Download Earthquakes and the Interior

Earthquakes and the Interior
 Earthquakes are definitely a geologic hazard for people
living in earthquake regions, but the seismic waves
generated by earthquakes are invaluable for studying the
interior of Earth.
 Earthquake waves indicate several things about Earth’s
Interior:
1) Earth appears to be layered, according to composition
and physical properties.
2) Density increases with depth.
3) Outer Core is a liquid.
Reference:
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Earthquakes and the Interior
 If composition (or physical properties) change abruptly at
some interface, then seismic waves will both reflect off the
interface and refract (or bend) as they pass through the
interface. Two cases of wave refraction can be recognized.
1) If the seismic wave velocity
increases when passing into
the rock below the interface,
the waves will be refracted or
bent upward relative to their
original path. (At Moho)
Earthquakes and the Interior
 If composition (or physical properties) change abruptly at
some interface, then seismic waves will both reflect off the
interface and refract (or bend) as they pass through the
interface. Two cases of wave refraction can be recognized.
2) If the seismic wave velocity
decreases when passing into
the rock below the interface,
the waves will be refracted
down relative to their original
path. (Mantle – Outer Core)
Earthquakes and the Interior
 If the seismic wave
velocities gradually
increase with depth in the
Earth, the waves will
continually be refracted
along curved paths that
curve back toward the
Earth's surface.
1) Earth Appears to Be Layered
 Scientists look for variations in Seismic wave velocities that
correspond to changes in rock properties. These changes indicate
that Earth has four major layers;
1) Crust - very thin outer layer ranging in thickness, 5km for
oceans and up to 60km for continents.
2) Mantle - Seismic wave velocities increase abruptly at the Moho.
(boundary between crust and mantle)
3) Outer Core - At a depth of 2900 Km P-wave velocities
suddenly decrease and S-wave velocities go to zero. This is the top
of the outer core. As discussed above, the outer core must be
liquid since S-wave velocities are 0.
4) Inner Core - At a depth of about 4800 km the sudden increase
in P-wave velocities indicate a solid inner core.
2) Density Changes With Depth
 Scientists also contribute the increase in seismic wave velocity to
an increase in density with depth into Earth’s interior.
 The four main Layers of Earth’s Interior differs in composition
and physical state. This contributes to the change in density
within Earth.
1) Crust – Continental crust composed of lower density silicates,
whereas the ocean crust is composed of higher density silicates.
2) Mantle – Increase in temperature and pressure causes the
silicates rocks to increase in density with depth.
3) Outer Core – Consist of very dense nickel and iron in a molten
(liquid) state.
4) Inner Core - Consist of very dense nickel and iron in a solid
state.
3) Liquid Outer Core
 Scientists look for variations in Seismic wave velocities at the
boundaries of each layer. These changes indicate that Earth has a
liquid outer core with a solid inner core within.
 The evidence for a liquid outer core was based on the behavior of
P- and S-waves at the mantle-core boundary.
 P-waves are bent (refracted) so to bend deeper into the outer core
which results in a P-wave shadow Zone.
 S-waves do not pass through the core and a S-wave shadow zone
results on the opposite side of Earth.
P-wave Shadow Zone
 P-waves are bent (refracted) at the mantle-outer core boundary
and bend deeper into the outer core. This causes these P-waves to
arrive at Earth’s surface some distance away from P-waves that
do not travel through the Outer core.
 This resulted in an area
from 105 degrees to 140
degrees from the focus of
an earthquake where no
P-waves were detected.
called a P-wave shadow
Zone.
105
E
105
E
 This is called a
P-wave shadow zone.
140
E
140
E
S-wave Shadow Zone
 The S-wave shadow zone occurs because no S-waves were
detected after 105 degrees from the focus. Since no direct Swaves arrive in this zone, it implies that no S-waves pass through
the core.
 Therefore, the core or at least
part of the core is in the liquid
state, since no S-waves are
transmitted through liquids.
 Thus, the S-wave shadow zone
is best explained by a liquid
outer core.
105
E
105
E