The Orkney earthquake (M5.5) occurred beneath the Moab Khotsong gold mine on the western edge of the Witwatersrand Basin in South Africa. The “Drilling into Seismogenic Zones of M 2.0-5.5 Earthquakes in Deep South African Gold Mines (DSeis)” project, which was adopted by the International Continental Scientific Drilling Program (ICDP), drilled from mine workings at a depth of 2.9 km. The project was successful in reaching the upper margin of the Orkney earthquake aftershock zone, extending from 3.5 km to 7 km below the surface. This study focused on seismicity within a 2 × 1 × 1 km 3 area surrounding the DSeis boreholes. Hypocenters were determined using the Double-Difference method, based on data recorded by 9 seismometers (with a sampling frequency of 6000 Hz) installed in mine tunnels at depth of 2.8–3.0 km, located within 1–2 km of the aftershock zone. Events ranged from Mw −1.8 to 1.6 and were recorded during the two weeks following the mainshock. The relocated aftershocks are distributed in a vertical NNW-SSE striking place that coincides with a lamprophyre dike and are concentrated in three sub-horizontal streaks that plunge at approximately 30 degrees. Integration of the aftershock distributions with legacy high-resolution 3D reflection seismic data confirmed that the streak pattern corresponds to the dip of strong seismic reflectors, likely sills. These results suggest that the intersections of the dike with multiple sills acted as strain accumulation zones, thereby contributing to the formation of the streak-like distribution of aftershocks. • Accurately located 1573–1.8 < Mw < 1.6 aftershocks of an M L 5.5 South African earthquake. • Aftershocks form a near-vertical fault plane with three sub-parallel streaks. • The near-vertical aftershock distribution coincides with a lamprophyre dike. • Streaks match the thickness and inclination of sills shown by reflection seismic data. • Dike-sill intersections likely controlled these streak-like aftershock clusters.
Fujita et al. (Fri,) studied this question.