A Look Inside the San Andreas fault at

Parkfield Through Vertical Seismic

Profiling

Chavarria, Malin, Catchings, and ShalevScience, 302, pp 1746-1748, 2003

Nick SchmerrApril 9, 2007

ASU EarthScope Seminar

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

Modified from 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

7.5 s 15 s

Chavarria et al., Science, 2003

ResultsChavarria 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

ResultsChavarria 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

Chavarria et al., Science, 2003

Unsworth et al., Geology, 1997

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?