1. Ch. 19.2 – Seismic Waves and Earth’s Interior Essential Questions
How does a seismometer work?
How have seismic waves been used to determine the structure and composition of Earth’s interior?
Seismic Waves and Earth’s Interior
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2. Vocabulary mantle seismometer seismogram Review New
Seismic Waves and Earth’s Interior
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3. Seismometers and Seismograms
Most of the vibrations caused by seismic waves cannot be felt at great distances from an earthquake’s epicenter, but they can be detected by sensitive instruments called seismometers, which measure horizontal or vertical motion during an earthquake.
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Seismic Waves and Earth’s Interior

4. Seismometers and Seismograms
The frame of a seismometer is anchored to the ground. When an earthquake occurs, the frame moves but the hanging mass and attached pen do not. The mass and pen record the relative movement as the recording device moves under them.
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Seismic Waves and Earth’s Interior

5. Seismometers and Seismograms
The record produced by a seismometer that can provide individual tracking of each type of seismic wave is a seismogram.
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Seismic Waves and Earth’s Interior

6. Travel-time curves
Travel-time curves provide the average time it takes for P- and S- waves to reach seismic stations located at different distances from an earthquake’s epicenter.
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Seismic Waves and Earth’s Interior

7. Distance from the epicenter
Waves recorded on seismograms from more distant stations are farther apart than waves recorded on seismograms at stations closer to the epicenter.
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Seismic Waves and Earth’s Interior

8. Distance from the epicenter
The separation of seismic waves on seismograms can be used to determine the distance from the epicenter of an earthquake to the seismic station that recorded the seismogram.
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Seismic Waves and Earth’s Interior

9. Clues to Earth’s Interior
Earth’s internal structure
Seismic waves change speed and direction when at the boundaries between different materials.
As P-waves and S-waves initially travel through the mantle, they follow fairly direct paths. When P-waves strike the core, they are refracted, which means they bend.
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Seismic Waves and Earth’s Interior

10. They then noticed that S-waves do not travel through Earth’s center.
Earth’s internal structure
Seismologists first determined that the back-and-forth motion of S-waves does not travel through liquid.
They then noticed that S-waves do not travel through Earth’s center.
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Seismic Waves and Earth’s Interior

11. Earth’s internal structure
The data collected for the paths and travel times of the waves inside Earth led to the current understanding that Earth has an outer core that is liquid and an inner core that is solid.
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Seismic Waves and Earth’s Interior

12. Imaging Earth’s interior
Because the speed of seismic waves depends on temperature and composition, it is possible to use seismic waves to create images of structures such as slabs and plumes. In general, the speed of seismic waves decreases as temperature increases.
Images like this one from Japan are generated by capturing the path of seismic waves through Earth’s interior.
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Seismic Waves and Earth’s Interior

13. Review Essential Questions How does a seismometer work?
How have seismic waves been used to determine the structure and composition of Earth’s interior?
Vocabulary
seismometer
seismogram
Seismic Waves and Earth’s Interior
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