ESTIMATION OF THE COMPLETE STRESS TENSOR TO 8-KM-DEPTH IN THE KTB-SCIENTIFIC-DRILL-HOLES - IMPLICATIONS FOR CRUSTAL STRENGTH

For many years, in situ stress in the brittle crust has been measured at relatively shallow depth and related to the mechanical behavior of the crust as inferred from laboratory studies and faulting theory. A c ontinuous profile of the magnitudes and orientations of the three prin cipal stresses has been estimated to depths of 7.7 km and 8.6 km in th e German Continental Deep Drilling Program (KTB). This was achieved by hydraulic fracturing tests at relatively shallow depth (1-3 km), esti mates of the magnitude of the least horizontal principal stress provid ed by modified hydraulic fracturing experiments at 6 km and 9 km depth s, and analysis of compressional (breakouts) and tensile (drilling-ind uced tensile wall fractures) failures of the borehole wall over nearly the entire depth of the KTB borehole. The orientation of the maximum horizontal principal stress was found to be uniform with depth with an orientation of N160 degrees+/-10 degrees E, which is consistent with the average orientation found throughout western Europe. The only sign ificant change in stress orientation was observed directly below a maj or fault zone crosscutting the borehole. The profile of stress magnitu des we have obtained demonstrates that to a depth of 8 km, the state o f stress in the brittle crust in southern Germany is in frictional equ ilibrium. That is, the ratio of shear to normal stress as resolved on preexisting faults which are well-oriented to the in situ stress field is comparable to their frictional strength based on predictions of Co ulomb faulting theory for a coefficient of friction of about 0.7 and n ear-hydrostatic pore pressure.