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Chapter 17
Earth’s Interior and
Geophysical Properties
Photo credit: NASA Apollo 17
Evidence from Seismic Waves
Earth’s Internal Structure
• The Crust
• The Mantle
• The Core
Seismic Reflection
Controlled by:
1) Snell’s Law
2) Elastic properties
of the layers
Vp 
K
4

3

Vs 


where
K  bulk or compressibility modulus
  rigidity or shear modulus
  density
Seismic Refraction
Ray paths are
“bent” across
velocity or seismic
discontinuities.
Bend away from
boundary normal
if V2 > V1
Bend toward
boundary normal
if V1 > V2
V1
V2
Ray Paths and Snell’s Law
V1
V2
sin i1 AC / BC AC V1



sin i2 BD / BC BD V2
V2 sin i1  V1  sin i2
sin i1 V1

sin i2 V2
In this example, V2 > V1
Modified from: Brown & Mussett, 1981
Seismic Ray Paths Animation
Locating Boundaries By Refraction
Radial Distribution of Ray Paths
Rays curve
upward because
of generally
increasing
velocity
with depth
Vp and Vs
Internal Earth Structure from Seismology
Crust: 0-60 km
Mantle: 60-2900 km
Core: 2900-6370 km
Seismic Moho and LVZ
Summary Crustal Properties
Mantle Tomography
Blue = Fast (cold)
Red = Slow (hot)
Deep Earth Velocity Structure
NB that velocity is measured from earthquake waves, but density must
Be inferred from inversion of the velocity combined with elastic modulii
P wave Shadow Zone
S wave Shadow Zone
Pallisite Meteorites - Rocks from a CMB?
Olivine
Fe-Ni
Metal Alloy
Non-seismological
evidence of
core - mantle
exchange!
Pallisite meteorites are classified as stoney-irons and are thought
to be derived from the CMB region of an asteroidal parent body.
D ‘’ Layer in Lower Mantle
From: Garnero, E.J., Ann. Rev. Earth Planetary Sci., 28 , 509-37, 2000.
Isostasy and Gravity
• Isostasy is the gravitational equilibrium
of bodies of different density.
• Isostatically compensated uplift zones
are stabilized by low density roots.
• Isostatic re-adjustment can be caused by
many different processes.
Isostasy
Low density
High density
Compensation depth is arbitrary and selected for convenience.
Total column mass (or gravitational acceleration) must be equal.
Isostatic Adjustment: Erosion Animation
Isostatic Readjustment: Erosion
Post-glacial Crustal Rebound
Polar Glaciers Melting Animation
QuickTime™ and a
GIF decompressor
are needed to see this picture.
From: http://www.uni-geophys.gwdg.de/~gkaufman/work/onset/onset_ice3g.html
Post-glacial Rebound Animation
Isostatic Uplift due to Crustal Thickening
NB that uplift would depend on the amount and density of
accumulated magma accreted to the base of the crust.
Gravitational Attraction
Gravity Meter’s Response to Density Variations
gravity
high
High density
gravity
low
Low density
Local gravity high can be related to a high density body within the crust,
while a gravity low can be related to a lower than average density.
Gravity Profile in Isostatic Equilibrium
Non-isostatically Compensated Profile
Negative Gravity Anomaly