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 orien tation and directions of maximum horizontal principal stress S-H for s ix areas west of the San Andreas fault in southern California: Santa B arbara, Ojai, Central Ventura Basin, East Ventura Basin, West Los Ange les 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 th e borehole is vertical and parallel to one of the principal stress dir ections, the breakouts will form parallel to the minimum horizontal pr incipal stress S-h, orthogonal to the maximum horizontal principal str ess S-H. Observations from deviated boreholes permit some constraints on the relative magnitudes of the principal stresses. In most cases th e data permit either a thrust faulting (S-v < S-h < S-H) or strike-sli p 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 mec hanism 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.