Compression directions in southern California (from Santa Barbara to Los Angeles Basin) obtained from borehole breakouts

Borehole elongation in 71 drill holes was used to infer breakout orientation and directions of maximum horizontal principal stress S H for six areas west of the San Andreas fault in southern California: Santa Barbara, Ojai, Central Ventura Basin, East Ventura Basin, West Los Angeles Basin, and East Los Angeles Basin. Breakouts were determined from analysis of oriented four‐arm caliper data. The breakouts form at the position of the maximum compressive stress on the borehole wall; if the borehole is vertical and parallel to one of the principal stress directions, the breakouts will form parallel to the minimum horizontal principal stress S h orthogonal to the maximum horizontal principal stress S H . Observations from deviated boreholes permit some constraints on the relative magnitudes of the principal stresses. In most cases the data permit either a thrust faulting ( S v < S h < S H ) or strike‐slip faulting ( S h < S v < S H ) stress regime with NE to NW directions of S H . These results are broadly consistent with results from focal mechanism studies [ Hauksson , 1990; Li , 1996] and with breakout and focal mechanism data present in the world stress map database [ Zoback , 1992]. However, we find systematic variations in S H directions suggestive of strong heterogeneity in the stress field at shallow depths, similar to that present in the Cajon Pass borehole [ Shamir and Zoback , 1992]. Anomalous NW directions of S H in the San Fernando Valley region and near the Whittier fault may be related to structural complexities and/or lateral ramps in nearby fault systems.