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Affiliation |
Graduate School of Engineering Science Center for Crossover Education |
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Laboratory Address |
Riko 3rd Bldg. #108, Akita University, 1-1 Tegata Gakuen Machi, Akita 010-8502, Japan |
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Mail Address |
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NAKAO Atsushi
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Research Interests 【 display / non-display 】
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Geodynamics
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Plate tectonics
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Subduction zone
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Data assimilation
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Numerical simulation
Graduating School 【 display / non-display 】
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2008.04-2012.03
University of Tsukuba School of Life and Environmental Sciences College of Geoscience Graduated
Graduate School 【 display / non-display 】
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2014.04-2017.03
Tokyo Institute of Technology Graduate School, Division of Science and Engineering Department of Earth and Planetary Sciences Doctor's Course Completed
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2012.04-2014.03
Tokyo Institute of Technology Graduate School, Division of Science and Engineering Department of Earth and Planetary Sciences Master's Course Completed
Degree 【 display / non-display 】
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Tokyo Institute of Technology - Doctor of Science
Campus Career 【 display / non-display 】
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2023.04-Now
Akita University Graduate School of Engineering Science Center for Crossover Education Assistant Professor
External Career 【 display / non-display 】
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2023.05
Japan Agency for Marine-Earth Science and Technology Research Institute for Marine Geodynamics Visiting Researcher
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2021.04-2023.03
Japan Agency for Marine-Earth Science and Technology Volcanoes and Earth's Interior Research Center, Research Institute for Marine Geodynamics Postdoctoral Researcher
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2021.01-2021.03
The University of Tokyo Earthquake Research Institute Project Researcher
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2019.09-2020.10
The University of Hong Kong Department of Earth Sciences Post-doctoral Fellow
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2017.04-2019.08
The University of Tokyo Earthquake Research Institute Project Researcher
Academic Society Affiliations 【 display / non-display 】
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2017.07-Now
United States
American Geophysical Union
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2014.10-Now
Japan
The Seismological Society of Japan
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2010.05-Now
Japan
Japan Geoscience Union
Research Areas 【 display / non-display 】
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Natural Science / Solid earth sciences / Geodynamics, Seismology, Data Science
Qualification acquired 【 display / non-display 】
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First Kind of High School Teacher License
Thesis for a degree 【 display / non-display 】
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Roles of water in subduction zone dynamics and mantle chemical evolution
Atsushi Nakao
2017.03
Single author
Research Achievements 【 display / non-display 】
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Adjoint-based data assimilation for reconstruction of thermal convection in a highly viscous fluid from surface velocity and temperature snapshots
Nakao, A., Kuwatani, T., Ito, S., & Nagao, H.
Geophysical Journal International 236 ( 1 ) 379 - 394 2024.01 [Refereed]
Research paper (journal) Domestic Co-author
It is a general problem in geoscience to estimate the time-series of velocity and temperature fields for a fluid based on limited observations, such as the flow velocity at the fluid surface and/or a temperature snapshot after flow. In this study, an adjoint-based data assimilation method (also known as four-dimensional variational data assimilation) was used to reconstruct the thermal convection in a highly viscous fluid (e.g., Earth’s mantle) to investigate which observations constrain the thermal convection and how accurately the convection can be reconstructed for different wavelengths. The data assimilated to the adjoint-based model were generated synthetically from forward models with convecting cells of different length-scales. Based on the surface velocity and temperature snapshot, our simulations successfully reconstructed thermal convection over 50 Myr in the case that the wavelength of the convective cells is sufficiently large. We obtained two main results from this parametric study. (1) When we only considered instantaneous thermal structure fitting in the cost function, the convection reconstruction tended to fail. However, there are some cases where the laminar thermal convection can be reconstructed by assimilating only the velocity along the fluid surface. (2) There is a limit to the reconstruction of thermal convection in the case that the convecting cells are small (∼1,000 km for a 50 Myr reconstruction). We propose that (1) is related to the balance of forces due to the thermal buoyancy and viscous stress around the thermal anomalies, and (2) is related to how information is preserved (i.e., how the previous thermal structure is maintained in the observable state throughout the convection process). The results enable the use of geological records to estimate time-series of velocity and temperature in Earth’s deep interior, even though the records may only contain information from shallow parts of Earth.
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Regression analysis and variable selection to determine the key subduction-zone parameters that determine the maximum earthquake magnitude
Nakao, A., Kuwatani, T., Ueki, K., Yoshida, K., Yutani, T., Hino, H., & Akaho, S.
Earth, Planets and Space ( Springer Nature ) 75 78 2023.05 [Refereed]
Research paper (journal) Domestic Co-author
Large variations in the maximum earthquake magnitude (Mmax) have been observed among the world’s subduction zones. There is still no universal relationship between Mmax and a given subduction-zone parameter, such as plate age, plate dip angle, or plate velocity, which suggests that multiple parameters control Mmax. Here we conduct exhaustive variable selections that are based on three evaluation criteria; leave-one- out cross-validation errors (LOOCVE), Akaike information criterion (AIC), and Bayesian information criterion (BIC) to determine the combination of subduction-zone parameters that best explains Mmax. Multiple linear regression analyses are applied using 18 subduction-zone parameters as potential candidates for the explanatory variables of Mmax. The minimum BIC is obtained when five variables (trench sediment thickness, existence of an accretionary prism, upper-plate crustal thickness, bending radius of the subducting oceanic plate, and trench depth) are selected as explanatory variables; each variable contributes positively to Mmax. Minimum LOOCVE and AIC values are obtained when eight variables (the five parameters for BIC, plus the along- strike plate convergence rate, age of the subducting plate, and maximum depth of the subducting plate) are selected. Our selection of the trench sediment thickness and plate bending radius contributing to Mmax is consistent with previous studies. The results show that increasing upper-plate crustal thickness results in a large Mmax. In addition to smoothing the subducting-plate interface via subducted sediments, along- dip extension of the crustal area along the convergent plate boundary would be important for generating a large earthquake.
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Subduction-zone parameters that control behavior of subducted oceanic plates at the 660-km depth boundary revealed by logistic regression analysis and model selection
Nakao, A., Kuwatani, T., Ueki, K., Yoshida, K., Yutani, T., Hino, H., & Akaho, S.
Frontiers in Earth Sciences ( Frontiers ) 10 2022.10 [Refereed]
Research paper (journal) Domestic Co-author
The potential mechanisms that drive the behavior of subducted oceanic plates at the 660-km discontinuity are subject to debate. Here we conduct logistic regression analysis and model selection to determine the key subduction-zone parameters in natural subduction zones that discriminate the plate behavior along the discontinuity. We select the key variables based on three information criteria: leave-one-out cross-validation score (LOO), Akaike Information Criterion (AIC), and Bayesian Information Criterion (BIC). Among the 17 subduction-zone parameters analyzed, only the trench velocity, convergence rate, and trench width are selected in the simplest model that minimizes BIC. The thermal parameter and several other variables are also selected to minimize AIC and LOO. Our results suggest that a stagnant slab occurs along the 660-km discontinuity when there is a narrow oceanic plate and a retreating trench in natural subduction zones, which has also been modeled in previous numerical simulations. Neither the stress nor the deformation rate of the upper-plate margin is selected in the three optimal models, which suggests that back-arc spreading in natural subduction zones does not globally characterize plate behavior at the 660-km discontinuity, although back-arc spreading and a stagnant slab coincide in some numerical simulations. The combination of subduction-zone data analysis and numerical simulations will therefore provide deep insights into the dynamics of Earth’s deep interior.
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Simultaneous analysis of seismic velocity and electrical conductivity in the crust and the uppermost mantle: A forward model and inversion test based on grid search
Iwamori, H., Ueki, K., Hoshide, T., Sakuma, H., Ichiki, M., Watanabe, T., Nakamura, M., Nakamura, H., Nishizawa, T., Nakao, A., Ogawa, Y., Kuwatani, T., Nagata, K., Okada, T., & Takahashi, E.
Journal of Geophysical Research: Solid Earth 126 ( 9 ) 2021.08 [Refereed]
Research paper (journal) Domestic Co-author
This study presents a forward model to quantify the P-wave velocity (VP), S-wave velocity (VS), and electrical conductivity (σ) of the solid-liquid mixtures for a given set of pressure, temperature, lithology, liquid phase (aqueous fluid or melt), liquid fraction, and geometrical parameters in relation to the aspect ratio and connectivity of the liquid phase. This is based on previous experimental and theoretical studies on seismic velocity and electrical conductivity of solid rocks and liquid phases. A total of 78 lithologies, an aqueous fluid with NaCl (∼0–10 wt.%), and mafic to felsic melt appropriate for the crust and the uppermost mantle conditions were described in terms of VP, VS, and σ, as per previous experimental measurements and molecular dynamics simulation. This forward model is provided as a Windows executable program, and generates synthetic VP, VS, and σ, referring to the seismic velocities and electrical conductivity observed in the northeast Japan arc. After generation of the synthetic VP, VS, and σ, the original lithology and liquid parameters (phase, fraction, aspect ratio, and connectivity) were searched by implementing the grid search algorithm to map the misfit over the broad parameter space. The mapping shows the presence of a global misfit minimum around the optimized solution and the possibility of resolving the lithology and the liquid phase parameters based on the observed VP, VS, and σ by using the forward model presented in this study.
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A possible roll-over slab geometry under the Caroline Plate imaged by Monte Carlo finite-frequency traveltime inversion of teleseismic SS phases
Fuji, N., Jang, H., Nakao, A., Kim, Y.-H., Fernández-Blanco, D., Lee, S.-M., Schroeder, A., & Konishi, K.
Frontiers in Earth Science ( Frontiers ) 9 2021.03 [Refereed]
Research paper (journal) International Co-author
The shape of a subducting slab varies as a function of trench motion. Two end-members of subduction modes are geodynamically possible: roll-back mode underneath neighboring plates and roll-over mode underneath the plate itself. Whereas most of major slabs seem to roll back while the Pacific plate shows a slab piling behavior down to ∼1,000 km depth under the Mariana trench, no clear evidence of slab roll-over in nature has been reported so far. Here we show a possible roll-over slab beneath the Caroline microplate, revealed from its three-dimensional seismic velocity structure derived by analyzing teleseismic reverberating SS phases. We suggest that slab roll-over is driven by at least two factors: 1) the overall buoyancy and fragility of the Caroline microplate at the surface, induced by a thin hot mantle plume that rises from depths ≥800 km; and 2) the pushing force of the Pacific plate acting on the trailing edge of the Caroline plate.
◆Original paper【 display / non-display 】
Grant-in-Aid for Scientific Research 【 display / non-display 】
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Modeling of mantle evolution and geochemical tracers
Grant-in-Aid for Early-Career Scientists
Project Year： 2022.04 - 2025.03 Investigator(s): Atsushi Nakao
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Grant-in-Aid for JSPS Fellows
Project Year： 2014.04 - 2017.03 Investigator(s): Atsushi Nakao
Presentations 【 display / non-display 】
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Data assimilation for reconstructing mantle thermal convection with geochemical tracers
Nakao, A., Kuwatani, T., Ito, S., & Nagao, H.
Water-Rock Interaction WRI-17/ Applied Isotope Geochemistry AIG-14 (Sendai) 2023.08 - 2023.08
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Reconstruction of mantle thermal convection using adjoint-based data assimilation
Nakao, A., Kuwatani, T., Ito, S., & Nagao, H. [Invited]
Japan Geoscience Union Meeting 2023 (Chiba) 2023.05 - 2023.05
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Relationship between maximum earthquake magnitudes and subduction-zone parameters revealed by multiple regression analysis and exhaustive variable selection
Nakao, A., Kuwatani, T., Ueki, K., Yoshida, K., Yutani, T., Hino, H., & Akaho, S. [Invited]
Asia Oceania Geoscience Society 19th Annual Meeting (Online) 2022.08 - 2022.08
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Regression analysis and variable selection that relate stagnation/penetration of subducting plates with subduction-zone parameters
Nakao, A., Kuwatani, T., Ueki, K., Yoshida, K., Yutani, T., Hino, H., & Akaho, S.
Japan Geoscience Union Meeting 2022 (Chiba) 2022.05 - 2022.05
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The importance of the measurement process in super-resolution and a toy model analysis of the back-projection
Yutani, T., Kuwatani, T., & Nakao, A.
Japan Geoscience Union Meeting 2022 (Chiba) 2022.05 - 2022.05