Have you ever thought about what type of ground buildings are constructed on? Rock, gravel, sand, soil and many others—there are lots of different types of "ground." And this issue becomes especially important in areas that are likely to get earthquakes. In this activity you will build a sweet building on a homemade shake table and find out how an earthquake impacts buildings constructed on sand. How will your structure perform in a model earthquake?
The earth's outer layer is like a puzzle made up of huge pieces of land. These pieces are called the tectonic plates, and they slide around slowly on the surface of the globe; they push against and slide alongside one another. Sometimes this friction generates sudden movements, which can trigger earthquakes. Earthquakes release a lot of energy, which is radiated out in the form of waves called seismic waves. One type of wave pushes and pulls the material it moves through, and people feel it as a back-and-forth or side-to-side motion. This type of wave is called the primary or P-wave. A P-wave can move through solid and liquid material.
The type of ground you stand on as seismic waves pass by also affects what you feel. Soft soils such as sand tend to amplify the shaking compared with hard soils such as bedrock. In addition to being soft, sand can also undergo a "liquefaction process" during an earthquake. This means the sand can change from behaving like a solid to behaving like a thick liquid, which further amplifies shaking.
In this activity you will construct a building and place it on material that mimics sand. You will test it on a homemade shake table that generates movement resembling the pushing and pulling of a P-wave. How well will your building withstand the quake?
- Butcher paper or newspaper
- Wax paper
- Damp cloth
- Starburst candy or sugar cubes (40 or more)
- No-stir peanut butter or a substitute, such as soy butter or sunflower seed butter
- Spoon or butter knife
- Measuring cup
- Shallow box
- Marbles or any small balls identical in size (1 to 2 dozen)
- Shallow tray that is a few inches smaller than your shallow box. (Aluminum cake-trays work well.)
- Yellow food coloring (optional)
- Play-Doh (optional)
- Grape-Nuts cereal (optional)
- Additional trays (optional)
- Protect your work area with butcher paper or newspaper. This will speed up cleanup time. Have a wet cloth ready to wipe off your hands when they get too sticky.
- Place a piece of wax paper five inches square or larger in front of you.
- Unwrap the Starburst candies, and open the peanut butter jar. The candies will be the bricks of your structure; the peanut butter will serve as the mortar that keeps the bricks together.
- Scoop up a little peanut butter with a spoon, a butter knife or a finger. Apply it to the side of a candy and stick another candy against it. Repeat this process to build a building. You can build any structure you like, but it needs to fit well inside your tray.
- Cut the wax paper so there is about one to two inches of paper left on each side of the building. Set the building aside on the wax paper until you are ready to test it.
- To prepare the mixture that will mimic the reaction of sand during an earthquake, scoop three cups of cornstarch into the shallow tray, and add one and a half cups of water. Add a few drops of yellow food coloring if available. Use a fork or your hands to mix it well. Getting the right consistency of the cornstarch mixture is important. It should be firm enough to support your test building—but still liquid enough that it slowly oozes down your fork or fingers when you scoop it up. Add more cornstarch if it is too liquid, or add more water if it is too firm. Set aside.
- To assemble your shake table, place the shallow box in front of you. Put the marbles into the box, and set the tray with the cornstarch mixture so that it is resting on top of the marbles. When you shake the box the tray should sway back and forth or side to side as it rolls over the marbles.
- Practice shaking your shake table. Shake it quickly but gently enough and with the right-sized motions so that the tray does not hit the sides of the box.
- Use the wax paper to carefully place your building on top of the cornstarch mixture. Look closely. Your building should sit on the cornstarch with only the wax paper in between, but it should not sink in. It is fine if your building makes a print in the mixture—similar to a footprint in wet sand. If it slowly sinks all the way down, however, your mixture is too wet. Remove your building on the wax paper, mix in more cornstarch, and try again.
- What do you think will happen to your structure when you shake the box quickly?
- Shake the box fast enough so the tray with the cornstarch mixture sways back and forth but does not hit the side of the box. As you shake the box count to 20 before you take a break to observe. Has the building changed in any way? Can you see signs of cracks? Did the building sink deeper into the mixture?
- What do you think will happen if you shake with more force—or for a longer time?
- Try it out! Gradually increase the force with which you shake the box, making sure not to let the tray hit the side of the box. Look at the cornstarch mixture while you shake. Can you see it moving almost like a very thick liquid?
- Assess the damage. Is your structure still standing? Did the building or the debris sink deep into the mixture or is it standing or laying on top of it? Why would this happen?
- Push a finger into the mixture. Does the mixture feel firm?
- Keep pushing while you shake the box vigorously with your other hand. Does your finger sink in the mixture? Does it feel like being in a very thick liquid? Why would that be?
- Extra: Use different materials to mimic different soil types, and use your shake table to see which soil types hold up a building best. Play-Doh can model bedrock, Grape-Nuts cereal can reproduce gravel, and a mixture of two to three parts of Grape-Nuts cereal with one part of water works well to mimic loose soil. Repeat the procedure with each of these soil types. Test identical buildings, and keep the strength and duration of your shaking consistent. Would some soils make the shaking generated by a P-wave more intense than others?
- Extra: Test different designs of structures. Keep the strength and duration of your shaking the same, and do not change the soil type, but try an alternative building design. Would a tall building be more vulnerable? Would it be better to have a large surface area, or would smaller buildings withstand the shaking generated by a P-wave better?
Observations and Results
Did the cornstarch mixture become a thick liquid after being vigorously shaken?
The cornstarch mixture acts like a solid when it is left alone, but it behaves more like a thick liquid when vigorously shaken. You could see and feel that it was like a solid at first, as it could hold the building, and you could feel the mixture resist your finger when you pushed on it. Shaking changed that picture. The building or its debris probably sank into the mixture while you shook the box—just like it would sink in thick mud. You could likely also feel the mixture giving way for your finger when you shook the box while pushing on the mixture—just like it would in a thick liquid.
Sand acts in a similar way. At rest it behaves like a solid, but when shaken forcefully it acts more like a thick liquid. Scientists call this the liquefaction process, and earthquakes are known to set this process in action.
The shake table in this activity recreates the side-to-side or back-and-forth movement created by a P-wave during an earthquake. But a real seismic wave generates more powerful movement. You didn't use real sand for this activity because you are trying to model the process by which sand changes from acting like a solid to acting like a liquid when it is shaken in an earthquake. With your hand-powered shake table you cannot shake sand fast enough to achieve liquefaction, so you used a cornstarch mixture instead.
If you tested other "ground" materials (such as Grape-Nuts or Play-Doh), you might have seen different results after shaking. Based on what you tested, what type of ground would you build a structure on in an earthquake-prone area? In most areas where earthquakes are common, engineers use many other strategies to help buildings withstand shaking, which you can learn more about in the "More to Explore" section.
Throw away your cornstarch-and-water mixture with the trash. Do not pour it down the drain as it will clog the pipes.
More to Explore
In the Wake of Shake, from Science Buddies
Earthquake-Proof Engineering for Skyscrapers, from Scientific American
Earthquake Rollers, from Scientific American
Make Your Own Seismograph, from Science Buddies
It's a Solid … It's a Liquid … It's Oobleck! from Scientific American
STEM Activities for Kids, from Science Buddies
This activity brought to you in partnership with Science Buddies