Fine-scale structure of the San Andreas fault zone and location of the SAFOD target earthquakes Thurber, Roecker, Zhang, Baher, and Ellsworth Geophysical Research Letters, 31, doi: /2003GL019398, 2004 Nick Schmerr April 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

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 simul D 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?