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

Abstract

AbstractWe 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.