Drilling into seismogenic zones of M2.0 – M5.5 earthquakes in deep South African gold mines (DSeis)

Abstract:

Mining-induced earthquakes with magnitudes equal to or larger than 2 take place only a few tens of meters away from active workings in South African gold mines several times a year. The largest event recorded in a South African mining region, a M5.5 earthquake, took place on 5 August 2014, with the upper edge of the activated fault being some hundred meters below the nearest mine workings (3.0 km depth). The depth and the strike-slip fault mechanism were very different from usual mining-induced events. This is one of the rare events for which detailed seismological data are available, both from surface and underground seismometers and strainmeters, allowing for a detailed analysis and comparison with in-situ observed data. This enables us to design a drilling program with a significantly better spatial coverage (potentially including rupture nuclei, strong motion sources, asperities, or rupture edges) than is possible when drilling in seismogenic zones of natural large earthquakes and with further advantages of lower drilling risk and costs.

In seismogenic zones in a critical state of stress, it is difficult to delineate reliably the local spatial variations of the directionand magnitude of the principal stresses. However, we have overcome this problem. We are able to numerically model stress better than before, enabling us to orientate boreholes so that the chance of stress-induced damage is minimized, and enabling us to measure the 3D full stress tensor reliably even when stresses are as large as those expected in seismogenic zones. Better recovery of cores with less stress-induced damage is also feasible. These factors will allow us to address key scientific questions in earthquake physics, namely the stress and strength in seismogenic zones, which so far have remained elusive.

We will report on the discussions on key scientific objectives, technical feasibility, and the best strategy at a workshop, which ICDP conditionally approved, that we hope will take place in South African in the fourth quarter of 2015.