Sedimentary Structure Derived From Multi‐Mode Ambient Noise Tomography With Dense OBS Network at the Japan Trench.

We derive the 3‐D S‐wave velocity structures of sediments and upper crust in the region off Ibaraki at Japan Trench subduction zone by applying ambient noise tomography to a dense array of short‐period ocean bottom seismometers (OBSs). The cross‐spectra were calculated using 27 or 142‐day continuous seismic data, and the phase velocities of the fundamental and the first‐higher Rayleigh wave modes are obtained in the frequency ranges of 0.1–0.25 Hz and 0.17–0.3 Hz, respectively. Our 1‐D S‐wave velocity inversion based on the trans‐dimensional Markov chain Monte Carlo method revealed multiple sedimentary layers above the acoustic basement and the upper crustal structure. The 1‐D structure was then used as a reference model to conduct ambient noise tomography and non‐linear inversion of the 3‐D S‐wave velocity structure by collecting data of the local 1‐D S‐wave velocity structure. Our 3‐D S‐wave velocity structure revealed three main points: (a) The acoustic basement is situated at a depth of ∼4 km depth; (b) the crustal structure is more complex than that of the sedimentary layers; and (c) the southern region has a complex crustal structure in which subducting seamounts were identified by previous P‐wave velocity tomographies. Plain Language Summary: Ambient noise tomography is a powerful tool to reveal seismic S‐wave velocity structure that provides important information for understanding the lithology, deformation, and earthquake processes of subduction zones. We applied the ambient noise tomography to a dense array of short‐period ocean bottom seismometers and obtained 3‐D S‐wave velocity structure of both the sediments and the upper crust in the region off Ibaraki at the Japan Trench subduction zone where a subducted seamount has been identified at 10–15 km depth. We obtained the phase velocities of the fundamental and the first‐higher Rayleigh wave modes in the frequency ranges of 0.1–0.25 Hz and 0.17–0.3 Hz, respectively. We performed inversion based on the trans‐dimensional Markov chain Monte Carlo method to obtain an initial averaged 1‐D S‐wave velocity structure with multiple sedimentary layers. The final 3‐D S‐wave velocity was obtained as the compilation of 1‐D structures adjusted for local heterogeneity through non‐linear inversion. Its structural features are summarized as (a) the acoustic basement is situated at ∼4 km depth, (b) the crustal structure is more complex than the sedimentary layers, and (c) the southern region has a complex crustal structure that may be caused by seamount subduction. Key Points: A 3‐D S‐wave velocity structure of sediments and upper crust is derived by applying ambient noise tomography using ocean bottom seismometers (OBS) data at 0.1–0.3 HzMulti‐mode Rayleigh wave analysis utilizing the Markov chain Monte Carlo method provides high‐resolution velocity structure of sediments and upper crustDense short‐period OBS array with station intervals of 6 km enables us to include high‐frequency components in the ambient noise analysis [ABSTRACT FROM AUTHOR]