Mechanics of Earthquakes and Faulting

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Mechanics of Earthquakes and Faulting
7 Mar. 2007
Mechanics of Faulting

• Wear and Gouge formation
• Fault Roughness
• Fault Rocks
• Fault Zone Fabrics
• Strength and Rheology of Faulting
• Thermo-mechanics of Faulting
• Seismological and Structural Evolution of
  Faulting

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• Andersons Theory of Faulting
• Free Surface and Principal Stresses

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Adhesive and Abrasive Wear Fault gouge is wear
material
Chester et al., 2005
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Fault Growth and Development
Fault gouge is wear material
run in and steady-state wear rate
Gouge Zone Thickness, T
Fault offset, D
This describes steady-state wear. But wear rate
is generally higher during a run-in period.
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• Fault Growth and Development
• Fault Roughness

Scholz, 1990
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Fault Growth and Development
Fault gouge is wear material
Gouge Zone Thickness
Fault offset
Scholz, 1987
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Fault Growth and Development
Scholz, 1990
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Fault Growth and Development
Cox and Scholz, JSG, 1988
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Fault Growth and Development
Fault zone width
Tchalenko, GSA Bull., 1970
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Fault Growth and Development
Marone Cox, 1994, PAGEOPH
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Fault zone roughness
Scholz, 1990
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Fault Growth and Development
Fault Zone Trapped Waves (aka. Head waves) show
a zone 150 m wide with 50 reduction in shear
wave speed.
Li, Vidale, Aki, Marone Lee, Science 1994.
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Fault Growth and Development
Seismic Productivity
Fault Structural Complexity
Wesnousky, BSSA, 1990.
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Strength and Rheology of Faulting
Scholz, 1989, 1990.
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Summary of laboratory and field observations
related to the updip stability transition
(a-b) gt 0 Always Stable, No Earthquake
Nucleation, Dynamic Rupture Arrested (a-b) lt 0
Conditionally Unstable, Earthquakes May Nucleate
if K lt Kc, Dynamic Rupture Will Propagate
Uninhibited
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Friction Laws and Their Application to Seismic
Faulting
???n (a ? b)
Frictional Instability Requires K lt Kc
Kc
Dc
(a-b) gt 0 Always Stable, No Earthquake
Nucleation, Dynamic Rupture Arrested
(a-b) lt 0 Conditionally Unstable, Earthquakes
May Nucleate if K lt Kc, Dynamic Rupture Will
Propagate Uninhibited
a ? b
Earthquake Stress Drop
Seismicity
( )
( - )
( )
( - )
Seismogenic Zone
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Laboratory data related to the updip seismic limit
Effect of Clay Mineralogy
Field Observations
a ? b
( - ) ( )
Seismicity
Smectite Illite
These data, collected at room temperature,
indicate that Illite-rich shales and mudstones
are unlikely to host earthquake nucleation
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Key Observations, Outstanding Questions

• Aseismic slip
• Slow earthquakes, Creep events, Tsunamogenic
  earthquakes
• Slow precursors to normal earthquakes
• Earthquakes with a distinct nucleation phase
• Afterslip and transient postseismic deformation
• Normal (fast) earthquakes

Seismic and Aseismic Faulting End Members of a
Continuous Spectrum of Behaviors What causes
this range of behaviors? One (earthquake)
mechanism, or several? How best do we describe
the rheology of brittle fault zones?
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Marone, 1998
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• Thermomechanics of faulting
• ?Energy balance of faulting
• Wf Q Es Us
• Us A ?, (? 103 erg/cm2 1J/m2. see Table
  1.1 of Scholz) and is negligible compared to
  frictional work WFr (?ND) Us /WFr 10-3.
• ?Wf ? v q, where q is the rate of heat
  production.
• ?Shear Heating, Inverted metamorphic gradients,
  fault rocks.
• Lab measurements show effect, but generally
  small.
• ?San Andreas fault strength, heat flow.
• Problem of finding very low strength materials.
• ?What is the state of stress in the lithosphere?
  Byerlees Law, Rangley experiments, Bore hole
  stress measurements, bore hole breakouts,
  earthquake focal mechanisms.
• ?Seismic stress drop vs. fault strength.

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Fault Strength, State of Stress in the
Lithosphere, and Earthquake Physics
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Is the San Andreas fault anomalously weak? Is
the frictional strength 10-20 MPa, e.g., µ
0.3, or is it 100-200 MPa, µ 0.6?
SAFOD The San Andreas Fault Observatory at Depth