The SAFOD Pilot Hole: Heat Flow Measurements

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The SAFOD Pilot Hole Heat Flow Measurements

• Todd Hansen
• Frontiers in Plate Boundary Deformation Seminar
• November 1, 2005

http//pangea.stanford.edu/research/stress/news.ht
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The basics
(Byerlee, 1978)

• Byerlee, 1978 lab measurements
• recall 100 bar 10 MPa
• at gt 5MPa
• rock type or roughness have little effect on
  friction
• Fault gouge significant?

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(Byerlee, 1978)
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Other Contributors

• Brune, 1969 upper bound ? 25 Mpa
• No heat flow anomaly
• Lachenbruch and Sass, 1980 weak SAF
• 100 heat flow measurements
• No anomaly
• Zoback et al, 1987 weak SAF
• Convergent plate motion, folding and reverse
  faulting sub-parallel to SAF
• Scholz, 2000 strong SAF
• Broad heat flow anomaly (convection)
• Borehole stress measurements

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Lachenbruch and Sass, 1980

• Along fault heat-flow variations

(Lachenbruch and Sass, 1980)
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SAF Strong Fault Arguments

• Pro
• Byerlees laboratory analysis of friction
• Borehole stress measurements (Scholz, 2000)
• Shallow, difficult to extrapolate
• Lithospheric stress required to support
  topography and flexure
• Cons
• Lack of a heat flow anomaly
• Most measurements shallow, uncorrected for
  topography
• Evidence for major stress normal to fault
• Minor stress axis can then be sub-parallel to SAF
• Weak fault relieves stress parallel to fault.
• Convergent motion is accommodated by folding and
  reverse faulting on neighboring faults

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SAFOD Heat Flow Measurements

• Heat flow measured along depth of pilot hole (2.2
  km)
• Regional heat-flow measurements corrected for
  topography

(Williams et al, 2004)
(Williams et al, 2004)
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SAFOD Pilot Hole measurements
(Williams et al, 2004)

• (Williams et al, 2004)

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SAFOD Pilot Holeresults from Williams et al, 2004

• No heat-flow anomaly apparent
• Along strike variation correlated with
  seismogenic depth
• No evidence of convective heat-flow
• Heat flow constant across fractures
• Results similar to Fulton, 2004
• Applied 3-d terrain corrections
• Modeled ground-water flow
• For strong fault
• a lot of advection would be required to mute
  anomaly
• Depth of seismicity would not match 350ºC
  isotherm predicted by heat-flow measurements.

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For future reference

• Brune et al, 1969, Heat Flow, Stress, and Rate
  of Slip along the San Andreas Fault, California
• Byerlee, 1978, Friction of Rocks
• Fulton et al, 2004 Re-evaluation of heat flow
  data near Parkfield, CA Evidence for a weak San
  Andreas Fault
• Lachenbruch and Sass, 1980, Heat flow and
  Energetics of the San Andreas Fault Zone
• Sass et al, 1997, Thermal regime of the San
  Andreas fault near Parkfield, California
• Sholz, 2000, Evidence for a strong San Andreas
  fault
• Sholz, 1998, Earthquakes and friction laws
• Townend and Zoback, 2004, Regional tectonic
  stress near the San Andreas fault in central and
  southern California
• Williams et al, 2004 Heat flow in the SAFOD
  pilot hole and implications for the strength of
  the San Andreas Fault
• Zoback et al, 1987, New evidence on the State of
  Stress of the San Andreas Fault System