Seismic structure of the lithosphere-asthenosphere system beneath the oldest seafloor – the broadband dispersion analysis of the Oldest-1 (Pacific Array) data

Abstract:

The international collaborative initiative, the Pacific Array, was launched in 2018. The first observation led by the Japan-South Korean joint team, Oldest-1 array, situates on the oldest Pacific seafloor (170 Ma) ~1000 km off the Marian trench. In November 2018, we deployed 12 broadband ocean bottom seismometers and 7 ocean bottom electro-magnetometers that were successfully recovered a year later. The seismic and electromagnetic analyses of this area are expected to reveal the lithosphere-asthenosphere structure, which is supposed to record the entire evolution process of the Pacific plates. Here we present results of the seismic analysis of the uppermost mantle structure revealed by the Oldest-1 array via the broadband Rayleigh-wave dispersion analysis using tilt noise and compliance noise corrected data. At short periods (7–40 s), phase velocities of fundamental-mode and first higher-mode Rayleigh waves are measurable at multiple components using ambient noise cross-correlation functions. At long periods (30­–100 s), we measure phase velocities of fundamental-mode Rayleigh waves using teleseismic waveform analysis. We then invert the broadband dispersion curves for a one-dimensional isotropic βv (Vsv) structure. A preliminary result indicates that the structure shallower than 150 km is similar to the one obtained for the 140 Ma northwestern Pacific seafloor by the NOMan project (Takeo et al., 2018, G-Cubed). We plan to extend our analysis to longer periods to delineate the deeper structure and measure azimuthal and radial anisotropy.