[S-SS03] Seismological advances in the ocean

[SSS03-P12] The Oldest-2 array observation: Second seafloor observation to study the lithosphere–asthenosphere system beneath the oldest Pacific Ocean

*Takehi Isse¹, Kiyoshi Baba¹, Ban-Yuan Kuo², PeiYing Patty Lin³, Wu-Cheng Chi², Chen-Hsiang Hung³, Ding-Jiun Lin⁴, Hogyum Kim⁵, Nozomu Takeuchi¹, Hitoshi Kawakatsu^1,2, Hajime Shiobara¹, Hisashi Utada¹, onboard scientists of 2022/2023 Oldest-2 cruises by R/V Legend - (1.Earthquake Research Institute University of Tokyo, 2.Institute of Earth Sciences, Academia Sinica, 3.National Taiwan Normal University, 4.National Central University, 5.Seoul National University)

Keywords:lithosphere-asthenosphere system, Pacific Array, ocean floor array observation, BBOBS, OBEM

Our understanding of the physical nature of the oceanic lithosphere–asthenosphere system (LAS) is still limited, although it takes nearly 60 years since the concept of plate tectonics was born. In-situ measurement of the physical property distributions in the mantle by seafloor geophysical array observations in all over the age will be one of the keys for further understanding, and the Pacific Array concept has been proposed. Since the Pacific Array has been launched in 2018, many broadband seafloor observations have been deployed.
We selected the oldest part (the mean age of about 170 Ma) of the Pacific Basin for the first deployment of the Pacific Array. In the study area, the seafloor isochron suggested by magnetic lineations is seen to converge to a small triangle which shows the shape of the Pacific plate when it was born in the middle Jurassic. One of the scientific purposes of this Oldest array is to reveal the early evolution of the Pacific LAS by estimating the anisotropic seismic structure from analysis of the BBOBS data. We also intend to examine the relation between the seafloor depths and mantle geotherms that might be recorded in the mantle to answer the long-lasting question why older seafloors are flat.
The observation consists of two array observations, one is called Oldest-1 Array by Japan-Korea collaboration between 2018 and 2019. Second one is called Odest-2 Array by Japan-Taiwan collaboration between 2022 and 2023. In this presentation, we will present the observation report of the latter array.
The deployment cruise was conducted from September 10 to October 8, 2022, by R/V Legend (Taiwan Ocean research Institute: TORI), started from and return to Taiwan. 14 sets of broadband ocean bottom seismometers (BBOBSs), five of them are Taiwanese BBOBSs and the other are Japanese BBOBSs, and 10 sets of Japanese ocean bottom electro-magnetometers (OBEMs) were deployed in the study area. The recovery cruise was performed between September 17 and October 14, 2023. All Japanese BBOBSs and OBEMs and three Taiwanese BBOBSs were recovered. Data recovery rate was 100 % for OBEMs, 77 % for seismometers, and 64 % for differential pressure gauge.