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Compare-Contrast-Connect: Seismic Waves and Determining Earth’s Structure

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Even though the technology does not exist to travel into all of Earth’s layers, scientists can still learn a great deal about Earth’s structure through seismic waves. Seismic waves are vibrations in the earth that transmit energy and occur during seismic activity such as earthquakes, volcanic eruptions, and even man-made explosions. There are two types of seismic waves, primary waves and secondary waves. Primary waves, also known as P waves or pressure waves, are longitudinal compression waves similar to the motion of a slinky (SF Fig. 7.1 A). Secondary waves, or S waves, are slower than P waves. The motion of secondary waves is perpendicular to the direction of the wave travel, similar to the motion of vigorously shaking a rope (SF Fig. 7.1 B).

<p><strong>SF Fig. 7.1.</strong> (<strong>A</strong>) Primary or "P" waves show longitudinal compression similar to a slinky.</p><br />

<p><strong>SF Fig. 7.1.</strong> (<strong>B</strong>) Secondary or "S" waves have motion perpendicular to the direction of the waive, similar to a rope.</p><br />

<p><strong>SF Fig. 7.1&nbsp;</strong>(<strong>C</strong>)&nbsp;Primary or P waves (on top) and secondary or S waves (on bottom) in motion</p><br />


SF Fig. 7.1 C shows primary or P waves (on top) and secondary or S waves (on bottom) in motion.

Scientists use seismometers (Fig. 7.2) to measure seismic waves. Seismometers measure the vibrations of the ground, relative to a stationary instrument. Data from a seismometer, also called a seismogram, shows velocity on the y axis and time on the x axis (Fig. 7.3). Note in SF Fig. 7.3 that the P wave occurs first, because they travel at a higher velocity.

<p><strong>SF Fig. 7.2.</strong> Seismometers are used to measure seismic waves.</p><br />
<p><strong>SF Fig. 7.3.</strong> A seismogram shows the data from a seismograph. Wave velocity is measured on the y axis, and time in seconds is measured on the x axis. P waves are recorded earlier than S waves, because they travel at a higher velocity.</p><br />

SF Table 7.1 shows that P waves have a higher velocity than S waves when traveling through several mineral types. The speed at which seismic waves travel depends on the properties of the material that they are passing through. For example, the denser a material is, the faster a seismic wave travels (SF Table 7.1). P waves can travel through liquid and solids and gases, while S waves only travel through solids. Scientists use this information to help them determine the structure of Earth. For example, if an earthquake occurs on one side of Earth, seismometers around the globe can measure the resulting S and P waves.

SF Table 7.1. Table of various minerals and their P and S wave velocities and density
Mineral P wave velocity (m/s) S wave velocity (m/s) Density (g/cm3)
Soil 300-700 100-300 1.7-2.4
Dry sand 400-1200 100-500 1.5-1.7
Limestone 3500-6000 2000-3300 2.4-2.7
Granite 4500-6000 2500-3300 2.5-2.7
Basalt 5000-6000 2800-3400 2.7-3.1


<p><strong>SF Fig. 7.4.</strong> This diagram shows hypothetical S and P wave propagation through the earth from an earthquake. P waves (arrows in yellow) can penetrate through the mantle and core, but S waves (arrows in red) can only travel through the mantle.</p><br />

SF Fig. 7.4 shows wave propagation through Earth. Note that P waves pass through all layers of the earth, while S waves cannot pass through the solid core of the earth, resulting in an S wave shadow on the opposite side of the earthquake.


Question Set: 
  1. What are seismic waves? Use your own words to describe them.
  2. Why do you think that waves traveling through basalt have a higher seismic velocity than a wave traveling through sand?
  3. How have scientists used seismic waves to determine structure of Earth?
  4. Think of additional objects, in addition to a slinky or rope tied to a tree, that have a similar motion to a P wave and an S wave.

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.