Reinterpretation of a legacy 3D seismic reflection data to elucidate an unusual sinistral M5.5 earthquake fault below a structurally complex deep gold mine
Abstract
In 2014, an M5.5 earthquake occurred near Orkney, South Africa. This earthquake was different from usual mining-induced earthquakes because (1) typical M>4 mining-induced earthquakes take place at depths < 3 km on the normal faults on the mining horizons;(2) the M5.5 earthquake ruptured within a depth range of 3 - 7 km below the Moab Khotsong mine; and (3) the M5.5 earthquake was a sinistral and occurred on an unknown nearly-vertical structure, striking NNW-SSE.
The M5.5 earthquake hypocenter occurred in the meta-sedimentary formations of the West Rand Group (2.9 Ga) in the Witwatersrand Supergroup that lies on the Kaapvaal Craton. The Supergroup is overlain by ~2.7 Ga Ventersdorp (dominantly basalt) and ~ 2.6-2.7 Ga Transvaal supergroups (dominantly dolostone, shale and quartzite). This study describes and re-interprets legacy 3D reflection seismic data that cover the West Rand Group and younger sequences, acquired by AngloGold Ashanti in 1996 prior to mining at Moab Khotsong. The data extend to 4 seconds TWT (~ 12 km), and cover a 15 km x 9 km area with 25 m x 25 m bins. The 3D cube covered a much wider area of the M5.5 aftershock zone than the legacy data of four 2D lines re-interpreted by Ogasawara et al. (2017 AGU). Very importantly, the 3D cube included the three holes successfully drilled by the ICDP DSeis project (Ogasawara et al. Deep Mining 2019, EGU 2020, JpGU 2020a) that intersected seismically reflective intrusions and the upper fringe of the aftershock zone of the M5.5 earthquake. Ogasawara et al. (2017 AGU) found a vertical zone of disruption (VZD) in the four 2-D lines that potentially suggested a structure responsible for the M5.5 earthquake. They also found that the VZD was not crosscut by other known faults that dislocated gold reefs more than several hundreds of meters both horizontally and vertically. However, the 2D data could not provide more details. With the 3D cube, Noda (2020 Master thesis; Ogasawara et al. 2020 EGU JpGU) investigated the geological structure on the northwestern side of the aftershock area, with the southeastern side not re-interpreted. Using the available information of the mine geology, DSeis drilling and logging, we look into more detail. This poster will report on our additional re-interpretation of the structural architecture and 3D seismic velocity field around the M5.5 aftershock zone.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMMR0190003S
- Keywords:
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- 1822 Geomechanics;
- HYDROLOGY;
- 5104 Fracture and flow;
- PHYSICAL PROPERTIES OF ROCKS;
- 7299 General or miscellaneous;
- SEISMOLOGY;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS