A Look Inside the San Andreas fault at Parkfield Through Vertical Seismic Profiling Chavarria, Malin, Catchings, and Shalev Science, 302, pp 1746-1748, 2003

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A Look Inside the San Andreas fault at Parkfield
Through Vertical Seismic ProfilingChavarria,
Malin, Catchings, and ShalevScience, 302, pp
1746-1748, 2003

• Nick Schmerr
• April 9, 2007
• ASU EarthScope Seminar

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Event Migration

• Migrate data from 43 microearthquakes and 11
  calibration shots to the Pilot Hole of the
  Vertical Seismic Profile array
• 3-component 15-Hz seismometers at 32 levels every
  40 meters in depth from 200-1400 m below sea
  level (900-1200 m below the surface)

Chavarria et al., Science, 2003
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Modified from Chavarria et al., Science, 2003
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7.5 s
15 s
Chavarria et al., Science, 2003

• Kirchoff Migration detects scattering features
• Direct P and S muted to prevent interference with
  secondary arrivals
• P-P , P-P with conversions, S-S energy

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Results
Chavarria et al., Science, 2003

• Two secondary faults dipping to the NE at 2-3 km
  depth (a,b)
• Previously detected fault from SAFOD (d)
• Fault extending several km in depth (c)
• San Andreas Fault Zone

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Results
Chavarria et al., Science, 2003

• Two secondary faults dipping to the NE at 2-3 km
  depth (a,b)
• Previously detected fault from SAFOD (d)
• Fault extending several km in depth (c)
• San Andreas Fault Zone

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Chavarria et al., Science, 2003
Unsworth et al., Geology, 1997
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Implications

• Scattering zone coincides with the location of
  low-resistivity along the SAFZ indicating the
  presence of fluid or altered materials
• The migration predicts SAFOD will pass through a
  previously unknown fault along the SAFZ
• Will help to improve structural models of SAFZ
• Additional Thoughts
• Are all interfaces necessarily faults and cracks,
  or are they geologic contacts?
• How robust are the undiscussed interfaces?