Attenuation tomography using large-scale seafloor and land network data in northeast Japan

Existence of a heterogeneous attenuation structure in northeast Japan has long been documented. Despite this, attenuation structure around the plate boundary in the offshore region could not be examined thoroughly owing to a lack of data. After the 2011 Mw 9.1 Tohoku-oki earthquake, a seafloor network of 150 observatories, known as S-net, was established spanning the entire Japan Trench subduction zone to reinforce infrastructure for earthquake and tsunami early warnings. In the present research, we computed S-wave Fourier spectra of accelerograms from the seafloor and land stations and applied the principle of seismic tomography to retrieve three-dimensional attenuation structure (Qs). Our results showed a prominent low-to-moderate Qs zone near the upper surface of the oceanic plate in the seafloor area, most likely indicating the presence of water-rich oceanic crust and subducted sediments. We also found that the Qs structures in both onshore and offshore regimes are moderately to strongly frequency-dependent. A comprehensive understanding of the frequency-dependent behavior of attenuation structure remains elusive. We anticipate that the findings presented here will contribute to resolving pertinent issues in the field of solid earth science and to a deeper understanding of earthquake hazards in subduction zones.