15:00 〜 15:15
[SSS02-06] Seismic structure of the lithosphere-asthenosphere system beneath the oldest seafloor revealed by the broadband dispersion analysis of the Oldest-1 Array
キーワード：Lithosphere-asthenosphere system、Broadband dispersion analysis、Pacific Array
Imaging of the oceanic mantle is a major undertaking to constrain the thermal evolution of the lithosphere-asthenosphere system. The Pacific oldest seafloor can help us reveal the age dependence of the lithospheric thickness and the underlying asthenospheric convection, as well as understanding the mantle flow dynamics of the early Pacific plate. Here we present seismic results from the Oldest-1 Array situated on the oldest (~170 Ma) seafloor ~1,000 km off Mariana Trench. Using ambient noise and teleseismic earthquakes, we measured the 6–200 s Rayleigh wave phase velocity and inverted them into a one-dimensional SV velocity structure from the crust to 300 km depth. We find that the low-velocity zone exists at the depth of ~80–250 km. The structure is similar to that obtained from the 140-Ma northwestern normal Pacific seafloor (NOMan B from Takeo et al., 2018), despite their 30-Ma age difference. The half-space cooling (HSC) model that fits the observation from the 140-Ma seafloor cannot fit that of the 170 Ma by only adjusting their age difference: the obtained phase velocity from the 170-Ma seafloor is ~1 % slower than the synthetic one based on the HSC model. If we assume that the NOMan B structure obeys simple thermal conduction, our Oldest-1 observation suggests that the underlying structure cannot solely be explained by simple thermal conduction. Additional thermal mechanisms, such as small-scale convection, might be required at the low-velocity zone. We will also investigate azimuthal and radial anisotropic structures to constrain the required mechanisms. Together with previously investigated local arrays in other oceanic regions, the results will give an exceptional opportunity to explore the oceanic lithosphere-asthenosphere system.