the body and surface waves.

Choose the Detail Window button. Next, use the Display option to put all the seismograms into the same units (in this case, velocity or nm/sec).

Use the magnifying cursor to analyze each seismogrm. Estimate the wave height of the largest surface wave of each seismogram. Add this estimate to your data table (see above). The data is consistent with your model.

This same procedure should work with most regional quake data sets.

dburch@indiana.edu

(812) 825-9596 home

(812) 825-5621 school

 

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