AY 101 Lecture Notes - Lecture 16: Transverse Wave, Outer Core, Asthenosphere

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18 Jun 2018
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Interior Structure: Core, Mantle, Crust
The interior of Earth is not subject to direct investigation, but its properties must be
indirectly deduced from the study of earthquake waves that propagate through the
interior rocks. From an earthquake near the surface, both pressure (compression)
waves and transverse (side to side) waves move outwards in all directions. Wave ‐ ‐
energy moving into the interior, however, has its path slowly changed by refraction as
the wave moves through regions of slowly changing properties. These waves reach the
surface after a time that depends on the length of the path and the velocity of
propagation at each point along that path. Careful analysis at seismographic stations of
the time of arrival of earthquake waves over the surface of Earth yields information
about the densities, temperatures, and pressures of Earth's interior. A thin crust (at its
thickest only 30 kilometers deep), which contains the continental masses and the ocean
floors, overlies a denser outer mantle. The uppermost layer of the mantle acts as solid
material, a lithosphere no more than about 80 kilometers deep. Most of the mantle
slowly flows under pressure and acts as a plastic, or malleable, asthenosphere.
In an annulus about the surface of Earth, opposite an earthquake, exists the shadow
zone, in which you cannot observe pressure waves. The path of pressure waves is
significantly affected by a sharp refraction that astronomers interpret as the point of
transition between the mantle and an interior core that is substantially different from the
outer part of the planet. The shadow zone for transverse waves, however, covers the
whole of Earth opposite the earthquake source. No transverse wave energy apparently
passes through the core, indicating that its physical state, in the outer regions at least,
must be liquid. The innermost core, however, though at higher temperatures, is likely
solid because of an even higher pressure there. As the center of Earth continues to
slowly cool over time, this inner core must be slowly growing in size at the expense of
the liquid outer core. Evidence also shows that this inner core is rotating faster than the
rest of the planet, completing one full turn in two thirds of a second less time than at the
surface. Applying other physical principles together with laboratory study of the nature
of different materials under high temperature and pressure suggests the
characterization of Earth's interior
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Document Summary

The interior of earth is not subject to direct investigation, but its properties must be indirectly deduced from the study of earthquake waves that propagate through the interior rocks. From an earthquake near the surface, both pressure (compression) waves and transverse (side to side) waves move outwards in all directions. Wave energy moving into the interior, however, has its path slowly changed by refraction as the wave moves through regions of slowly changing properties. These waves reach the surface after a time that depends on the length of the path and the velocity of propagation at each point along that path. Careful analysis at seismographic stations of the time of arrival of earthquake waves over the surface of earth yields information about the densities, temperatures, and pressures of earth"s interior. A thin crust (at its thickest only 30 kilometers deep), which contains the continental masses and the ocean floors, overlies a denser outer mantle.

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