EARTHS CRUST

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Summer 2008

• EARTHS CRUST
• and
• INTERIOR

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• HOW DO WE KNOW WHAT WE KNOW ABOUT THE EARTHS
  DEEP INTERIOR?

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• Headlines

• Epicenters (map view) indicate plate boundaries
  
• Hypocenters (add depth) map subduction zones
• Shadow zones indicate presence of liquids
• Wave arrival times indicate several properties
  at depth
• density
• elasticity
• boundaries between materials of different
  properties

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• EARTHQUAKES

Earthquakes are the way that Earth releases
stress and passes it on to people. Kate
Hutton An earthquake is a sudden,
violent of movement of the Earth that releases
stored elastic strain In general, materials must
be brittle elastic solids to store and release
energy in this way i.e. materials cannot be
too warm
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EARTHQUAKE!
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• Earthquake energy

• The amount of energy released (MAGNITUDE) is
  indicated
• by the AMPLITUDE and FREQUENCY of the waves
• by the DURATION of shaking
• by the AREA of the fault, and the DISTANCE it
  slipped
• The magnitude scale is LOGARITHMIC this means
  that every increase of ONE POINT on the scale,
• means TEN TIMES bigger shaking

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Aftershocks reveal most detail about fault
extents
They typically occur close to, but not directly
on, the fault plane, and represent the adjustment
of the crust to the sudden movement of the
earthquake
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RECAP Earthquake epicenters
Epicenter  point on ground above the focus
(hypocenter)
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Wadati-Benioff zones
Earthquakes are seen down to 670 km The bands
of hypocenters point all the way back to the
trench
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• Hypocenter distribution varies with boundary type

• DIVERGENT (mid-ocean ridges)
• Narrow zones, normal faults
• Shallow earthquakes (above melting depth)
• Usually not of great magnitude up to M6 (thin
  weak crust)
• TRANSFORM
• Narrow zones, strike-slip faults
• Shallow earthquakes (0-10km in ocean crust,
  0-30 in continent)
• Up to magnitude 8 (full strength crust)
• CONVERGENT (subduction zones)
• Wide zones, thrust faults
• Shallow to very deep earthquakes (below
  melting depth)
• (tracks subduction of cold brittle material)
• Up to magnitude 9.5 (Chile, 1960), could be
  larger in theory

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Three types of plate boundary
From Davis et al.



divergent convergent
transform
MELTING reduced pressure water
added none
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EARTHQUAKE WAVES
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Seismic wave velocities determined by density and
elasticity at depth
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Effect of Earths internal structure
Changes in material properties with depth,
i.e. lithosphere asthenosphere mesosphere
outer core inner core
Hypothetical homogeneous Earth
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Surface Waves
S
P
Rayleigh
Love
Body Waves
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• SEISMIC WAVES

• P-Waves (body wave)
• Push or Pressure waves (like sound waves)
• Primary or fastest body waves
• Travel through solids and fluids motion
  parallel to propagation
• S-Waves (body wave)
• Shake or Shear waves
• Secondary or slower body waves
• Travel through solids only motion
  perpendicular to propagation
• Surface Waves
• Love Waves (horizontal shear motion)
• Rayleigh Waves (vertical motion elliptical
  orbits)

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Locating an earthquake
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Changes in propagation direction
Reflection
Refraction
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Refraction and reflection
At a boundary between different densities, rays
are reflected or refracted depending on the
angle This happens for seismic rays as well as
light rays
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Diving waves
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S-wave shadow zones
Liquids have zero rigidity  S-waves cannot pass
through liquid Thus S-wave shadows suggest the
outer core is liquid
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P-wave shadow zones
Liquids have zero rigidity  P-wave velocity
drops dramatically Thus P-wave shadows suggest
the outer core is liquid
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• VELOCITY STRUCTURE OF MANTLE

• LOW VELOCITY ZONE
• 100 300 km depth
• Partial melting in Asthenosphere
• VELOCITY INCREASES
• 410 kilometers
• 660 kilometers
• Phase changes in Olivine to denser lattices

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• Headlines

• Epicenters (map view) indicate plate boundaries
  
• Hypocenters (add depth) map subduction zones
• Shadow zones indicate presence of liquids
• Wave arrival times indicate several properties
  at depth
• density
• elasticity
• boundaries between materials of different
  properties