[M-GI28] 地球掘削科学

[MGI28-10] Probing the heart of an earthquake and life in the deep subsurface (PROTEA) -Progress in 2024 toward the international workshop-

*矢部 康男¹、Manzi Musa²、Gomez-Arias Alba³、Nisson Devan⁴、Castillo Julio⁵、Durhheim Raymond²、小笠原 宏⁶、Kieft Thomas⁷、Perl Scott⁸、Maleke Maleke⁹、林 立虹¹⁰、Wu Hung-Yu¹¹、Colombero Chiara¹²、Wadas Sonja¹³、Liebenberg Bennie¹⁴ (1.東北大学大学院理学研究科附属地震・噴火予知研究観測センター、2.School of Geosciences, University of the Witwatersrand、3.Institute of Natural Resources and Agrobiology(IRNAS), Spanish National Research Council (CSIC)、4.NASA Ames Research Center、5.Department of Integrated Science, University of Huelva、6.立命館大学総合科学技術研究機構、7.Biology Department, New Mexico Institute of Mining and Technology、8.Earth, Planetary, and Space Sciences, University of California Los Angeles、9.Department of Life Sciences, Central University of Technology, Free State、10.国立台湾大学地球科学科、11.国立成功大学資源工学科、12.Department of Environmental, Land and Infrastructure Engineering, Politecnico di Torino、13.Department 1: Geophysical Exploration, LIAG - Institute for Applied Geophysics、14.ICDP-DSeis)

キーワード：震源断層掘削、地下生命圏、緑色片岩相、ICDP、PROTEA、南アフリカ

A number of ICDP/IODP projects have reached the seismogenic zone of active earthquake faults; however, a sample from the hypocenter of an earthquake greater than M5 in hard rock formation has never been obtained. Hypocenters of earthquakes are usually at depths of 5 to 20 km and are very difficult drilling targets. The 2014 Orkney earthquake (M5.5), South Africa, occurred at 5 km depth, just beneath Moab Khotsong mine, one of the world’s deepest mines. The ICDP-DSeis (Drilling into seismogenic zones of M2.0-M5.5 earthquakes in deep South African gold mines) project drilled holes from a tunnel at 2.9 km depth and succeeded in recovering rock samples from an active, seismogenic fault that hosted high aftershock activity following the Orkney earthquake. The fault was formed in an altered lamprophyre dike rich in talc, biotite, Ca/Na amphibole and calcite and intruded into the 2.9 Ga altered basaltic andesite lava of the Crown Formation. The aftershocks aligned linearly and sub-parallel to seismic reflectors that correspond to sills intruded into the West Rand Group. The DSeis holes also intersected a fissure that hosts hypersaline (23wt%), moderate temperature (54℃) and high pressure (10 MPa) brine. The brine has been isolated for 1.2 Ga from near-surface meteoric water and hosts a microbial ecosystem. Therefore, Moab Khotsong is the only mine in the world that is known to have an ensemble of drillable targets (Figure) that enable us to address the key scientific questions:
Q1. How do variations in the mechanical properties and conditions along the fault affect the nucleation, propagation and termination of an earthquake?
Q2. How do the interactions between continental and mantle-derived rocks and fluids under the pressure and temperature (P-T) condition at the bottom or below the natural seismogenic zones generate diversity of rheology?
Q3. How great is the diversity of microbial ecosystems in an environment that mimics the expected Martian underground environment?
Q4. What is the balance between H₂-based chemoautotrophy and heterotrophic metabilism of ancient, recalcitrant organic C in this long-sequenstrted hypersaline aquifer?

To answer these questions, PROTEA (Probing the heart of an earthquake and life in the deep subsurface) led by 14 scientists from 7 countries and regions plans to drill into the hypocenter of the Orkney earthquake from 2-3 km deep tunnel. The physical, geological and hydrological analyses of recovered cores will significantly deepen understanding of earthquake generation from the perspective of physics, mechanics and material science.
PROTEA also provides an opportunity to study the water-rock interaction at P-T conditions towards the base of the seismogenic zone using rock samples that have never been exposed to the surface environment; hence the crystal structure and mineral composition are unaffected by weathering. Post-drilling active and passive seismic surveys using DAS and nodal seismometers will elucidate fine structures of the dike-sill complex.
PROTEA will drill and core multiple holes intersecting the hypersaline brine fissure, employing contamination control and quantification measures. This initiative will facilitate the study of microbial diversity in uncontaminated microfractures of the core samples. The ancient and radiolytically enriched hypersaline brine is one of the most physiologically challenging microbial habitats studied in the deep subsurface. PROTEA provides a unique opportunity to investigate a pristine and metabolically active microbial ecosystem analog to Mars subsurface environment, allowing us to postulate how cellular life may thrive under such extreme geobiological conditions.