PHYSICAL PROPERTIES of the EARTHS INTERIOR: SEISMIC VELOCITY

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PHYSICAL PROPERTIES of the EARTHS INTERIOR
SEISMIC VELOCITY

• Seismic Velocities of Rocks
• Global Patterns of the Earths Seismicity
• Seismic Waves as Probes of the Earths Interior
• Distribution of the P- and S- Wave Velocities in
  
• the Crust, Mantle and Core
• Seismic Wave Velocity Discontinuties

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Seismic Velocities of Rocks

• The factors affecting the seismic velocity are
• 1) Elastic properties of rock forming minerals
  (rock composition)
• 2) Cracks, fractures and pores
• 3) Anisotropic effects (direction dependency of
  velocity)
• 4) Temperature
• 5) Pressure
• 6) Fluid content
• 7) Saturation
•  

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• The seismic velocity parameters of the Earths
  interior are obtained
• from the following sources
• Meteorites
• Xenoliths
• Seismological observations
• Deep drilling on continents ( maximum depths
  Underground Mining 3 km Petroleum wells 8 km
  Research wells 12 km)
• Laboratory measurements
• Seismic sources (artifical, natural)
• Overthrusts (Ivrea Zone, Italy)

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(Plummer McGeary, 1991)
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(Plummer McGeary, 1991)
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(Plummer McGeary, 1991)
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(Plummer McGeary, 1991)

• Olivine (Mg,Fe)2 SiO4
• Pyroxene (Mg,Fe) SiO8
• Amphibole Group
• (Ca2Mg5)Si8O22 (OH)2
• Mica Group
• Clay Group
• Kaolinite Al2Si2O5(OH)4

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Diagram of the crystal structure of olivine
(Plummer McGeary,1991)
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P? H400 km gt The molecules collapses into a
denser form (Geller Stein, 1986)
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The common extrusive rocks (Plummer and McGeary,
1991)
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Most common igneous rocks (Plummer and McGeary,
1991)
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(Press and Siever, 1997)
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Sedimentary rocks (Smith, 1981)
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The metamorphic facies (Plummer McGeary, 1991)
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Mean-value range of longitudinal and transeverse
wave velocities for groups and types of some
commonly occuring rocks (Schön, 1998)
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The velocity of elastic waves in different rock
types

• Magmatic and metamorphic rocks
• The velocity variations are caused by the
  influences of
• differences of the rock composition,
• effects of cracks, fractures and pores,
• anisotropic effects (especially metamorphic
  rocks),
• temperature and pressure.
• Sedimentary rocks
• The velocity variations are caused by the
  influence of
• mineralogical composition of the rock matrix,
• consolidation and cementation of the rock
  matrix,
• porosity, pore space and pore content,
• pressure and temperature.

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• (Schön, 1998)

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The dependency of elastic wave velocity on
pressure and temperature

• T ? ? V ?
• P ? ? V ?

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(Plummer McGeary, 1991)

• Global Patterns of the Earths Seismicity

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(Plummer and McGeary, 1991)
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• Divergent Plate Boundaries
• A narrow belt of shallow-focus earthquakes
• Oceanic ridges
• Shallow earthquakes
• 15 km depth
• Small magnitude
• Normal Faulting
• Intrusion of basaltic magmas
• Transform Plate Boundaries
• Transform Faults
• Shallow focus earthquakes in the area between
  the ridges
• Subduction Zones
• Intense earthquake activity
• Convergent plate boundaries
• Shallow-intermediate deep earthquakes
• Trenches
• 670 km (max)

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• Collision Zones
• A wide belt of shallow earthquakes
• The Himalayas the Tibetan Plateau
• No deep earthquakes (no subduction zone)
• Interplate Seismicity
• Plate boundaries
• Continental platforms
• Infrequent scattered shallow focus earthquakes
• Eastern Africa
• Western United States
• Minor shallow earthquakes (only 0.5 of total
  energy releases by this way)
• (Hamblin and Christiansen, 1998)

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(Press Siever, 1997)

• Seismic Waves as Probes of the Earths Interior

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(Press Siever, 1997)
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(Press Siever, 1997)

• Distribution of the P- and S- Wave Velocities
• in the Crust, Mantle and Core

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• (Bott, 1982)

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• THE CRUST (Press Siever, 1997)
• Continental crust
• Thickness 25-70 km
• Rock types Felsic (Granitic)
• P-wave velocity 6 km/s
• Density 2.7 g/cm3
• Oceanic crust
• Thickness 8 km
• Rock types Mafic (Basalt Gabbro)
• P-wave velocity 7 km/s
• Density 3.0 g/cm3
• Crust-Mantle Boundary Mohorovicic
• (Andrija Mohorovicic, Yugoslavian
  seismologists, 1909 )
• Upper Mantle
• Rock types Ultramafic (Peridotite)
• P-wave velocity 8 km/s
• Density 3.3 g/cm3

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(Plummer and McGeary, 1991)
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• THE MANTLE
• Rock type Peridotite (Olivine and Pyroxene
  minerals)
• Olivine Fe2SiO4 Fayalite
• Mg2SiO4 Forsterite
• Pyroxene MgSiO3 Enstatite
• FeSiO3 Orthoferrosilite
• Depth ?? P ? T ? ? Atoms of Olivine and
  Pyroxene become closer. ? More
• compact structures
• a) Lithosphere (100 km) The S-wave passes
  easily.
• b) Asthenosphere (100-200 km) Vs ?
• The waves are partially absorbed. ? Zone of
  weakness (partially fluid solid)
• At mid-ocean ridges, the asthenosphere is close
  to the surface.
• The asthenosphere is the source of basaltic
  magma.
• c) 200-400 km Depth ?? Vs ? gradually
• d) 400 km P ? T ? ? olivine ? spinel ?
  perovskite (a change in crystal
• structure a repacking of atoms more closely)

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• e) 450-650 km The velocity of S-wave is almost
  constant.
• f) 670 km The velocity of S-wave increases
  rapidly. The atoms must be
• packed even more closely.
• Spinel phase ? Magnesiowustite (Mg,Fe)O
  Perovskite (Mg, Fe) SiO3
• g) 700-2900 km Lower mantle
• Vs ? gradually (little change in composition and
  crystal structure)
• THE CORE
• 2900-5100 km Outer core (86 Fe,12 S, 2 Ni) ?
  fluid
• P-wave travels but S-wave doesnt.
• 5100-6370 km Inner core (80 Fe, 20 Ni) ?
  solid
• P ? T ? ? The increase in velocity ? solid iron
  core
• Mantle Core Boundaries ? Compositional changes
• 400 600 km Boundaries ? Phase changes ( denser
  minerals)

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• Press Siever, 1997

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Moho and Mantle-Core boundary Compositional
changes400 and 670 km discontinuties Phase
changesOlivine Fe2SiO4 (Fayalite), Mg2SiO4
(Forsterite)Pyroxene MgSiO3 (Enstatite), FeSiO3
(Orthoferrosilite)Garnet A3B2(SiO4)3 A Ca,
Mg, Fe B Al, Fe, CrMgWustite (Mg,
Fe)OPerovskite (Mg,Fe)SiO3
(Hamblin Christiansen, 1998)
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(Bott, 1982)

• Seismic Wave Velocity Discontinuties

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S-wave velocity versus depth (Smith, 1981)
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(Hamblin Christiansen, 1998)
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Appendices
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(Plummer and McGeary, 1991)
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(Plummer and McGeary, 1991)
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(Plummer and McGeary, 1991)
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(Plummer and McGeary, 1991)
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REFERENCES

• Bott, M.H.P., 1982, The Interior of the Earth
  its structure, constitution and
• evolution, Elsevier, p 5-10, 30-49, 87-98,
  119-124, 150-161, 229-239
• (ITU Mustafa Inan Library, QE 28.2 .B68).
• Geller and Stein, 1986, Chapter 3, Seismology and
  Earth Structure.
• Hamblin and Christiansen, 1998, Earths Dynamic
  System, Brigham Young
• University, Provo, Utah, p 470-495 (ITU Mustafa
  Inan Library, QE 28.2 .H36)
• Plummer C.C. and McGeary, D. 1991, Physical
  Geology, Wm.C. Brown Pub.,
• p 345-380 (ITU Mustafa Inan Library, QE 28.2
  .P58).
• Press and Siever, 1997, Understanding Earth, W.H.
  Freeman and Company,
• p 459-495 (ITU Mustafa Inan Library QE 28 .P74
  1997)
•  
• Schön, J.H., 1998, Handbook of Geophysical
  Exploration, Seismic Exploration, V 18 Physical
• Properties of rocks Fundamentals and Principles
  of Petrophysics, Pergamon press,
• p 149-205 (ITU Mustafa Inan Library, 431.6 P5 S34
  1998).