Experiment

This activity created in partnership with AGI.

The crust of the Earth is made up of plates that are in constant—though very slow—motion along faults in the lithosphere. There are also faults within the plates, not associated with the plate boundaries. Sometimes faults get stuck and stress builds up. That stress gets released with a sudden jerky and irregular movement of the rock on either side of the fault, causing strong vibrations. The vibrations travel away from the fault in the form of seismic waves. Above the fault, the vibrations cause up-and-down motions and side-to-side motions of the ground surface. That movement is an earthquake.

Earthquakes vary greatly in their strength. Most earthquakes are so small that they can be detected only with special instruments. Some earthquakes, however, release an enormous amount of energy. They can cause ground motions so strong that people who are out in the open can’t even stand up! 

The most serious hazard associated with earthquakes is the destruction of buildings. Many structures cannot withstand the prolonged shaking of the ground during an earthquake. As a result, they may collapse or tip over and fall onto neighboring structures. Collapsing or falling buildings are responsible for many earthquake-related deaths.

One type of damage that can occur during an earthquake is related to the height of a building. All buildings have a natural oscillation period—the time it takes for the building to sway back and forth one time. The oscillation period depends on the size and shape of the building. Generally, taller buildings have longer periods of oscillation than shorter buildings. If the natural oscillation of a building has the same period of oscillation as the back-and-forth shaking of the ground during an earthquake, then the building’s sway will be greater. This is called resonance. This increases the probability that the building will collapse. If the ground oscillation is either more rapid or slower than a building’s natural oscillation, then the building will be less affected.

Our Experiment

In this activity, you will calculate the natural oscillation period of an object. You will use your findings to explore the relationship between height and stability of a structure during an earthquake. We suggest that you work in groups of two or three on this activity.

Tools and materials

• Two square pieces of wood measuring about 10 cm (4 in) across
• 30-cm-long (12-in) thin metal strip—we used a thin metal ruler
• Small C-clamp
• Lump of modeling clay, approximately half the size of a fist, about 100 g (3.5 oz)
• Ruler or meter stick
• Hammer and nails, or screwdriver and screws, for wood
• Stopwatch or timer
• Safety goggles

What to do

Note: Please wear safety goggles during this activity while constructing your model and performing the activity.

Attach the two pieces of wood together with nails or screws to make an L-shaped base. Set it on a table with one side horizontal and the other side upright. Then, clamp the metal strip to the upright piece of wood so that the strip is vertical, as shown in the photo. Make sure the clamp is at the top of the wood; this will ensure that your results are accurate.

Set up a data table similar to the following: Height of clay ball Oscillation period

Mold the modeling clay into a ball and push it onto the top of the metal strip. Measure the distance from where the blade is clamped to the center of the ball. Record this as the first clay ball height.

Have one partner hold the base, pull the ball to one side (about 5 cm, or 2 in), and release it so it can swing back and forth. At the same time the other partner should start the timer. You are modeling how a tall building sways back and forth when it is disturbed by the ground motion of an earthquake, or even by a strong gust of wind. Each back-and-forth movement of the clay ball is one oscillation. The period of the oscillation is the time it takes to go out from its released position and return. At the end of three oscillations, stop the timer. Caution: Stand clear of the swinging metal strip. Avoid pulling back the metal strip to extreme bends. It is possible for all or part of the clay ball to come off. Position yourself to avoid being hit.

Divide your time by three to calculate the period of one oscillation. Record this as your first oscillation period.   Push the ball down the metal strip to simulate a shorter building height and repeat your measurements. Do this for three or four different heights. This simulates how buildings of different heights might sway during an earthquake. Record your measurements and observations.   Plot a graph with the oscillation period on the vertical axis and the height of the ball on the horizontal axis. Draw a smooth curve to connect your data points.   Use your graph to explain how the oscillation period varies depending on the height of the ball.   Use the results of your investigation to answer the following questions: Will earthquakes affect all buildings equally? What evidence do you have for your answer?

	Digging Deeper Find out more about earthquakes and resonance.