Beschreibung
Several times a year, small (M2) mining-induced earthquakes occur only a few tens of meters from active workings in South African gold mines at depths of up to 3.4 km. The source regions of these events are accessible with short boreholes from the deep mines, and provide a very cost-effective method to directly study the earthquake sources. Recently, the largest event (M5.5) recorded in a mining region, took place near Orkney, South Africa on 5 August 2014, with the upper edge of the activated fault being several hundred meters below the nearest mine workings (3.0 km depth). This event has rare detailed seismological data available both from surface and underground seismometers and strainmeters, allowing for a detailed seismological analysis. Drilling to the source area of this earthquake while aftershocks are still occurring, will enable important near-field seismological observations, as well as a rare opportunity to study possible H2 that is important for microbiological activity. This project proposes to drill 40 holes into and around seismogenic zones to study the rupture details and scaling of both small (M2.0) and larger (M5.5) earthquakes. An advantage of the relatively low cost of the drilling, is that multiple holes can be drilled. Past fault zone drilling projects have been limited to 1 or 2 boreholes, severing limiting the ability to resolve spatial variability. The value of the project will be maximized if we combine results from a number of boreholes drilled into the source area of the M5.5 seismogenic zone, and also compare with boreholes in source regions of small earthquakes of other mining horizons. Additionally, the combination of logging, fault sampling, and earthquake monitoring, will be enhanced in some cases by the direct visual observations of exhumed faults, leading to a unique complete picture of the earthquake source. In seismogenic zones in a critical state of stress, it is difficult to delineate reliably the local spatial variation in both the directions and magnitudes of principal stresses (3D full stress tensor). We have overcome this problem and can numerically model stress better, enabling orientations of boreholes that minimize stress-induced damage during drilling and overcoring. We can also reliably measure the full 3D stress tensor 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 studies will allow us to address key scientific questions in earthquake science and deep biosphere activities.