Potential borehole observatory in the aftershock zone at about 3.5 km depth of a 2014 M5.5 sinistral earthquake that the ICDP DSeis project has probed

It is invaluable if we can deploy an observatory in boreholes that have intersected a seismogenic zone to understand mechanical, chemical, hydrological, and geo-microbiological interactions better. We propose an observatory in the zone elucidated as described below.

In 2016, ICDP approved the project, Drilling into the Seismogenic zone of M2.0-M5.5 earthquakes in South African gold mines (DSeis). During 2017-2018, the DSeis team completed three full-core NQ drillings from 2.9 km depth at an active gold mine (817 m Hole A, 700 m Hole B, and 96 m Hole C branched from Hole B), with Holes B and C successfully intersecting the upper fringe of the aftershock zone of an earthquake of M5.5. The team could recover a damage zone (several tens of meters thick) with fault core zone (a few meters thick) with velocity-strengthening, substantially weak materials. They probed spatially heterogeneous lithologies, physical properties, ruptures, and rock stresses almost throughout the holes above the fault intersection, successfully revealing that the West Rand Group (metamorphosed sedimentary hard rock formations, 2.8-2.9 Ga), occasionally intruded by mafic sills and dikes (2.0 ~ 2.2 Ga), hosted the M5.5 capable fault. 10 MPa hypersaline brine and abiogenic gases have been detected and continuously monitored in Hole A.

Seventeen strong-motion meters on the earth's surface with several km's station intervals, forty-six in-mine sensitive seismometers with less than several hundreds of meters' intervals, and legacy data of seismic reflection exploration were available for comparison. The comparison elucidated the following characteristics of the aftershock zone: (1) aftershocks align as stripes dipping to the south on the M5.5 fault, (2) a sharp cut-off of aftershock zones also dips to the south, (3) the M5.5 mainshock slip centroids are located below and away from the aftershock zone, and (4) the M5.5 rupture and aftershocks span a range of 3-8 km depth from earth's surface, along a NNW-SSE oriented dike that intruded into the Central Rand Group (2.8 Ga) that hosts gold reefs and a 2.5-2.6 Ga near-surface dolomite formation, while earthquakes in South African gold mines are typically induced on mining horizons on normal faults striking NE-SW that offset gold reefs sometimes as significantly as about 1 km horizontally or vertically.