fine-scale structure of the san andreas fault zone and location of the safod target earthquakes...
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Fine-scale structure of the San Andreas fault zone and location of
the SAFOD target earthquakes
Thurber, Roecker, Zhang, Baher, and EllsworthGeophysical Research Letters, 31, doi:10.1029/2003GL019398, 2004
Nick SchmerrApril 9, 2007
ASU EarthScope Seminar
SAFOD • San Andreas Fault Observatory at Depth at Parkfield, California
• San Andreas fault transitions from creep to locked
• Vertical borehole ~2km deep (Pilot Hole)
• Subhorizontal drilling into SAFZ
• Primary goal is to drill into the rupture patch of a ~2.0 magnitude earthquake
• Requires accurate location of target earthquake
http://earthquake.usgs.gov/research/parkfield/Images/SAFOD-3D-new2-lrg.html
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Data• Earthquake and explosion arrival times from
short period instruments
• 90,000 P and S arrival times from 800 earthquakes and 100 explosions
• SAFOD PH array
• PASO (Parkfield Area Seismic Observatory)
• Berkeley High Resolution Seismic Network
• USGS Central California Seismic Network
• Gravity data of [McPhee and Jachens, 2004]
Methods
• Joint inversion for velocity and earthquake locations using 3 tomographic methods
• simul2000 - 3D model with 1-km grid spacing and a finer 2D model near the fault with 500 m grid spacing
• tomoDD - double difference inversion to determine earthquake location and structure on 1-km grid model
• tomoGRAV - 1D starting model on a 200 m grid that jointly fits arrival times and gravity data
• Accuracy also tested using a “virtual” earthquake method from explosions recorded at borehole seismometers
Tomography Results
• Vp velocity structure with depth, loses resolution below 5 km
• Earthquake locations similar between models
• Strong velocity gradient near the SAF
Thurber et al., GRL 2004
Tomography Results
• 2D model projects all earthquakes onto the section
• Earthquake locations similar between models
• Velocity reversal at depth in tomoGRAV results
Thurber et al., GRL 2004
Tomography Locations
•Scatter of 100 m horizontally, 180 m vertically
•Desired accuracy of 100 m resolution for the SAFOD event
Thurber et al., GRL 2004
Virtual Source Results
• Each explosion is treated as a station from a source located at the Pilot Hole seismometer (~50 m accuracy)
• Locate the explosions with the array (~100 m near center of array, ~200 m accuracy at edges)
Implications
• Further improvement of location accuracy is needed for properly targeting the rupture patch
• Virtual modeling suggest higher resolution is achievable with further data coverage
• Additional Thoughts:
• Future active source experiments should improve the locations in the tomographic inversions
• 3D synthetic modeling of waveforms and traveltimes?