[S-CG67_1PM1] Ocean Floor Geoscience

[SCG67-03] Preliminary result of the oceanic mantle structure revealed by "Normal Oceanic Mantle Project"

*Takehi ISSE¹, Hajime SHIOBARA¹, Hiroko SUGIOKA², Aki ITO², Akiko TAKEO¹, Hisashi UTADA¹, Hitoshi KAWAKATSU¹, Takashi TONEGAWA², Satoru TANAKA² (1.ERI, Univ. of Tokyo, 2.Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology)

Keywords:upper mantle, BBOBS, surface wave, tomography

The oceanic mantle is an important region to understand the Earth system, because more than 2/3 of the surface is covered by oceanic area. Since 1990s, we have operated several seafloor observations by using newly developed long-term broadband ocean bottom seismometers (BBOBSs) in the northwest and central part of the Pacific Ocean. These observations have revealed the structures in and around the subduction zone in the Pacific Ocean and in the Pacific superswells, respectively. However, we have no observation and result in the normal oceanic regions. To reveal the normal oceanic mantle structure from observational approach, we conduct a long-term observation called “Normal Oceanic Mantle Project” (NOMan project) from 2010, deploying ocean bottom geophysical (seismic and electromagnetic ) instruments in the northwestern Pacific Ocean. We conduct two arrays in that region. One is northwest side of the Shatsky Rise (Area A) and the other is southeast side of the Shatsky Rise (Area B). Sea floor age of Area A is 125 - 135 Ma, that of Area B is 135 - 145 Ma, so that the shear wave structures of those area should be similar. By using seismograms of the NOMan project, other BBOBS observations, and permanent broadband seismic stations on land, we have determined the three-dimensional shear wave velocity structure of the upper mantle in the northwestern Pacific Ocean to reveal this area to be really normal. We used a surface wave tomography technique in which multimode phase speed of the surface wave are measured and inverted for a 3-D shear wave velocity structure by incorporating the effects of finite frequency and ray bending. Our obtained model shows that lateral heterogeneity of each area is not strong and that average structures are different in two areas. Area A is consistent with previous models, whereas Area B is faster than previous models. We think for the present that Area A seems to be normal oceanic mantle, though Area B is not. We will recover all BBOBSs in this year. These BBOBS's data enable us to obtain more reliable mantle structures.