Drilling into seismogenic zones of M2.0 - M5.5 earthquakes in deep South African gold mines
Africa, South Africa, North West, Orkney
New Full-proposal: ICDP-2016/08
For the funding-period starting 2016-01-15
For the funding-period starting 2016-01-15
by Hiroshi Ogasawara, Yasuo Yabe, Takatoshi Ito, Gerrie van Aswegen, Artur Cichowicz, Raymond john Durrheim, Tullis C. Onstott, Thomas L. Kieft, Margaret S. Boettcher, Stefan Wiemer, Martin Ziegler, Serge A. Shapiro, Harsh Gupta, Phillip Dight
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.
- We plan 40 boreholes into source regions of earthquakes in South Africa gold mines. Results will provide information for fundamental understanding of earthquake sources. Targets are (A1) Active aftershock zone below the deepest mining horizon (3 km) activated by M5.5 earthquake (A2) Seismogenic zones where the M5.5 ruptures stopped,
- (B) M~2 rupture zones on mining horizons quasi-statically evolved and exhumed by mining,
- (C) M2.8 rupture on mining horizon where additional events are induced and fault will be exhumed,
- (D) Significant seismically active dike on a mining horizon. We will integrate existing data with results of drilling, logging, core investigation, post-drilling monitoring to address key seismological questions.
- - What physical properties control rupture evolution and termination? We will drill into the main rupture zone, edge areas, and surrounding regions outside of the rupture. We will also study in-situ data to explore the aftershock areas.
- - How do seismogenic zones and surroundings vary with time between larger events? Post-drilling measurements and local sensitive monitoring network will elucidate changing rock and fluid properties.
- - What are structural and physical differences for different size earthquakes? Source parameters and local stress conditions will study scaling relationships.
- - How do the fluid and chemical properties of faulting influence microbiological communities? In-situ monitoring will enable geomicrobiologists to discuss relationship between physical properties of active faults and deep biosphere activities.
- - How to explain the M5.5 Orkney earthquake? It is important to unravel the unusual properties for this earthquake, which is one of the largest mining induced events.
Earthquakes, Geomicrobiology, Gold Mines, Orkney, Seismogenic Zone Drilling, South Africa