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Stress orientations and magnitudes in the SAFOD pilot hole

January 1, 2004

Borehole breakouts and drilling-induced tensile fractures in the 2.2-km-deep SAFOD pilot hole at Parkfield, CA, indicate significant local variations in the direction of the maximum horizontal compressive stress, SHmax, but show a generalized increase in the angle between SHmax and the San Andreas Fault with depth. This angle ranges from a minimum of 25 ?? 10?? at 1000-1150 m to a maximum of 69 ?? 14?? at 2050-2200 m. The simultaneous occurrence of tensile fractures and borehole breakouts indicates a transitional strike-slip to reverse faulting stress regime with high horizontal differential stress, although there is considerable uncertainty in our estimates of horizontal stress magnitudes. If stress observations near the bottom of the pilot hole are representative of stresses acting at greater depth, then they are consistent with regional stress field indicators and an anomalously weak San Andreas Fault in an otherwise strong crust. Copyright 2004 by the American Geophysical Union.

Citation Information

Publication Year 2004
Title Stress orientations and magnitudes in the SAFOD pilot hole
DOI 10.1029/2004GL020043
Authors S. Hickman, M. Zoback
Publication Type Article
Publication Subtype Journal Article
Series Title Geophysical Research Letters
Index ID 70026795
Record Source USGS Publications Warehouse