On Shaky Ground

Great Sichuan Earthquake of May 12, 2008AP Photo/Kyodo News, Takanori Sekine

Prof. Tina Niemi

Dr. Jack AlpertGEOPATHS - 2008

How a seismograph works

Lesson #1

Exploration: Activity 1—Frequency and Amplitude

To get a feel for measuring vibrations lets play with pendulums because it is easy to measure the frequency and amplitude of their motions. We will make pendulums of different lengths and make measurements on how many times it swings back and forth.

Materials — string, weights, tape measure or ruler, stopwatch

Experiment 1: In this experiment you will take strings of different lengths with the same weight attached and measure the number of cycles it makes in 10 seconds.

Make a prediction of how the number of swings of the pendulum relates to the string length.

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You will need three pieces of string measuring 20 cm, 60 cm, and 1 m in length. Tie a weight to the bottom of each string. Pull the string about 10° from vertical and count the number of back and forth swings in 10 seconds. Record the data in the table below.Experiment

no.String length

Starting angle of weight from

vertical

Number of cycles

1 a 20 cm 10°1 b 60 cm 10°1 c 1 m 10°

Observations—How does the lengths of the string relate to the number of cycles the weight swings back and forth?

Experiment 2: In this experiment you will take a 1-m-long string with a weight attached to it and measure the number of cycles it makes in 10 seconds when the starting point of the swing is changed from 10° to 20° and 30° from the vertical.

Make a prediction of how the number of swings of the pendulum relates to the angle over which it is swinging.

Now take the 1 m string and tie a weight to the bottom. Pull the string about 10° from vertical and count the number of back and forth swings in 10 seconds. Record the data in the table below. Report the experiment by pulling the string about 20° from vertical and 30° vertical. Record the data below.

Experiment no.

String length

Starting angle of weight from

vertical

Number of cycles

2 a 1 m 10°2 b 1 m 20°2 c 1 m 30°

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Observations— How does the number of swings of the pendulum relates to the angle over which it is swinging?

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Concept IntroductionFrequency, amplitude, sine wave, Seismograph, inertia, ground motion

To understand how a seismograph works we need to know: 1) what kind of information is being passed to it (e.g. ground motions created by an earthquake).2) the mechanics of measuring these motions, and3) how the recorded data represent the ground motion.

When an earthquake occurs, it is like a big hammer hitting a rocks. It compresses the rock, which then expands and compresses the rock next to it, which expands and compresses the rock next to it, etc. This series of compressions travels outward from the earthquake, like the ripples created when a stone is thrown into water.

When these waves pass a seismometer, the ground vibrates or travels forward and back, as each vibration passes by. The number of vibrations per second is called frequency, and the amount of forward and back motion is called the amplitude of the vibration.

A pendulum swings or vibrates has a cycle per second or frequency and amplitude – the size of each vibration.

What would happen if we put a pen on our pendulum and a piece of paper beneath it?

When pendulum is swinging, if we move the paper downward, what kind of line would it trace?

Where is the pendulum going the fastest?at that point, would the drawn line be horizontal or vertical?

Where is the pendulum going the slowest? at that point, would the drawn line be horizontal or vertical?

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Application: Seismogram Generator

Now it is time to put this all together and see how it works in a seismometer. I need something to make the ground move in a series of waves. I need a large inertial mass that does not move when the ground moves. And I need a strip of paper to capture the movements.

We will experiment with the seismogram-generating machine pictured above. One of the parameters we will be changing is the input vibrations (Point A in the image). We will experiment with changing the amplitude of the source wave, and the form of the source wave. This would be analogous to changes in the source earthquake.

The paper will have to be pulled at a constant rate (i.e. a specific distance over a specific time). We will look at how this functions on the seismograph. One person will mark the paper every second. Since seismic data are dependent on an accurate clock, this is a very important part of the experiment.

Finally, we will analyze the output data and compare it to data acquired with the pendulum.

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A. Place to input the earthquake vibrations

B. Paper and pen for recording vibrations

C. Paper is pulled at a constant rate and marked every second

Arm and inertial mass in back

Seismic WavesLesson #2

Adapted from: http://www.geo.mtu.edu/UPSeis/making.html

Exploration: Activity 1—How do waves move?

How Do I Make My Own P and S Waves?You can imitate the motion of P and S waves using a Slinky® (the metal ones work best). The S wave can also be simulated using a piece of rope in place of a Slinky®. These activities work best with a partner and on a flat surface such as a table or the floor.

Materials — 6 Slinkys®, tape measure, stop watch

Making P WavesP waves consist of a compressional (shortening) motion and a dilational (expanding) motion that both lie along a line. As you make your own P wave in this exercise, try to identify the compressions and dilations in the Slinky®. Here's how you do it:1 Place the Slinky® on a flat surface. (The floor works well). Have your

partner hold the opposite end of the Slinky®. If you don't have a partner, you can tie the Slinky® onto a hook in the wall or onto a door knob (close the door first) and try this activity in the air.

2 Holding the other end of the Slinky®, walk away from your partner, or from the wall or door.

3 Stop walking away when the Slinky® isn't sagging anymore (if in the air) or there is no more slack. Don't pull the Slinky® too tight; just take up the slack.

4 Push your end of the Slinky® towards your partner in one, quick motion (if the Slinky® is suspended in the air, quickly jerk your end of the Slinky® towards the wall and then back). Don't let go of the Slinky®.

Observations—how is each part of your Slinky moving? (for example, are all parts moving the same way; is the Slinky moving horizontally or vertically). What happens once the

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wave reaches the other end?

Making S Waves

When making your S wave, notice how the Slinky® itself moves in a direction perpendicular to the direction that the energy is traveling in (perpendicular to the direction of wave propagation). S waves are more complex than P waves, but they should be easier to simulate in this activity:1 Place the Slinky® on a flat surface, and have your partner hold the

opposite end of the Slinky®. If working alone, tie one end of the Slinky® to a hook on the wall or a doorknob (close the door first).

2 Holding the other end of the Slinky®, walk away from your partner, or from the wall or door.

3 Stop walking when the Slinky® has only some slack left. If working alone and the Slinky® is suspended in the air, you want to stop walking only when the Slinky® no longer sags in the air. Don't pull the Slinky® tight; just take up most of the slack.

4 Quickly jerk your end of the Slinky® from side to side once. If the Slinky® is suspended in the air, a quick jerk up and down once is sufficient. Don't let go of the Slinky®.

Observations—how is each part of your Slinky moving? (for example, are all parts moving the same way; is the Slinky moving horizontally or vertically)

Exploration: Activity 2—How fast do waves move?

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Which type of wave moves faster?Now go through the same two experiments you did in Activity 1, but this time measure the length of the stretched Slinky®. Use a stop watch to time low long it takes for each type of wave to move from one end, where the wave is generated, to the other end, where the wave is received. This experiment works best if two or three Slinkys are taped together to make them longer.

Observations—How long does it take for the P wave to travel from one end of the Slinky to the other end? How long does it take for the S wave to travel from one end of the Slinky to the other end? If both P and S waves are generated at the same time, which wave would arrive at a location first? Calculate the velocity of each wave type.

Exploration: Activity 3—How can we determine where a wave came from?

How far away was that earthquake?Use three Slinkys with different lengths. Have one person hold all three Slinkys and generate P waves (as in Activities 1 and 2). Have the other three people hold the receiving ends of the three Slinkys. Time the amount of time it takes for each Slinky to move from the generating end to the receiving end.

Observations—How does the length of the Slinky relate to the amount of time it takes for the P wave to travel from one

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end to the other? Think about a wave being generated at one location and traveling outward to multiple locations. How could the timing be used to figure out where the wave was generated?

Exploration: Activity 4—Seismic Wave Dance

The class will now demonstrate how P waves and S waves travel through different media. First, everyone should stand side-by-side with their feet together, knees locked, and their arms outstretched to the shoulders of the student on either side. The class is now rigidly-linked particles as in a solid.

Now a P wave will be initiated at the end of the line by a gentle firm push. Next, an S wave is initiated by moving the student at the end of the line forward and backwards.

Finally, the students should drop their arms and stand shoulder-to-shoulder in a line. Each student is a particle that is no longer rigidly-linked to the neighboring particle as in a liquid. Now, the same P wave and S wave pulse is initiated.

Observations—What was the difference between the linked and non-linked experiments? How does this model the propagation of seismic waves through solids and liquids?

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Concept Introduction

P wave, S wave, compression, dilation, shear, reflection, refraction, velocity, S-P lag time, focus, epicenter, magnitude, intensity, seismograph, seismogram, seismic shadow, triangulate, nomogram

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Application: Epicenter and Magnitude

Now go to a web site where you can put into practice what you’ve learned. On this site you will use P and S waves received at three different locations on Earth to figure out where the earthquake was generated. Upon completion of this activity you will be given the opportunity to receive a personalized certificate as a "Virtual Seismologist"!

Geology Online Labs (produced by geologists at the California State University, Los Angeles)— http://nemo.sciencecourseware.org/VirtualEarthquake/

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“We all fall down”

Lesson #3

Exploration: Activity 1—Tacoma Narrows Bridge video

On November 7, 1940, the collapse of the Tacoma Narrows Bridge was captured on film. Let’s view the bridge video on www.you.tube.com. Pay close attention to which part of the bridge moves and which part does not.

Observations — Describe the motion of the bridge.

Let’s view the video again. The wind was only blowing at 40 mph the day of the collapse. What do you think caused the bridge to fail?

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Exploration: Activity 2 — Rubber bands and weights

Let’s explore what happens when we shake a weight on a rubber band slowly and vigorously. First, take two long rubbers and cut them open and tie them together. Attach a weight to the end of the rubber band. Now do the following experiments.

Experiment 1 Hold the rubber band at its end and let the weight hang down.

Move your hand up and down about 20 cm (8 inches) Moving your hand very slowly up and down.

Observations — Describe what happens. For example, does the weight move up and

down about at the same, faster or slower rate, direction, and amount as your hands motion?

Experiment 2 Hold the rubber band at its end and let the weight hang down.

Move your hand up and down about 3 cm (1 inches) Moving your hand very slowly very rapidly.

Observations — Describe what happens. For example, does the weight move up and

down about at the same, faster or slower rate, direction, and amount as your hands motion?

Experiment 3Hold the rubber band at its end and let the weight hang down.

Move your hand up and down about 3 cm (1 inches) Moving your hand up and down slowly at first, but increase the speed of up and

down

Observations — Describe what happens. For example, does the weight move up

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anddown about at the same, faster, or slower rate, direction, and amount as your hands motion?

How do you think these experiments relate the Tacoma bridge failure?

How do you think these experiments might relate to earthquakes and building failures?

Concept IntroductionAmplitude, displacement, frequency, cycle, compression, tension, spring, vibrate, oscillation, resonance frequency, ground acceleration, ground motion, attenuation, dampen, center of gravity

Application: Earthquake-proof Buildings

In order to explore what building styles perform best in an earthquake, we will run several experiments with a shake table. This exercise was developed by developed by Ted Latham, physics and science and technology teacher, Watchung Hills Regional High School, Warren, New Jersey and adapted from the Discovery Education website: http://school.discoveryeducation.com/lessonplans/programs/earthquakeproof/index.html

The following factors contribute to the durability of a structure:• Distribution of weight• Variation in shape• Variation in height• Variation in foundation material

Each group will build different structures and then see how durable each is by placing it on the shake table and simulating an earthquake by shaking the generator. Each group will be testing one of the variables discussed above. Before conducting the experiment, each group should make a prediction of which structure has the best chances of surviving an earthquake. A diagram of the shake table is shown below. You will find details on how to construct the shake table following this application.

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Experimental Group 1: How does the distribution of weight within a structure affect its stability during an earthquake?a Students in this group will make two rectangular, solid blocks with dimensions

approximating 15 × 15 × 20 centimeters from light materials such as Styrofoam, cardboard, or foam board. The third block should be made of a heavier material, such as wood. The wood block, placed at different positions during each trial, is the dark-colored block in the drawings below.

b Predict which structure has the best chance of withstanding an earthquake and explain why.

c Place each structure on the earthquake generator and simulate an earthquake by shaking the generator.

d Observe which structure was the most durable and withstood the earthquake.e. Write your conclusion and revise your original explanation if you disproved your prediction.

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Experimental Group 2: How does variation in shape and placement of objects within a structure affect its stability during an earthquake?a Students in this group will use three different rectangular, solid blocks made from the same material (either Styrofoam, foam board, or cardboard). Make one block 15 × 15 × 20 centimeters, one block 10 × 10 × 20 centimeters, and one block 5 × 5 × 20 centimeters. In each trial, the blocks will be stacked in a different order, as shown in the diagrams below.

b Predict which structure has the best chance of withstanding an earthquake and explain why.

c Place each structure on the earthquake generator and simulate an earthquake by shaking the generator.

d Observe which structure was durable enough to withstand the earthquake.e Write your conclusion and revise your original explanation if you disproved your

prediction.

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Experimental Group 3: How does the variation in the height of each structural element and its placement affect the structure’s stability during an earthquake?a Make three different rectangular, solid blocks out of the same material (either

Styrofoam, foam board, or cardboard). Make one block 15 × 15 × 30 centimeters, one block 15 × 15 × 20 centimeters, and one block 15 × 15 × 10 centimeters. Have students conduct an earthquake trial in the orders shown in the diagrams below.

b Predict which structure has the best chance of withstanding an earthquake and explain why.

c Place each structure on the earthquake generator and simulate an earthquake by shaking the generator.

d Observe which structure withstood the earthquake.e Write your conclusion and revise your original explanation if you disproved your

prediction.

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Experimental Group 4: How does variation in foundation material affect the stability of a structure during an earthquake?a Make two equal rectangular, solid blocks out of the same material. The blocks should

have the dimensions 15 × 15 × 20 centimeters. In each earthquake trial, the blocks will be stacked in the same way but placed on different foundation materials. In the first trial, put marbles or ball bearings in a shallow box and place the structure on top of the marbles. In trial 2, replace the marbles with several short wooden dowels or round pencils. In trial 3, use a large sponge. (You can try other foundation materials, such as sand or Teflon-coated cooking sheets.)

b Predict which structure has the best chance of withstanding an earthquake and explain why.

c Place each structure on the earthquake generator and simulate an earthquake by shaking the generator.

d Observe which structure survived the earthquake the best.e Write your conclusion and revise your original explanation if you disproved your prediction.

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