[MIS11-13] The next step following the accomplished drilling into seismogenic zones of M2.0-5.5 earthquakes in South African gold mines (ICDP DSeis)
Keywords:altered lamprophyre dike, fault material recovered, stress measured, hypersaline brine and gas, deep life
Previous projects of deep fault drilling from the Earth's surface or the ocean bottom include the North Anatolian, Alpine, Chelungpu, Nojima, and San Andreas, Wenchuan Faults, Eger Rift, as well as the JFAST and NanTroSEIZE drilling projects offshore Japan, respectively. The unprecedented outcomes from these investigations have enhanced the understanding of fault behavior significantly, yet nobody has yet elucidated seismogenic zones. Therefore, the ICDP Science Plan 2014-2019 raised several open fundamental questions to deepen our understanding of seismogenic processes and associated deep life. To address them, DSeis accomplished wire-line full-core drilling of three holes with a total length of 1.6 km. This approach was fundamentally different from the previous fault drilling projects which mainly used rotary or other non-coring drilling methods using drilling mud and with limited spot coring. The target of DSeis was the aftershock zone of an M5.5 earthquake below a deep gold mine in South Africa that several tens of near-field seismic sensors identified. Although seismicity of the DSeis's drilling target was more than two to four orders of magnitude lower than that of the projects mentioned above, the previous projects were challenged by identifying the location of the seismogenic zone accurately enough for detailed coring. In the DSeis project, the important differences included: (1) drilling from 2.9 km depth at the mine allowed design of the drilling directions with minimal risk of borehole breakouts or core discing; (2) a 3 m double and a 1.5 m triple-tubes were used for the critical sections, resulting in successfully recovering samples of not only intact core but also fragile lithologies in the fault zone; (3) drilling without mud resulted in better core recovery, e.g. not losing tiny rock fragments to be washed away. Petrological and geochemical analyses revealed an altered lamprophyre dike with talc hosted the M5.5 earthquake (Hirono et al. 2020 JpGU). The structure exhibited distinctively large density and magnetic susceptibility.
Funato and Ito (2017) developed a new non-destructive method that needs only a drilled core to measure stress, by Diametrical Core Deformation Analysis (DCDA), allowing us more than 200 measurements of differential stress. We supplement 3D core stress measurement at three depths in Hole A with Deformation Rate Analysis.
Ogasawara et al. (2020 JpGU) analyzed the 3D seismic reflection data, revealing a nearly vertical geological structure that crosscuts all the older formations and structures up to Karoo-age sediments near the Earth's surface. This structure is interpreted as a Karoo-age dike that was emplaced when the Gondwana supercontinent broke up at ca. 180 Ma.
Hole A intersected a hypersaline brine fissure rich in abiogenic volatile hydrocarbons (Rusley et al. 2018 AGU; Wiersberg et al. 2018 AGU; Nisson et al. 2019 AGU). These new findings will allow us to discuss how deep life can be fueled.
Additional effort to analyze the existing critical sections of the core at Kochi Core Center is crucial. As Holes A and B/C are still kept uncased and open except near the M5.5 structure, additional down-hole investigation increases critical data. We can plan an additional drilling campaign to traverse or reach an M2.7 aftershock source zone and the M5.5 strong-motion sources. To make the best plan, we have proposed ICDP a workshop at KCC.
ICDP, JSPS Core-to-Core Program, US NSF, German DFG, South African NRF, MEXT, KCC, and Ritsumeikan Univ. financially support the project.