Faulting

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Faulting
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Faults

• They result from brittle fracture in rock and
  involve rapid movement of
• crust on either side of a fault plane.
• Fault planes are usually not vertical, they dip
  (are inclined) at an angle
• into the crust.
• Faults are classified based on the movement
  which has occurred relative
• to the fault plane, as in up-down motion
  (dip-slip) or horizontal motion
• (strike-slip).

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Faults develop in response to stress or forces
(p. 17, top) Stress force applied to
rock Strain change in rock due to stress
(deformation)
Reverse dip slip fault
Normal dip slip fault
Strike slip fault
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See pg. 15 and 16 in CP
East
West
Fault _at_ surface
(rock above fault plane)
(Rock below fault plane)
Fault plane (dips east)
Dip-slip Fault up/down motion Strike-slip Fault
side by side motion
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rift valley
Normal Faults
Fault scarp
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Reverse Fault
Thrust Fault
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Reverse Fault
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Thrust Fault
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fig. D p. 16

• Face fault
• Cross fault
• Direction you go (left or right)
• to find rest of displaced feature

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Earthquakes / Seismology

• Earthquakes occur when stress overcomes the
  strength of the rock.
• Sudden movement occurs along a fault plane.
• Seismic waves propagate out from ruptured area
  along the fault plane.

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Faults

• Active faults last 10,000 yrs
• Potentially active last 1.65 million years
• Types of faults
• Reverse, normal, left-lateral, right lateral
• Buried or surface faults

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Elastic Rebound Theory
Rocks are subjected to stress

• Theory explains how rocks respond to stress
• Elastic energy is stored in rocks as they are
  subjected to stress, strain builds.
• Earthquakes occur when stress exceeds strength of
  rock, strain is released as rock breaks.
• Rock will vibrate as energy is released, then
  return to original shape, spring back or
  rebound.

Strain builds
Stored energy released, Fault ruptures--gt
Earthquake
Rocks slip and snap back
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Dilatancy

• At 5 km, the overburden pressure is about equal
  to the strength of the rocks. Rocks should
  behave plastically at this depth however,
  earthquakes (rupture) still occurs here and at
  greater depths. How?
• Answer Water in the pore spaces

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Surface location directly above focus
(seismic waves)
Origin of earthquake or area of fault rupture.
Most are shallow.
(Hypocenter)
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Seismic Waves

• Body waves travel through Earths interior, 2
  types
• 1) P wave primary wave, fastest wave (5km/sec or
  12,000 miles/hr)
• Compressional wave, moves like a slinky.
• Propagates through all mediums (liquid, solid,
  and gas).
• 2) S wave secondary wave, slower than P wave
  (1.7 X slower)
• Shear wave, moves perpendicular to direction of
  propagation.
• Only propagate through solid material.

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S waves
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Surface Waves

• These are not classified as body waves.
• Surface waves propagate in crust near the
  surface.
• Slower than body waves
• 2 types
• Highest amplitude on seismogram

Love wave
Rayleigh Wave
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Seismogram record of seismic wave energy
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Epicenter

• All wave energy originates _at_ the
• focus.
• Wave velocity depends on temperature of
• rock, and the type of rock.

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Seismic Tomography Image middle Earth
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Finding earthquake epicenters by triangulation
Travel-time graph
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Magnitude Measure of energy released during an
earthquake
.
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Richter Magnitude

• Richter Magnitude (ML)
• The largest amplitude produced by an earthquake
  on a seismograph 100 km from the epicenter
• Logarithmic ground motion increases by 10 times
  for each unit of M
• Energy released increases 32X for each unit of M

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• Richter Magnitude scale
• measures largest amplitude on seismogram.
• Nomagraph used to calculate magnitude
• Logarithmic scale magnitude 5 is 10x stronger
  than a 4 and 100x stronger than a 3.

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• Moment Magnitude scale
• Newer scale and more reliable for large
  earthquakes.
• Strength of rock
• Rupture area
• displacement

1979, Imperial Valley Fault
1964, Magnitude 9.2 in Anchorage Alaska
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Great Earthquakes

• 15 largest earthquakes in the US
• 1) Alaska, 1964 9.2 6) Alaska, 1899 8.2
• 2) Alaska, 1957 8.8 7) Alaska, 1986 8.0
• 3) Alaska, 1965 8.7 8) California, 1906 8.1
• 4) Alaska, 1938 8.3 9) Missouri, 1811 8.1
• 5) Alaska1958 8.3