Engineering Geology and Seismology

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Lecture 2
Engineering Geology and Seismology Origin and
Inferiors of the Earth Instructor Dr.
Attaullah Shah
Department of Civil Engineering Swedish College
of Engineering and Technology-Wah Cantt.
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Geology literally means "study of the
Earth. Physical geology examines the materials
and processes of the Earth. Historical geology
examines the origin and evolution of our planet
through time. Engineering geology is the
application of geological data, techniques and
principles to the study of rock and soil
surfacing materials, and ground water.
 Seismology is study of the generation,
propagation and recording of the elastic waves
and the source that produce them.
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Importance of engg geology in Civil Engineering
practice

• What is Engineering Geology?
• Engineering geology is the application of
  geological data, techniques and principles to the
  study of rock and soil surficial materials and
  ground water.
• This is essential for the proper location,
  planning, design, construction, operation and
  maintenance of engineering structure.

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Importance of engg geology in Civil Engineering
practice

• What does Engineering Geology study?
• Rock, soil, water and the interaction among these
  constituents, as well as with engineering
  materials and structures.

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Importance of engg geology in Civil Engineering
practice

• Why Engineering geology?
• Serve civil engineering to provide information in
  3 most important areas
• Resources for construction aggregates, fills and
  borrows.
• Finding stable foundations
• Mitigation of geological hazards Identify
  proplems, evaluate the costs, provide information
  to mitigate the problem

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Origin of Earth

• Various Theory
• Nebular Hypothesis
• Planetesimal Hypothesis
• Gaseous Tidal Hypothesis
• Binary Star Hypothesis
• Gas Dust Clout Hypothesis

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Nabular Hypothesis

• German philosopher, Kant and French
  mathematician, Laplace
• Earth, planets and sun originated from Nebula.
• Nebula was large cloud of gas and dust. It
  rotates slowly.
• Gradually it cooled and contracted and its speed
  increased.
• A gaseous ring was separated from nebula
• Later the ring cooled and took form of a planet
• On repetition of the process all other planets
  came into being
• The central region, nebula became sun.

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• Objections to Nabular Hypothesis
• Sun should have the greatest angular momentum
  because of its mass and situated in the center,
  however, it has only two percent of momentum of
  the solar system
• How the hot gaseous material condensed in to rings

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Planetesimal Hypothesis

• Chamberlin and Moulton proposed the theory in
  1904
• The sun existed before the formation of planets
• A star came close to the sun.
• Because of the gravitation pull of the star,
  small gaseous bodies were separated from the sun
• These bodies on cooing became small planet's
• During rotation the small planets collided and
  form planets

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• Objections to Planetesimal Hypothesis
• The angular momentum could not be produced by the
  passing star.
• The theory failed to explain how the
  planetesimals had become one planet

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Gaseous Tidal Theory

• Jeans and Jeffrey proposed the theory in 1925
• Large star came near the sun. Due to
  gravitational pull a gaseous tide was raised on
  the surface of the sun.
• As the star came nearer, the tide increased in
  size.
• Gaseous tide detached when star move away.
• The shape of the tide was like spindle.
• It broke into pieces-forming nine planets of the
  solar system.

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Interior of earth

• Crust
• Continental crust (25-40 km?)
• Oceanic crust (6 km)
• Mantle
• Upper mantle (650 km)
• Lower mantle (2235 km)
• Core
• Outer core liquid (2270 km)
• Inner core solid (1216 km)

? Values in brackets represent the approximate
thickness of each layer
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Layers of the Earth

• The earth is divided into three main layers
  Inner core, outer core, mantle and crust.
• The core is composed mostly of iron (Fe) and is
  so hot that the outer core is molten, with about
  10 sulphur (S). The inner core is under such
  extreme pressure that it remains solid.
• Most of the Earth's mass is in the mantle, which
  is composed of iron (Fe), magnesium (Mg),
  aluminum (Al), silicon (Si), and oxygen (O)
  silicate compounds. At over 1000 degrees C, the
  mantle is solid but can deform slowly in a
  plastic manner.

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THE CRUST

• The crust is much thinner than any of the other
  layers, and is composed of the least dense
  calcium (Ca) and sodium (Na) aluminum-silicate
  minerals. Being relatively cold, the crust is
  rocky and brittle, so it can fracture in
  earthquakes.
• The shell of the earth, the crust, can be said to
  have two different thicknesses.
• Under the oceans, it is relatively thin. It
  varies in thickness from 5 to 8 km. Under the
  land masses, it is relatively thick. The
  thickness of the continental crust varies from 10
  to 65 km.

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THE CRUST

• The eggshell analogy for the crust is not an
  exaggeration. It is paper thin compared with the
  radius of the earth which is approximately 6400
  km.
• The total weight of the continental crust is less
  than 0.3 of the weight of the earth.
• Variations in the crust thickness are compensated
  by the weight of the water and the differences in
  the specific gravities of the crust under the
  oceans (3.0 to 3.1) and under the continents(2.7
  to 2.8).

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THE CRUST

• If one thinks of the crust as virtually
  floating on the mantle, one is less likely to
  wonder why the earth does not wobble as it
  rotates about its axis.
• The weight of the crust plus the mantle has a
  reasonably uniform distribution over the globe.

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THE MOHO

• The Moho, or the Mohorovicic Discontinuity,
  refers to a zone or a thin shell below the crust
  of the earth that varies in thickness from 1 to 3
  km.

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THE MOHO

• In seismology, the term "discontinuity" is used
  in its general sense. It refers to a change over
  a short distance of a material property. In this
  case, the "short distance" may be as long as 3
  km, a trifle compared with the radius of the
  earth.
• In that zone, the P-wave velocity has been
  observed to increase from approximately 6 to
  approximately 8 km/sec.
• The Moho is considered to be the boundary between
  the crust and the mantle.
• The increase in P-wave velocity is ascribed to
  change in composition of the medium. Rocks of the
  mantle are poorer in silicon but richer in iron
  and magnesium

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THE MANTLE
The mantle can be thought of having three
different layers. The separation is made because
of different deformational properties in the
mantle inferred from seismic wave
measurements. (1) The upper layer is stiff. It is
presumed that if the entire mantle had been as
stiff, the outer shell of the earth would stay
put. This stiff layer of the mantle and the
overlying crust are referred to as the
lithosphere. The lithosphere is approximately
80-km thick
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THE MANTLE

• (2) Beneath the lithosphere is a soft layer of
  mantle called the asthenosphere.
• Its thickness is inferred to be several times
  that of the lithosphere.
• One may think of this as a film of lubricant
  although film is not exactly the word for
  something so thick. It is assumed that the
  lithosphere, protruding (meaning extending
  beyond) parts and all, can glide over the
  asthenosphere with little distortion of the
  lithosphere

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THE MANTLE

• (3) The mesosphere is the lowest layer of the
  mantle.
• Considering the vagueness in defining the lower
  boundary of the asthenosphere it would be
  expected that the thickness and material
  properties of the mesosphere are not well known.
• It is expected to have a stiffness somewhere
  between those of the lithosphere and the
  asthenosphere.

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THE CORE

• At a depth of approximately 2900 km, there is a
  large reduction (on the order of 40) in the
  measured velocity of seismic waves. The boundary
  between the mantle and the core is assumed to be
  at this depth.

• Because no S-wave has been observed to travel
  through the material below this boundary for a
  thickness of approximately 2300 km, it has been
  inferred that the core comprises two layers.
• The 2300-km thick outer layer which is in a
  molten state and an 1100-km thick inner layer
  which is solid.

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THE CORE
It is known that the pressure increases toward
the center of the earth. So does the temperature.
The liquid outer layer versus the solid inner
layer is rationalized by recognizing that the
melting point of the material increases (with
pressure) at a faster rate than the temperature
as the center of the earth is approached.