Elucidation of the lithosphere-asthenosphere system of oceanic mantle via broadband ocean bottom seismology – Pacific Array

Hitoshi Kawakatsu*1

1Earthquake Research Institute, The University of Tokyo, Japan (hitosi@eri.u-tokyo.ac.jp).

Although plate tectonics started as a theory of the “ocean” nearly 50 years ago, the detail of the mechanism how it works is still poorly known. It has been hampered partly by our inability to determine in-situthe structure of the lithosphere-asthenosphere system in the ocean. Recent advances in ocean bottom broadband seismometry, together with advances in the seismic analysis methodology, have now enabled us to resolve the regional 1-D structure of the entire lithosphere-asthenosphere system, from the surface to a depth of ~250km, including seismic anisotropy (azimuthal), with deployments of ~15 broadband ocean bottom seismometers (BBOBSs) for 1~2 years (Takeo et al., 2013, 2016, JGR; 2018, G-cubed). We have thus succeeded to model the entire oceanic lithosphere-asthenosphere system without a priori assumption for the shallow-most structure, the assumption often made for the global surface wave tomography. We will cover the background of the research, multi-band approach for the “broadband ocean bottom seismology”,and findings from the Normal Mantle project (Takeuchi et al., 2017, Science) in which we conducted a broadband seismic and EM array survey in the northwest Pacific, and hopefully convince the audience that we are now at an exciting stage that a large scale array experiment in the ocean, Pacific Array (http://eri-ndc.eri.u-tokyo.ac.jp/PacificArray/) is becoming approachable to elucidate how plate tectonics might have worked in the past ~180 Ma beneath the Pacific ocean. In fact, the first two Pacific Array deployments by Japanese-Korean and US teams should be completed by the time of the workshop.