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

Unsworth and Bedrosian., GRL 2004

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?