In-Situ Stress State at the Proximity of the Rupture Zone of an M2.2 Earthquake in Tautona Mine, South Africa

The Natural Earthquake Laboratory in South African Mines (NELSAM) utilizes the infrastructure of deep gold mines to study earthquake processes at focal depth. As part of this analysis in the Tautona mine, Western Deep Levels, South Africa, we determine the in-situ stress field by camera logging of boreholes. We logged seven short boreholes (10-40 m long) and one 418 m long borehole around the Pretorius fault, which is the largest fault in the region of NELSAM laboratory. We present here the analysis of in-situ stresses around the rupture zone of the M2.2 earthquake of December 12, 2004, at a depth of 3.6 km. The analysis is based on in- situ stress estimates from three independent sources: (1) In-situ stresses estimated from breakouts and drilling induced tensile fractures in three vertical boreholes located tens of meters away from the rupture zone. The breakouts and tensile fractures indicate Shmin = 30 to 36 MPa, and SHmax = 70 to 94 MPa; the later at azimuth 335 degrees. (2) The geometry and slip direction of the rupture zone as exposed in tunnels, some of which were mined months after the earthquake. The M2.2 earthquake reactivated four segments of the ancient Pretorius fault that display dextral-normal slip with maximum displacement of 2.5 cm during the earthquake slip. (3) The mining induced stresses that were calculated with a 3D boundary element program (MAP3D) for the mine layout at the earthquake time. These calculations show stress state of strike-slip faulting with horizontal stress orientations similar to the orientations indicated by the breakouts. We develop an optimization scheme for a stress tensor which is simultaneously compatible with the above three stress estimates as well as failure conditions. The results indicate conditions of dextral-normal faulting, low (~0.12) [shear stress/normal stress] ratio in the slip direction, and orientation of maximum compression close to verticality. We will discuss the implications of these results to fault weakening and stress state induced by mining. The study was supported by NSF Continental dynamics grant 0409605.