[S-SS04] Seismological advances in the ocean

[SSS04-P04] Upper Mantle P-wave Velocity Structure Beneath the Oldest Pacific Seafloor Revealed From Finite-Frequency Travel-Time Tomography Using Oldest-1 and Oldest-2 OBS Data

*Jun-Jie Ke¹, Shu-Huei Hung¹, Takehi Isse², Hitoshi Kawakatsu^2,3, Ban-Yuan Kuo³, Pei-Ying Patty Lin⁴, Hisashi Utada², Nozomu Takeuchi², Hajime Shiobara², Hiroko Sugioka⁵, YoungHee Kim⁶, Sang-Mook Lee⁶, Ping-Yu Chang^7,8,9, Yih Yang¹⁰, Daisuke Suetsugu⁵, Hiroko Sugioka⁵, Aki Ito¹¹, Kazunori Yoshizawa¹² (1.Department of Geosciences, National Taiwan University, Taipei, Taiwan, 2.Earthquake Research Institute, The University of Tokyo, Tokyo, Japan, 3.Institute of EarthSciences, Academia Sinica, Taipei, Taiwan, 4.Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan, 5.Department of Planetology, Graduate School of Science, Kobe University, Kobe, Japan, 6.School of Earth and Environmental Sciences, Seoul National University, Seoul, Republic of Korea, 7.Department of Earth Sciences, National Central University, Taoyuan, Taiwan, 8.Earthquake-Disaster, Risk Evaluation, and Management Centre, National Central University, Taoyuan, Taiwan, 9.Center for Astronautical Physics and Engineering (CAPE), National Central University, Taoyuan, Taiwan, 10.Taiwan Ocean Research Institute, National Applied Research Laboratories, Taiwan, 11.Japan Agency for Marine-Earth Science and Technology, Japan, 12.Department of Earth and Planetary Sciences, Hokkaido University, Japan)

Keywords:Pacific Plate, Oceanic Lithosphere, Ocean-Bottom Seismometers, Oldest-1, Oldest-2, Finite-Frequency Tomography

The world's oldest seafloor preserves the birth record of the Pacific Plate, making it essential for understanding the evolution of the oceanic lithosphere. However, its remote location has posed significant challenges challenging for detailed investigations. From 2018 to 2019, the Oldest-1 experiment, part of the Pacific Array project, deployed ocean-bottom seismometers (OBSs) in this region to probe deep seismic velocity structures, revealing enigmatic features of Pacific Plate evolution. Building on this effort, the Oldest-2 OBS experiment was conducted from 2022 to 2023 on a younger seafloor to the west (160–170 Ma). In this study, we integrate teleseismic P wave data from both experiments to construct an expanded three-dimensional P-wave velocity model of the upper mantle.

We select teleseismic earthquakes with magnitude greater than 5.5 at epicentral distances of 30-90^o for relative P-wave traveltime measurements. The vertical-component P-wave seismograms are pre-processed by removing the trend, mean, and instrument response, then filtered into three frequency bands: 8–16 s, 12–24 s, and 16–32 s. We visually inspect waveforms and retain more than 200 events with high signal-to-noise ratios for further analysis. An interactive python package, Aimbat, is implemented to measure relative traveltime residuals between stations based on the iterative cross-correlation and stack (ICCS) and multi-channel cross-correlation (MCCC) algorithms. The residuals from multiple frequency bands along with their finite-frequency sensitivity kernels will be incorporated into a multiscale tomographic inversion. This approach aims to enhance both lateral and depth resolution of the upper mantle structure beneath the oldest seafloor, providing new insights into the dynamic evolution of the Pacific oceanic lithosphere.