How Earthquakes Reveal Earth's Interior
Seismic Waves Probe Structure & Composition of Planetary Interiors
Jan 21, 2009
Earth's interior structure and composition consists of a thin mostly solid rocky crust, a slowly flowing rocky mantle, a liquid metallic outer core, and a solid metallic inner core. It is possible to drill and directly sample only the top few miles of the crust. So how do scientists deduce the composition and structure of Earth's deep interior?
There are three major clues that scientists can use to probe Earth's interior. These same clues can also help scientists deduce the interior structures of other planets. The clues are:
- The planet's bulk density.
- Seismic waves from earthquakes.
- The planet's magnetic field.
This article discusses how seismic waves allow scientists to deduce Earth's interior structure.
Seismic Waves
When an earthquake strikes, seismometers across Earth pick up the seismic waves from the earthquake. Because these seismic waves have passed through Earth's interior they make a good probe to the interior structure and provide seismologists with clues to Earth's structure.
In addition to the surface waves that damage buildings, there are two types of seismic waves that pass through Earth's interior. They are P or primary waves, and S or secondary waves. It also helps to think of them as push and sideways waves.
Visualize the two types of waves with a slinky. Hold the top of the slinky allowing it to hang down. Moving one's hand up and down will generate a wave through the slinky in which the individual wires move up and down as the wave propagates downward through the slinky. This is like a P wave. Moving one's hand back and forth sideways propagates a wave downward through the slinky, but the individual wires move sideways, or perpendicular to the direction the wave propagates, like an S wave.
P waves can propagate through a liquid. S waves cannot. On the other side of the Earth from an earthquake, seismometers detect P but not S waves from the earthquake. Therefore Earth has a liquid outer core. Calculations then tell us that the high pressure in the inner core makes it solid again.
Just as a glass lens will bend, or refract, light, Earth's interior will bend seismic waves. the precise amount that a lens refracts a light wave depends on the exact shape and composition of the glass lens. Similarly the precise amount that a seismic wave bends while traveling through Earth's interior depends on conditions such as composition, density, etc in Earth's interior. Therefore seismic waves that have passed through Earth's interior tell geophysicists about the interior structure and composition.
Application to Other Planets
Seismic waves could in principle give astronomers clues to the interior structures of the solid planets: Mercury, Venus, Mars, and the Moon. To date NASA has landed seismometers on the Moon. Moonquakes are very weak and occur deep in the Moon's interior. Only there is the Moon warm enough to be partially molten and allow quake activity.
Seismic waves passing through Earth's interior provide geophysical clues to Earth's interior structure and composition. The same technique has also been used to probe the Moon's interior.
Further Reading
Zeilik, M. Astronomy: The Evolving Universe, 9th ed. Cambridge, 2002.
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