A detailed profile of the orientation of the apparent maximum horizontal compressive stress was constructed over the depth interval 1.75--3.46 km in the Cajon Pass drill hole, located 4.2 km from the San Andreas fault in southern California. The profile is based on the analysis of stress-induced well bore breakouts and consists of 32.616 orientation determinations at a basic sampling interval of ~4 cm. The azimuth of the apparent maximum horizontal compressive stress around the borehole is 057¿ (s.d.=19¿). Depth-dependent variations in breakout azimuths, from a few degrees to as much as 100¿, occur over depth intervals of several centimeters to hundreds of meters in the borehole, with wavelengths showing a self-similar distribution over a range of depth intervals. The variations in breakout orientation at depth seem to reflect stress fluctuations associated with active faults penetrated by the drill hole. Assuming linear elastic behavior, perturbations in stress magnitude and orientation were computed for predrilling slip on these faults. Fault stress drops limited to the ambient shear stress (i.e., up to total stress drop), along with occasional, local extreme stress drops and geometrical complications, can explain the variations in breakout orientations in the drill hole. The penetrative stress inhomogeneity may suggest that the average orientation of borehole breakouts in Cajon Pass, which indicates left-lateral shear on planes parallel to the San Andreas fault, may not be representative of the state of stress near the fault at depth in this region.

Rather, the stress orientation profile indicates the superposition of numerous local stress perturbations on an average stress field which is characterized by an absence of appreciable right-lateral shear on planes parallel to the San Andreas. Combining these observations with the highly variable stress state in shallow boreholes in the western Mojave Desert, it is suggested that the heterogeneous stress field in this region, representing a spectrum of seismic events, may extend over substantial distances across and along the San Andreas fault zone. If the average measured breakout orientation in Cajon Pass is representative of the state of stress near the San Andreas in this region, then substantial left-lateral shear stress is resolved on planes parallel to the fault, in contradiction to the sense of its long-term motion. We use an elastic dislocation model to compute the net stress change near the Cajon Pass drill site since just prior to the large 1812 earthquake, taking into account coseismic slip in major earthquakes and aseismic accumulation of slip on the San Andreas and San Jacinto faults. The model suggests that the net change in fault-parallel shear stress during the current earthquake cycle in the Cajon Pass area is nearly negligible. The difference between the measured and computed results requires that additional left-lateral shear stress be superimposed on a generally fault-normal maximum horizontal compressive stress. The results suggest that during the great Fort Tejon earthquake of 1857, left-lateral shear stress parallel to the San Andreas fault in the region of Cajon Pass may have played a role in terminating the southeastward extent of the rupture propagation. ¿ American Geophysical Union 1992