In-situ stress measurements at Hi Vista, California; continuation of a deep borehole profile near the San Andreas Fault

Abstract

Hydraulic fracturing stress measurements were made in an approximately 600-m-deep well at Hi Vista, California, 32 km from the San Andreas Fault in the western Mojave Desert. The stress regime at this site is transitional from thrust faulting at shallow depths to strike-slip faulting below about 400 m. Analysis of the measured stress magnitudes using Byerlee's Law indicates that the frictional failure on favorably oriented pre-existing faults at this site is unlikely below about 200 m, in accord with the low levels of seismicity observed in this region of the western Mojave Desert. The direction of the maximum horizontal principal stress at Hi Vista is approximately north-south to north-northwest, although the azimuths determined from individual tests exhibit considerable scatter (± 31°). The measured magnitudes of the horizontal principal stresses and the horizontal deviatoric stress in this well are less than or equal to those measured in a nearby well of comparable depth 4 km from the San Andreas fault. This result is counter to the increase in these stress components with distance from the San Andreas fault that was observed in a shallower borehole profile in the same area. Marked fluctuations in both stress magnitudes and orientations with depth in the Hi Vista Well, however, may result from a localized perturbation to the regional stress regime at this site. The nature of the presumed perturbation is unclear, as no correlation was found to exist in this well between stress magnitudes and either P-wave velocities or natural fracture densities, although the low stresses measured at a depth of about 540 m and the variability in stress orientations may reflect proximity to an intensely fractured and permeable zone at the bottom of the well.