ASHI Juichiro
(Associate Professor/Division of Environmental Studies)
Department of Natural Environmental Studies/geology of accretionary prism
Career Summary
1991: Post-Doctoral Fellow of JSPS at the Ocean Research Institute, the University of Tokyo
1993: Research Associate at the Geological Institute, the University of Tokyo
2001: Associate Professor at the Ocean Research Institute, the University of Tokyo
2006-present: Associate Professor at Graduate School of Frontier Sciences, the University of Tokyo
Educational Activities
Graduate School: Hadal Environmental Science, Fundamentals of Natural Environmental Studies
Research Activities
1. Studies of fault activities and tectono-sedimentary processes in plate subduction zones
2. Studies of methane hydrates in active tectonic regions
3. Growth process of mud volcanoes and its implications for tectonics and paleoenvironments
4. Origin, migration and venting of fluids in accretionary prisms
Literature
1) Kanamatsu, T., Ashi, J. and Shiraishi, K. (2024) Controlling factors of a submarine landslide on the Kumano-nada continental slope, West Japan, Tectonophysics 883 230370-230370.
2) Nakanishi, R., Ashi, J., Okamura, S., Yokoyama Y. and Miyairi, Y. (2024) Understanding paleo-earthquakes in the Kuril Trench based on Late-Holocene tsunami deposits in the distal region from wave sources, northern Hidaka, Hokkaido, Japan, PLOS ONE, https://doi.org/10.1371/journal.pone.0298720.
3 Nakanishi, R. and Ashi, J. (2022) Sediment Transport Modeling Based on Geological Data for Holocene Coastal Evolution: Wave Source Estimation of Sandy Layers on the Coast of Hidaka, Hokkaido, Japan, Journal of Geophysical Research-Earth Surface 127(8).
4) Okuma, Y., Noda, A., Koge, H., Yamada, Y., Yamaguchi, A. and Ashi, J. (2022) Surface friction of subducting seamounts influences deformation of the accretionary wedge, Tcctonophysics 845, https://doi.org/10.1016/j.tecto.2022.229644
5) Misawa, A., Ashi, J., Tara, K. Yamashita, M. and Kinoshita, M. (2020) Active deformation of Sagami Bay triggered by approach of the Izu island arc, Geo-Marine Letters 40, 741–753.
6) Okutsu, N., Ashi, J., Yamaguchi, A., Irino, T. , Ikehara, K., Kanamatsu, T., Suganuma, T. and Murayama (2018) M. Evidence for surface sediment remobilization by earthquakes in the Nankai forearc region from sedimentary records, Geological Society Special Publication, 4773745https://doi.org/10.1144/SP477.222019.
7) Kioka, A., Tsuji, T., Otsuka, H. and Ashi, J. (2019) Methane concentration in mud conduits of submarine mud volcanoes: A coupled geochemical and geophysical approach. Geochemistry, Geophysics, Geosystems, 20. https://doi.org/10.1029/2018GC007890.
8) Koge, H., Yamada, Y., Ohde, A., Bauville, A., Yamaguchi, A. and Ashi, J. (2018) Dynamic formation process of thick deformation zone on the shallow plate boundary fault of the Japan Trench: insight from analog experiment of half-graben subduction, Progress in Earth Planetary Science, https://doi.org/10.1186/s40645-018-0230-5.
1) Ashi, J., Sawada, R., Omura, A. and Ikehara, K. (2014) Accumulation of an earthquake-induced extremely turbid layer in a terminal basin of the Nankai accretionary prism, Earth, Planets and Space, 66:51, doi:10.1186/1880-5981-66-51.
9) Ikehara, K., Irino, T., Usami, K., Jenkins, R., Omura, A. and Ashi, J. (2014) Possible submarine tsunami deposits on the outer shelf of Sendai Bay, Japan resulting from the 2011 earthquake and tsunami off the Pacific coast of Tohoku, Marine Geology 358, 120-127, doi:10.1016/j.margeo.2014.11.004.
10) Tsuji, T., Ashi, J. and Ikeda, Y. (2014) Strike-slip motion of a mega-splay fault system in the Nankai oblique subduction zone, Earth, Planets and Space, 66:120, doi:10.1186/1880-5981-66-120.
11) Ashi, J., Ikehara, K., Kinoshita, M. and KY04-11 and KH-10-3 shipboard scientists (2012) Settling of Earthquake-Induced Turbidity on the Accretionary Prism Slope of the Central Nankai Subduction Zone, Submarine Mass Movements and Their Consequences, Advances in Natural and Technological Hazards Research, Springer, 31, 561-571.
12) Ashi, J., Tokuyama, H. and Taira, A. (2002) Distribution of methane hydrate BSRs and its implication for the prism growth in the Nankai Trough, Marine Geology, 187, 117-191.
13) Ashi, J. and Taira, A. (1993) Thermal structure of the Nankai Accretionary Prism as inferred from the distribution of gas hydrate BSRs: Geological Society of America Special Paper 273, 137-149.
Other Activities
Geological Society of Japan
Seismological Society of Japan
American Geophysical Union
Future Plan
Fluids in subduction zones exert significant influence on deformation, temperature structures, and material cycles. To clarify the relationship between the growth of accretionary prisms and fluid behavior, we are studying current geological phenomena in the shallow to surface layers of the Nankai Trough that indicate the movement of fluids and gases, such as methane hydrates, cold seeps, and mud volcanoes. Additionally, we are attempting to clarify the distribution of active faults associated with large earthquakes along the Nankai Trough and estimate their activity history. The main research themes are outlined below.
Submarine active faults
The distribution of submarine active faults is the most fundamental information for understanding the growth process of accretionary prisms, fluid outbursts, and disaster prevention measures for tsunamis and earthquakes. We are mapping the distribution of active faults in the Nankai Trough based on swath bathymetric surveys, side-scan sonar images, reverse-dip seismic survey profiles, and submarine observations.
Methane hydrate
Methane hydrate is important for considering hydrocarbon circulation, global warming, and the stability of submarine slopes. Furthermore, BSR (bottom-seam-like reflection surfaces) observed on seismic sections considered to be the boundary between methane hydrate-bearing strata and underlying free gas layers can be used to estimate geothermal gradients and gas migration processes. We are investigating the distribution of BSR in the Nankai Trough accretionary prism and forearc basin to estimate heat flux and methane gas accumulation processes.
Cold seeps
Cold seeps in subduction zones are closely related to tectonics. The geological structures beneath the seafloor that serve as fluid discharge pathways can be inferred from the distribution and chemical composition of cold seeps. In the Nankai Trough, we are investigating the distribution of chemolithotrophic communities associated with methane seeps using submersibles and deep-sea video. Previous studies have shown that most cold seeps are distributed along active seafloor faults or mud volcanoes.
Messages to Students
We still have very limited knowledge of what is happening at the bottom of the vast ocean. However, recent advancements in observation technology have enabled the collection of high-quality data on substances, topography, and underground structures from the seafloor surface to the subsurface. Why not join us on a research vessel to explore the seafloor, which is far more active than imagined and closely interconnected with the Earth's environment? Our Department boasts faculty members and graduate students with diverse perspectives. We encourage you to cultivate a broad outlook and pioneer new research fields that prioritize the societal application of research outcomes.