Geology

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Geology Earth Science

• Earthquake
• Power Point Notes

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Background

• For centuries, people explained earthquakes with
  superstition and folk lore
• Native Americans thought that the Earth was
  carried on the back of a mighty turtle a quake
  occurred when the beast took a step

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Earthquake Definition

• We now recognize earthquakes as the sudden
  movement of a crustal part of the earth (or
  lands surface), generally along a fracture,
  caused by the rapid release of slowly accumulated
  strain in crustal rocks

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Origin of Earthquakes

• The violent motion experienced during an
  earthquake can have one of several causes
• plate tectonics (no duh!)
• faulting
• volcanic explosions
• pressure changes
• lubrication

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Plate Tectonic Boundaries

• Since we will cover this in the next chapter, for
  now we will cover the major types of plate
  boundaries
• divergent
• convergent
• transform

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Divergent Plate Boundaries

• Divergent plate boundaries occur when plates are
  moving apart magma wells up towards the surface
  from depth to fill in the void left by the
  separation

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Convergent Plate Boundaries

• Convergent Plate Boundaries are characterized by
  plates moving toward one another and colliding
• the processes of subduction and obduction occur
  at this type of boundary

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Subduction and Obduction

• As two plates meet, density differences will
  determine which plate is more likely to be shoved
  underneath the other the process of this plate
  diving under the other bully plate is called
  subduction

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Subduction and Obduction

• Sometimes as the plate is going underneath the
  over-riding plate, pieces of the downward
  traveling plate are scraped off onto the top of
  the bully plate this process is called obduction

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Transform Plate Boundary

• Transform plate boundaries occur when two plates
  are sliding past one another causing shallow
  earthquakes
• The rate of sliding is determined by friction
  (when enough pressure builds up to overcome
  friction, movement occurs)

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Fault Vs Joint

• A fault is a crack in a rock body along which
  movement occurs
• A joint is a crack in a rock body along which no
  movement occurs

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Faulting

• Faulting occurs along all plate boundaries and
  many other places worldwide
• Each side of the rock body slips and moves
  relative to the other rock body
• Movement can be in any direction according to the
  stress applied
• If this movement is sudden, its a quake

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Types of Faults

• Normal hanging wall moves down relative to the
  foot wall
• Caused by tensional forces (a pulling apart)
• The first declination of the wave at the
  seismograph station goes down
• Illustrate a normal fault

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Types of Faults

• Reverse hanging wall moves up relative to the
  foot wall
• Caused by compressional forces
• first wave at seismograph station is deflected
  upwards
• Illustrate a reverse fault

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Types of Faults

• Thrust similar to a reverse fault in that the
  hanging wall still moves upwards relative to the
  foot wall but the angle of the fault plane is
  low less than fourty-five degrees (low angle
  fault plane)
• Illustrate a thrust fault

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Types of Faults

• Strike-Slip has no hanging or foot walls all
  movement occurs horizontally along a vertical
  fault plane and can be classified as
• right lateral strike-slip fault or
• left lateral strike-slip fault
• Illustrate these types of SS faults

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Volcanic Explosions

• Usually due to rapid depressurization of the
  magma because of the escaping volatiles
• Can also be caused by the movement of the magma
  within the chamber seeking areas of low pressure
  (the surface)
• These quakes are rarely felt over a wide area

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Pressure Changes

• A change in pressure can occur from pumping out
  such things as water or petroleum or pumping in
  such things as nuclear waste, waste fluids, etc.
• Pressure changes can also be caused simple
  sedimentation over time

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Lubrication

• Associated with the pumping of liquid sludge
  waste into cracks in the ground (inactive faults)
  for disposal or storage

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Effects of Earthquakes

• Sounds
• Cracks
• Landslides
• Destruction By Fire
• Tsunami
• Loss of Life

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Landslides

• When regolith or other materials have become
  steepened over time or whose pore spaces have
  become waterlogged (acts as a lubricant) and are
  subjected to shaking they tend to slide downslope
  under the influence of gravity

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Destruction by Fire

• Gas mains are ruptured by breakage and fires
  result water mains are also ruptured so the
  fires cant be fought
• only 5 of the devastation of the 1906 San
  Francisco quake was caused by the actual shaking
  the rest (which leveled countless city blocks)
  was from fire

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Loss of Life

• People are able to check out in many different
  ways during an earthquake most die from man-made
  structures collapsing on them
• From earliest recorded history over 16 million
  folks have checked out due to earthquake activity

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Tsunami

• Caused by rapid movement of the ocean floor
  these waves appear as a simple rise in sea level
  and are not noticed by ships at sea they only
  become visible when they pile up as the wave
  comes ashore as the wave bunches up, the water
  recedes for miles to come crashing back
• Illustrate tsunami formation

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Earthquake Measurement

• Earthquakes generate waves we can measure these
  waves to estimate the ferocity of the earthquake
• These waves are measured on seismographs...

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Measurement Magnitude

• When an earthquake occurs, a seismograph is able
  to record a measure of the total seismic energy
  released in the form of earth waves producing a
  seismogram...

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Types of Seismographs
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Seismograph Vs Seismogram

• Remember that a seismograph is the machine that
  records the data and the data that is recorded is
  the seismogram

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Earthquake Waves

• Movement originates deep within the earth at a
  point from this point, the waves caused by the
  movement travel outwards in all directions the
  point where movement first occurs deep within the
  earth is the focus the point directly above the
  focus on the earths surface is the epicenter

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Earthquake Waves

• The waves generated are of two different types
• BODY WAVES and
• SURFACE WAVES

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Body Waves

• P waves Primary Pressure arrive at a given
  seismograph station first, can be transmitted
  through solids, liquids and gases
• S Waves Secondary Shear arrive at a given
  seismograph station second, can only be
  transmitted through solids

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Surface Waves

• Rayleigh waves and Love Waves arrive at the
  seismograph station thirdly, following the P and
  S waves these surface waves cause the most
  destruction to man-made structures and cause the
  surface of the earth to roll like ocean waves...

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Seismogram of 06 SF Quake
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Earthquake Measurement

• Earthquakes can be measured in two ways
• Intensity the strength of the quake
• Magnitude the amount of energy released during
  the quake

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Measurement Intensity

• The intensity is the strength of the earthquake
  it is a persons measure (opinion) of how the
  earthquake affects that individual and the
  earths surface (primarily man-made structures)

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Measurement Intensity

• A number is assigned to different intensity
  levels, but it should be noted that the effects
  of shaking are different at different locations
  so one earthquake can have many different
  intensities
• An Italian seismologist named Mercalli proposed
  the scale that we still use to classify
  Intensities

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Measurement Magnitude

• The total energy released during an earthquake is
  the magnitude while there are many intensities
  for one given earthquake (depending upon
  location) , there can be only one magnitude
• The scale we use to classify an earthquakes
  magnitude is the Richter Scale (allows world-wide
  comparison)

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