TY - JOUR
T1 - High Fluid-Pressure Patches Beneath the Décollement
T2 - A Potential Source of Slow Earthquakes in the Nankai Trough off Cape Muroto
AU - Hirose, T.
AU - Hamada, Y.
AU - Tanikawa, W.
AU - Kamiya, N.
AU - Yamamoto, Y.
AU - Tsuji, T.
AU - Kinoshita, M.
AU - Heuer, V. B.
AU - Inagaki, F.
AU - Morono, Y.
AU - Kubo, Y.
N1 - Funding Information:
This research used samples and data provided by the International Ocean Discovery Program (IODP). The authors thank the ship's crew, technicians, and scientific party aboard D/V Chikyu during Expedition 370. The authors also special thanks to Y. Nanba for the underwater TV operations and D. Saffer, J.D. Bedford and K. Okazaki for their helpful discussions. We are also grateful to reviewers M. Collignon and K. Kanagawa and anonymous associate editor for their constructive comments that helped to improve this manuscript. This work was partly supported by the Japan Society for the Promotion of Science KAKENHI Grant Numbers JP19H02006, JP19K21907, 17H06455, and JP16H06476, by the Scientific Research on Innovative Areas “Science of Slow Earthquakes”, and by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy – EXC-2077 – 390741603.
Funding Information:
This research used samples and data provided by the International Ocean Discovery Program (IODP). The authors thank the ship's crew, technicians, and scientific party aboard D/V during Expedition 370. The authors also special thanks to Y. Nanba for the underwater TV operations and D. Saffer, J.D. Bedford and K. Okazaki for their helpful discussions. We are also grateful to reviewers M. Collignon and K. Kanagawa and anonymous associate editor for their constructive comments that helped to improve this manuscript. This work was partly supported by the Japan Society for the Promotion of Science KAKENHI Grant Numbers JP19H02006, JP19K21907, 17H06455, and JP16H06476, by the Scientific Research on Innovative Areas “Science of Slow Earthquakes”, and by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy – EXC‐2077 – 390741603. Chikyu
Publisher Copyright:
© 2021. The Authors.
PY - 2021/6
Y1 - 2021/6
N2 - Pore pressure plays a key role in the generation of earthquakes in subduction zones. However, quantitative constraints for its determination are quite limited. Here, we estimate the subsurface pore pressure by analyzing the transient upwelling flow of drilling mud from borehole C0023A of the International Ocean Discovery Program (IODP) Expedition 370, in the Nankai Trough off Cape Muroto. This upward flow provided the first direct evidence of an overpressured aquifer in the underthrust sediments off Cape Muroto. To estimate the pre-drilling pore pressure in the overpressured aquifer around a depth of 950–1,050 m below sea floor, we examined the measured porosities of core samples retrieved from nearby IODP wells; we then proceeded to explain the observed time evolution of the flow rate of the upwelling flow by modeling various sized aquifers through solving a radial diffusion equation. It was observed that for a permeability of 10−13 m2, the aquifer possessed an initial excess pore pressure of ∼5–10 MPa above the hydrostatic pressure, with a lateral dimension of several hundred meters and thickness of several tens of meters. The overpressure estimates from the porosity-depth profile at Site C0023 differ from those at other drill sites in the region, suggesting the possible existence of multiple overpressured aquifers with a patchy distribution in the underthrust sediments of the Nankai Trough. As pore pressure is relevant in maintaining fault stability, the overpressured aquifers may be the source of slow earthquakes that have been observed around the drilling site.
AB - Pore pressure plays a key role in the generation of earthquakes in subduction zones. However, quantitative constraints for its determination are quite limited. Here, we estimate the subsurface pore pressure by analyzing the transient upwelling flow of drilling mud from borehole C0023A of the International Ocean Discovery Program (IODP) Expedition 370, in the Nankai Trough off Cape Muroto. This upward flow provided the first direct evidence of an overpressured aquifer in the underthrust sediments off Cape Muroto. To estimate the pre-drilling pore pressure in the overpressured aquifer around a depth of 950–1,050 m below sea floor, we examined the measured porosities of core samples retrieved from nearby IODP wells; we then proceeded to explain the observed time evolution of the flow rate of the upwelling flow by modeling various sized aquifers through solving a radial diffusion equation. It was observed that for a permeability of 10−13 m2, the aquifer possessed an initial excess pore pressure of ∼5–10 MPa above the hydrostatic pressure, with a lateral dimension of several hundred meters and thickness of several tens of meters. The overpressure estimates from the porosity-depth profile at Site C0023 differ from those at other drill sites in the region, suggesting the possible existence of multiple overpressured aquifers with a patchy distribution in the underthrust sediments of the Nankai Trough. As pore pressure is relevant in maintaining fault stability, the overpressured aquifers may be the source of slow earthquakes that have been observed around the drilling site.
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U2 - 10.1029/2021JB021831
DO - 10.1029/2021JB021831
M3 - Article
AN - SCOPUS:85109201555
SN - 2169-9313
VL - 126
JO - Journal of Geophysical Research: Solid Earth
JF - Journal of Geophysical Research: Solid Earth
IS - 6
M1 - e2021JB021831
ER -