Stress state in the upper margin of aftershock zone of the 2014 Orkney earthquake (M5.5), South Africa, measured from boring cores of ICDP-DSeis project
Abstract
The 2014 Orkney earthquake (M5.5) occurred beneath a gold mine in South Africa. The shallowest aftershocks were located only several hundred meters below the deepest tunnel in the mine. Two boreholes (Holes A and B; 817 and 700 m respectively) toward the upper margin of aftershock zone were drilled from a tunnel at 2.9 km depth in the mine by the ICDP-DSeis project. Hole A deflected from the aftershock zone, while Hole B intersected the aftershock zone. Except for the intersection, boring core recovery from these holes was ~100%.
The focal mechanism solutions of mining induced earthquakes shallower than ~3 km are the normal faulting ones, while those of the Orkney earthquake and its aftershocks deeper than 3.5 km are the strike-slip ones. In this study, we applied the Deformation Rate Analysis (DRA) and the Diametrical Core Deformation Analysis (DCDA) techniques to boring cores recovered from Holes A and B of the ICDP-DSeis project to explore the depth variation in the stress state that would cause the depth variation in the faulting regime. The DRA can determine the normal stress from hysteresis of the stress-strain curve under a cyclic loading. The DCDA can evaluate the differential stress in the plane normal to a borehole from the ellipsoidal cross-section shape of boring cores. Boring cores of Hole A are oriented by correlating joints and veins on the borehole wall observed by an optical televiewer and the boring cores. The normal stress parallel to the borehole axis down to ~500 m from the hole collar are measured by the DRA. The DCDA is applied to boring cores down to ~700 m from the hole collar. Localized enhancements of stresses are detected at ~450 m and ~700 m distances from the hole collar of Hole A. The former anomaly coincides with fractures from which high salinity water outflows. The latter anomaly corresponds to the depth of shallowest aftershocks. Significant perturbation of stress orientation is also observed at this depth.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMNH11D0803Y
- Keywords:
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- 4315 Monitoring;
- forecasting;
- prediction;
- NATURAL HAZARDS;
- 7212 Earthquake ground motions and engineering seismology;
- SEISMOLOGY;
- 7294 Seismic instruments and networks;
- SEISMOLOGY;
- 8194 Instruments and techniques;
- TECTONOPHYSICS