Lecture

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Lecture 11- Faults and Faulting
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Faults Bound the Major Plates
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Rock Deformation

• Rocks slowly deform as a result of Earths
  convection and thermal cooling.
• Where the rocks are cool and brittle, they can
  stick together along faults that fracture causing
  earthquakes.
• Deep below Earths surface, where it is hot, the
  rocks bend and flow like taffy.

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Rock Deformation

• The specific style of rock deformation depends on
• Composition
• Temperature
• Pressure
• Strain Rate

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Rock Behavior

• In general, rocks will either flow or fracture
  depending on their temperature.
• However, even cold rocks can sustain some strain
  before they break.
• If the strain is released before the strength of
  the rock is exceeded, the rock returns to its
  original shape.

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Rock Fractures (Faults Joints)

• A joint is a fracture across which the sides have
  not moved.
• A fault is a crack across which the two sides
  have moved.

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Joints
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Joints and Weathering
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Fault Scarps

• An exposed fault surface is called a scarp.

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Faults (Fairview Valley, NV)
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San Andreas (central CA)
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Fault Structure

• We dont have many examples of what faults look
  like far below Earths surface, but heres an
  example.

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CaliforniaFaults Large Ruptures
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Earthquakes and Faults

• Earthquakes occur on faults, but not all of the
  fault ruptures during each earthquake.
• The hypocenter (or focus) is the place where the
  rupture begins, the epicenter is the place
  directly above the hypocenter.

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Hypocenter and Epicenter
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Rupture Size

• Larger earthquakes rupture larger faults or
  larger fractions of faults.

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Fault Geometry Terminology

• We need some definitions and concepts that we can
  use to discuss faults.
• Important Terms
• Hanging Wall / Foot Wall
• Strike
• Dip
• Slip

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Dip ( Hanging Wall/Foot Wall)

• The orientation of the fault surface with respect
  to Earths surface is defined by the fault dip.

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Strike

• Strike is an angle use to describe the
  orientation of the fault surface with respect to
  North.

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Slip

• Slip is the angle used to describe the
  orientation of the movement of the hanging wall
  relative to the foot wall.

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Faulting Styles

• There are four basic styles of faulting
• Normal
• Reverse
• Strike-Slip
• Oblique
• The type of faulting depends on the slip
  direction (the movement of the hanging wall with
  respect to the foot wall).

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Slip Direction

• If the slip is in the direction of the dip, we
  call it a dip-slip motion.
• If the slip is in the direction of strike, we
  call it a strike-slip (or transform) motion.

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Normal Faulting

• The hanging wall slides down the fault - as you
  would expect (thats why its called normal).

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Faults (Fairview Valley, NV)
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Reverse Faulting

• The hanging wall is pushed up the fault - not
  what you would expect (thats why its called
  reverse).

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Strike-Slip Faulting

• The hanging wall horizontally (no motion in the
  direction of fault dip).
• There are 2 cases depending on how the rocks on
  the other side of the fault move - right lateral
  and left lateral.

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SF 06 - Left or Right Lateral?
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Oblique Faulting

• A combination of dip-slip and strike-slip motion.

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Faulting Summary
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Stress

• Stress is a force per unit area.
• Examples
• Pressure
• Friction
• Stress is an important parameter in faulting. A
  fault fails when the stress on the fault
  becomes larger than the frictional forces holding
  the fault together.

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Faulting and Stresses

• The style of faulting (normal, reverse, etc.)
  also tells us about the stresses acting within
  Earth.
• We describe the stresses by considering three
  stresses, two horizontal and the vertical.

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Normal Faulting Stresses
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Reverse Faulting Stresses
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Strike-Slip Faulting Stresses
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Stress Faulting Summary

• At any place there are three principal stresses
  acting on a fault.
• If the vertical stress
• Largest - Normal Faulting
• Smallest - Reverse Faulting
• Intermediate - Strike-Slip Faulting

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Faulting and Seismograms

• The nature of faulting affects the amplitudes and
  shapes of seismic waves (this allows us to use
  seismograms to study the faulting).
• We call the variation in wave amplitude with
  direction the radiation pattern.

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Radiation Patterns

• Radiation patterns are common in the study of
  sources of just about anything.

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Faulting Shear Waves

• Faulting generates large shear waves because
  earthquakes release shear strains stored in the
  rocks around the fault.

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Radiation Patterns in 3D
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Seismic Beach Balls

• We use the radiation patterns of P-waves to
  construct a graphical representation of
  earthquake faulting geometry.
• The symbols are called Focal Mechanisms or
  Beach Balls, and they contain information on
  the fault orientation and the direction of slip.

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Representing a Plane
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The Principal Mechanisms
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Faults and Plates

• The style of faulting tells us something about
  the forces acting in a particular part of Earth.
• Along plate boundaries, faulting reflects the
  motion of plates.
• Divergent Boundary Normal Faulting
• Convergent Boundary Reverse Faulting
• Transform Boundary Strike-Slip Faulting

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Example East Africa
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Summary

• Faults are rock fractures across which the rocks
  have moved.
• Earthquakes occur along faults.
• Faulting geometry (strike, dip, slip) is related
  to the stresses acting on the rocks.
• The three faulting styles (normal, strike-slip,
  and reverse) reflect the deformation occurring
  within Earth and vary systematically with plate
  boundary style