A Ground-Breaking Revelation: Testing Longitudinal Waves in Different Soil Types *

Abstract

Did you know that waves travel through the Earth’s crust all the time? One major source of these waves is earthquakes, although ground motion can also be caused by something man-made, such as a mine blast or nuclear explosion, or other natural events, such as landslides or volcanic activity.

How does an earthquake cause these waves? The entire outer shell of the Earth, known as the lithosphere, is made up of tectonic plates that are constantly moving. There are seven or eight large tectonic plates and many more minor ones. Where the tectonic plates meet and bump together, it is common to find mountains, volcanic activity, mid-ocean ridges, and earthquakes. (What forms depends on how exactly the tectonic plates are moving against each other at the plate boundary.) The movement of the tectonic plates also causes faults to form, which are cracks in the Earth’s surface where a plate, or parts of a plate, moves in different directions. Faults are usually near the edge of a plate. When two tectonic plates (or parts of the same plate) bump or catch as they slide past each other at the fault, earthquakes usually occur. Specifically, as the plates rub together, when they catch and get stuck it results in a build up of pressure because the rocks want to move but cannot. Eventually, some rocks break and the pressure is released as the plates suddenly move. This causes waves of energy, known as seismic waves, to travel through the Earth, making the ground shake. Where the rocks broke is known as the earthquake’s focus, and right above this point, up on the ground, is called the earthquake’s epicenter.

There are two main types of seismic waves: body waves, which can travel through the inner layers of the Earth, and surface waves, which can only travel on the surface. Body waves are the fastest and have the highest frequency. One type of body wave is called the primary or P-wave. (It is called “primary” because it is the first one you feel in an earthquake.) It pushes and pulls the solid rock or liquid matter that it is moving through, and people feel it as a back-and-forth or side-to-side motion. The P-wave is also what is known as a longitudinal wave, or compression wave. A longitudinal wave is a wave that travels in the same direction as the direction of vibration. Longitudinal waves are around us all the time; other examples of longitudinal waves include an oscillation in a spring, a tsunami wave, and a sound wave.

How well do longitudinal waves, such as from an earthquake, travel in different soil types? In this geology science project, you will answer this by investigating how different types of soil change the amplitude of compression waves. To do this, you will construct a solids wave tank and fill it, one at a time, with different types of soils, such as sandy, clay, or silty soil. For the longitudinal wave source, mount a loudspeaker to the base of the solids wave tank and connect a sound generator and amplifier to the loudspeaker to create vibrations similar to that of an earthquake. (There are softwares available that could be used to initiate the waves and control the amplitude of the loudspeaker, such as FreqGenie.) Measure the amplitude of the waves using an accelerometer connected to computer software that can measure this, such as LoggerPro. You may want to try placing the accelerometer in different positions to ensure that your conclusions are accurate. Which type of soil leads to the smallest amplitudes? Which type leads to the largest amplitudes? How do you think this correlates to how well P-waves from an earthquake travel through different parts of the Earth?

Credits

Alexander Pherson; Sophie Scheidlinger

Teisha Rowland, Ph.D., Science Buddies

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Bibliography

Here are a few websites that will help you start gathering information about seismic waves, seismology, and different types of soil:

• Nave, C. R. (n.d.). Seismic Waves. HyperPhysics. Department of Physics and Astronomy, Georgia State University. Retrieved March 4, 2013, from http://hyperphysics.phy-astr.gsu.edu/%E2%80%8Chbase/waves/seismic.html
• The Seed Site. (n.d.). Soil. Retrieved March 4, 2013, from http://theseedsite.co.uk/soil.html
• Wood, M. M., and Pennington, W. (2007, April 16). UPSeis: An educational site for budding seismologists. Department of Geological and Mining Engineering Sciences, Michigan Technical University. Retrieved December 20, 2012, from http://www.geo.mtu.edu/UPSeis/index.html

Variations

• In this science project you investigated how well longitudinal waves travel through different types of soil, but instead of using soil you could try different kinds of substances in your solids wave tank, such as rocks, gelatin, water, or many other possibilities. How well do the longitudinal waves travel through these different substances?
• A seismograph is an instrument used to detect and record seismic waves from earthquakes. Do some background research on seismographs and design a way to use a seismograph to investigate how well longitudinal waves travel through different types of soil.
• If you are interested in this science project, you may also be interested in these other science projects:
  • Is There a Whole Lot of Shaking Going On? Make Your Own Seismograph to Find Out!
  • How Fast do Seismic Waves Travel?
  • Measuring the Diameter of the Earth’s Core with Seismic Waves Around the Globe

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