Stress State in the Source Region of Mw2.2 Earthquake in a Deep Gold Mine in South Africa Determined from Borehole Cores

Abstract

In December 2007, an Mw2.2 earthquake occurred in a gabbroic dike at 3.3 km depth in a deep gold mine in South Africa. The fore- and aftershock activity was analyzed in an effort to understand the preparation and generation processes of earthquakes; these findings have already been published. The present paper focuses on the stress state in the source region of the mainshock. A 90-m-long borehole across the mainshock fault was drilled ~ 1.5 years after the mainshock and logged by an optical televiewer. The fault can be identified by severe damage to the borehole wall at the point where the borehole intersected the aftershock cluster. Except for a 10-m section in the hanging wall of the fault, borehole cores were fully recovered. Borehole breakout (BO) and core disking (CD) were found to occur. Two stress measurement techniques [Deformation Rate Analysis (DRA) and Diametrical Core Deformation Analysis (DCDA)] were applied to the borehole cores. By combining their results with occurrence criteria for BO and CD, the principal stress state in the source region of the mainshock was determined. The principal directions in the hanging wall of the fault were nearly identical to the virgin stress state, while it was significantly disturbed in the footwall. The vertical stresses were 106 MPa and 40 MPa in the hanging wall and footwall, respectively. The significant difference in the vertical stress between the two sides of the fault can be explained by the stress redistribution associated with a nonuniform slip on a nonplanar fault.

Frictional properties of rocks recovered form aftershock cloud of the 2014 Orkney earthquake (M5.5), South Africa, by ICDP-DSeis

“…Drilling into a seismogenic fault provides good opportunities to investigate micro-structures (Bradbury et al, 2011;Bouller et al, 2009;Chester et al, 2013) and to determine its mechanical (Carpenter et al, 2015;Chéry et al, 2004;Fulton et al, 2013;Wu et al, 2007;Yabe et al, 2008;Yabe et al, 2022), chemical (Li et al, 2013;Moore, 2014;Schleicher et al, 2015), and hydrological (Fujimoto et al, 2007;Tanikawa et al, 2013) properties. As these knowledges are fundamental to understand and evaluate hazards induced by earthquakes and to mitigate its risks, fault-zone drilling has been a centerpiece of the International Continental Scientific Drilling Program (ICDP) since its foundation in 1996 (ICDP Science Plan 2020-2030.…”

Factors influencing caving

“…This was explained by DCDA and HF using elastic methods, while DRA used inelastic methods. It should be noted that DCDA is a non-destructive test and DRA is destructive Yabe et al (2022). report on parallel DRA and DCDA testing at the Mponeng gold mine in South Africa.…”