Overburden and/or tectonic forces expel fluids and gases from sedimentary layers. Such fluids and gases physically and chemically control sediment compaction and deformation. Roles of fluids and gases are especially important in the subduction zones, where active sedimentation and deformation proceed. I am interested in interactions between wet-sediment deformation and fluid/gas migration. Methods are divided into three: field survey, studies on samples, analysis of geophysical data. Firstly, field survey using submersible has been conducted on the Nankai Trough to examine migration paths of fluid/gas along active faults. Distributions of cold-seepage off Tokai were revealed by discovery of chemosynthetic biological community such as Calyptogena associated with methane seepage during the international program of the Japanese-French KAIKO-Tokai project. Future work will focus on relationship between historical earthquakes, fault activities and cold seepage. Secondly, wet-sediment deformations are studied on samples obtained by submersible and piston coring (Nankai Trough), deep-sea drilling (Cascadia and Barbados Ridge), field survey (Shimanto Accretionary Complex) based on rock physical properties and structures. Thirdly, analysis of geophysical data have been conducted using previously obtained seismic-reflection profiles to examine distributions of active faults and folds, and methane hydrate BRSs. BSRs provides information about not only occurrence of methane gas in sedimentary sequences, but also geothermal structures from their positions. Present BSR study focuses on the following two topics: migration and accumulation of methane gas in subduction zones, and relationships between thermal structure of the Nankai Trough and age of subducting plate. Mapping of heat flow using BSRs, moreover, is one of my important works to know conditions of seismogenic zones in subduction zones.
1. Ashi, J., R. Sawada, A. Omura and K. Ikehara, 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, 2014.