09:45 〜 10:00
[MGI32-08] ICDP DSeis Report 1: 南アフリカ金鉱山地震発生場掘削計画2021年度の活動報告
キーワード：地震発生場、Full-core掘削、上部地殻質母岩、超苦鉄質変質貫入岩
We overview our activity in FY2021 in the project ”Drilling into Seismogenic zone of M2.0-5.5 earthquakes in South African gold mines (DSeis; Ogasawara et al. Afrirock 2017)”. International Continental Scientific Drilling Program (ICDP) approved this project in 2016, accomplishing drilling and downhole logging in 2018 (Ogasawara et al. Deep Mining 2019; ICDP Thrill to Drill). In 2019, we imported the critical section (a hundred and several tens of meters in total length) to Kochi Core Center and completed a non-destructive investigation for all the imported core and a 1m-interval XRD and XRF analysis (Yokoyama, 2020 Osaka Univ. Master Thesis).
The DSeis project (1) targeted the seismogenic zones elucidated by the dense seismic monitoring networks in the forerunning project, successfully (2) reached seismogenic zones in country rocks with density and Vp as high as those in rock mass in the upper crust (2.7 g/cm3 and 6 km/s, respectively), not seismic faults in sedimental covers or fault zones in unconsolidated formation (Vp<4 km/s);
(3) recovered full-core of a total of 1.6 km length with a wire-line NQ and BQ diamond drilling (with the drilling rigs underground; we used a 1.5m triple-tube for the most critical section);
(4) on the mining horizon, characterized seismogenic zone, measuring several tens of meters across, evolved ahead of mining faces in Archean felsic metasedimental rock mass;
(5) below the mining horizon, intersected altered mafic or ultramafic intrusives in felsic Archean metasedimental formations. Some sill-dyke complexes are associated with the Ventersdorp (2.7Ga) Large Igneous Province activity (LIP) and additional later mantle plume activities in several generations. The ~N-S altered ultramafic dykes (thinner than several meters) adjacent to each other host the aftershock zone of the M5.5 earthquake and the non-meteoric hypersaline brine rich in abiogenic dissolved organic carbon, respectively.
The outcomes in FY2021 include
(1) Industrial XCT, XRF scan, EPMA, FTIR, and core piece by piece Vp measurement at Kochi Core Center successfully characterize the difference between the ultramafic altered dykes mentioned above. We could elucidate the significant spatial variation in XCT-value, density, magsus, velocity, and chemical composition in the altered ultramafic dyke adjacent to the core loss zone (ICDP DSeis 2 and 3 in JpGU2022 S-CG46)
(2) We could better calibrate in the 3D velocity field in 15 km x 9 km x 7 km volume, including most of the aftershock zone of the M5.5 earthquake. As a result, we could much better elucidate the relationship between the streaks of the aftershock and the intersection of the mafic sills and the altered ultramafic dyke (ICDP DSeis 4 in JpGU 2022 S-SS07). Furthermore, with the calibrated velocity model with much faster dolomite formation and sills, we expect to address an outstanding problem that very near-field M5.5 seismograms/straingrams have not been able to be accounted for by any faulting model (ICDP DSeis 5 in JpGU 2022 S-SS07).
(3) With additional measurements in 2021, we could compile all available DCDA core stress measurements to have detailed stress spatial variation around the upper fringe of the aftershock zone (ICDP DSeis 6 in JpGU2022 S-SS007)
(4) JpGU2022 B-GM02 summarizes advantages in studying deep life or early life with the DSeis core and the hypersaline fissure deep underground including 2022 down hole logging.
The DSeis team consists of seismologists, geologists, geomicrobiologisits, rock mechanists, mining engineers from Japan, South Africa USA, Switzerland, Germany, India, and Australia. The DSeis project is build on JST-JICA SATREPS project and Kakenhi (21224012) and is supported by ICDP, JSPS Core-to-Core Program, Ritsumeikan University, MEXT 2nd Earthquake and Volcano Hazard Reduction Research, and Kochi Core Center.
The DSeis project (1) targeted the seismogenic zones elucidated by the dense seismic monitoring networks in the forerunning project, successfully (2) reached seismogenic zones in country rocks with density and Vp as high as those in rock mass in the upper crust (2.7 g/cm3 and 6 km/s, respectively), not seismic faults in sedimental covers or fault zones in unconsolidated formation (Vp<4 km/s);
(3) recovered full-core of a total of 1.6 km length with a wire-line NQ and BQ diamond drilling (with the drilling rigs underground; we used a 1.5m triple-tube for the most critical section);
(4) on the mining horizon, characterized seismogenic zone, measuring several tens of meters across, evolved ahead of mining faces in Archean felsic metasedimental rock mass;
(5) below the mining horizon, intersected altered mafic or ultramafic intrusives in felsic Archean metasedimental formations. Some sill-dyke complexes are associated with the Ventersdorp (2.7Ga) Large Igneous Province activity (LIP) and additional later mantle plume activities in several generations. The ~N-S altered ultramafic dykes (thinner than several meters) adjacent to each other host the aftershock zone of the M5.5 earthquake and the non-meteoric hypersaline brine rich in abiogenic dissolved organic carbon, respectively.
The outcomes in FY2021 include
(1) Industrial XCT, XRF scan, EPMA, FTIR, and core piece by piece Vp measurement at Kochi Core Center successfully characterize the difference between the ultramafic altered dykes mentioned above. We could elucidate the significant spatial variation in XCT-value, density, magsus, velocity, and chemical composition in the altered ultramafic dyke adjacent to the core loss zone (ICDP DSeis 2 and 3 in JpGU2022 S-CG46)
(2) We could better calibrate in the 3D velocity field in 15 km x 9 km x 7 km volume, including most of the aftershock zone of the M5.5 earthquake. As a result, we could much better elucidate the relationship between the streaks of the aftershock and the intersection of the mafic sills and the altered ultramafic dyke (ICDP DSeis 4 in JpGU 2022 S-SS07). Furthermore, with the calibrated velocity model with much faster dolomite formation and sills, we expect to address an outstanding problem that very near-field M5.5 seismograms/straingrams have not been able to be accounted for by any faulting model (ICDP DSeis 5 in JpGU 2022 S-SS07).
(3) With additional measurements in 2021, we could compile all available DCDA core stress measurements to have detailed stress spatial variation around the upper fringe of the aftershock zone (ICDP DSeis 6 in JpGU2022 S-SS007)
(4) JpGU2022 B-GM02 summarizes advantages in studying deep life or early life with the DSeis core and the hypersaline fissure deep underground including 2022 down hole logging.
The DSeis team consists of seismologists, geologists, geomicrobiologisits, rock mechanists, mining engineers from Japan, South Africa USA, Switzerland, Germany, India, and Australia. The DSeis project is build on JST-JICA SATREPS project and Kakenhi (21224012) and is supported by ICDP, JSPS Core-to-Core Program, Ritsumeikan University, MEXT 2nd Earthquake and Volcano Hazard Reduction Research, and Kochi Core Center.