1) Dynamics of mid-ocean ridge
2) Structure and evolution of ocean lithosphere and asthenosphere
3) Geophysical process of hydrothermal system
My approach is based on geophysical observations and my target is ocean floor; which includes mid-ocean ridges, back-arc basins, ocean basins, and subduction system. I have also developed instruments in order to obtain a new type of data and/or higher quality of data.
I do sea surface geophysical surveys using research vessels to obtain bathymetry, gravity anomaly, and geomagnetic anomaly data, which are primal information to understand the tectonic evolution of the ocean floor.
Gravity anomaly data is used to derive the thickness of the oceanic crust or the density structure of the upper mantle, depending on predictable components of the gravity field. The predictable components include attraction of sea-floor topography and crustal density structure, which are estimated from the P-wav e velocity structure of the crust using velocity-density relations when the velocity structure is available. The predictable components of the gravity field are subtracted from the free-ai r anomalies to produce residual gravity field, which can be related directly to the oceanic cru stal thickness or the upper mantle and allows us to model them. These results are important to u nderstand their tectonic process.
Geomagnetic anomaly is used to identify crustal ages leading the tectonic evolutions, and to estimate magnetic st ructure of the oceanic lithosphere, which is closely related to its formation. Our characteristic feature is that we measure geomagnetic field as a vector, although the conventional method allows us to measure only its intensity. The vector data pro vide more detailed information than total intensity data for understanding the magnet ic structure of oceanic crust and we has been developed new methods and algorithms to d erive magnetic source structure from the vector data. There are two main advantages in using vector geomagnetic anomaly field data. First, total intensity anomaly amplitudes are often much reduced, depending on the orientation of the ambient geomagnetic field and magnetic l ineation, while these have no effect on vector anomalies. This advantage permits easily identified magnetic lineation even near the magnetic equator and the vector magnetic field intensity variation directly shows a variation in the magnetized layer variation. Second, vector geomagnetic anomaly fie ld data provide the positions, strikes and characters of magnetic boundaries, allowing changes i n these boundaries to be identified along individual ship tracks. This advantage permits tectonic interpretations to be well constrained, even in areas of sparse ship track data coverage.
I have developed a new deep-tow three component ma gnetometer. Three main features of the system are 1) the high accuracy measurement of magnetic vector field, attitude and position, 2) acoustic communication, and 3) that this system requires only a winch for deep towing and would be easily used by any vessels with the winch. This system was first used for a geomagnetic investigation of the East Pacific Rise around 18 S aboard the R/V ATLANTIS in September 1998. Analysis of the data will lead results of the magnetic structure and we will interpret them with related to crustal accretion process at mid-ocean ridge, fine-scale tectonic evolution and/or paleo-geomagnetic field intensity.
I also use magnetotelluric method for the seafloor, which is sensiti ve to high electrical conductivity structure and is good for the investigating 1) lithosphere-asthenosphe re transition, 2) melting structure under mid-ocean ridges, and 3) undergoing slab structure. Since it is very important to include seafloor topography as known information for a conductivity modeling, we dev eloped a new idea to express it for the modeling. Our method has three advantages compared to conventiona l ones: those are 1) accurate calculation, 2) small burden on computer, and 3) application for a 3D model ing. Further, we have developed a new type of ocean bottom electro-magnetometer, which has four main feat ures: 1) easy to handle, 2) high performance, 3) low cost, and 4) measurement of three components of electric fields.
Matsuno, T., Y. Nogi and N. Seama, Electrical resistivity structure under the western Cosmonauts Sea at the continental margin of East Antarctica inferred via a marine magnetotelluric experiment, Polar Science 9, 2, 221-234, 2015.
Seama, N., H. Sato, Y. Nogi and K. Okino, The mantle dynamics, the crustal formation, and the hydrothermal activity of the Southern Mariana Trough back-arc Basin, Subseafloor Biosphere Linked to Global Hydrothermal Systems, TAIGA Concept, edited by Ishibashi, J., K. Okino, and S. Sunamura, Springer Japan, Tokyo, 215-227, DOI: 10.1007/978-4-431-54865-2_17, 2015.
Seama, N. and K. Okino, Asymmetric seafloor spreading of the Southern Mariana Trough back-arc basin, Subseafloor Biosphere Linked to Global Hydrothermal Systems, TAIGA Concept, edited by Ishibashi, J., K. Okino, and S. Sunamura, Springer Japan, Tokyo, 53-260, DOI: 10.1007/978-4-431-54865-2_20, 2015.
Matsuno T., M. Kimura and N. Seama, Electrical resistivity structure of the Snail hydrothermal site at the Southern Mariana Trough spreading center, Subseafloor Biosphere Linked to Global Hydrothermal Systems, TAIGA Concept, edited by Ishibashi, J., K. Okino, and S. Sunamura, Springer Japan, Tokyo, 241-251, DOI: 10.1007/978-4-431-54865-2_19, 2015.
Sato, T., M. Mizuno, H. Takata, T. Yamada, T. Isse, K. Mochizuki, M. Shinohara and N. Seama, Seismic structure and seismicity in the Southern Mariana Trough and their relation to hydrothermal activity, Subseafloor Biosphere Linked to Global Hydrothermal Systems, TAIGA Concept, edited by Ishibashi, J., K. Okino, and S. Sunamura, Springer Japan, Tokyo, 229-240, DOI: 10.1007/978-4-431-54865-2_18, 2015.
Kawada, Y., M. Yamano and Seama, N., Hydrothermal heat mining in an incoming oceanic plate due to aquifer thickening: Explaining the high heat flow anomaly observed around the Japan Trench, Geochem. Geophys. Geosyst., 15, 1580-1599, DOI: 10.1002/2014GC005285, 2014.