Geophysical Research Letters

In situ stress orientation near the San Andreas Fault: Preliminary results to 2.1 km depth from the Cajon Pass Scientific Drillhole



Stress-induced wellbore breakouts were analyzed in the Cajon Pass Scientific Drillhole (4.3km NE of the San Andreas fault) to determine the orientation of the principal horizontal stresses, using borehole televiewer digitized data. In the depth interval 1750-2115 m the mean orientation of the maximum horizontal stress is 073°. The scatter in the orientation of breakouts is as much as ±37°, and variations occur with wavelengths of few hundred meters in this interval, reflecting either true changes in stress orientation with depth or the variability of rock anisotropy. All the measured orientations, however, result in left-lateral shear stress resolved onto planes parallel to the San Andreas fault, contrary to its post Miocene right-lateral slip of ∼ 300 km. The stress orientation is consistent, however, with reported left lateral displacement across the Cleghorn fault (3 km northeast of, and possibly subparallel to the San Andreas near Cajon Pass) since the Pleistocene. The Cajon Pass stress orientation may reflect the current state of stress across the San Andreas fault, in which case it should change prior to right lateral slip on this fault, or a shallow local perturbation to the San Andreas stress field, in which case it should rotate couterclockwise with depth. Determination of the stress orientations at greater depths in the Cajon Pass drillhole will better sample the near field of the San Andreas fault and help resolve this question.