[S-SS09] Crustal Structure

[SSS09-02] Estimation of three-dimensional S-wave velocity structure in the northern Japan Trench using a dense ocean-bottom seismic network

*Ryota Takagi¹, Ryosuke Azuma¹, Ryota Hino¹, Kiwamu Nishida², Masanao Shinohara² (1.Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, 2.Earthquake Research Institute, The University of Tokyo)

Keywords:Japan Trench, Seismic velocity structure, Ambient noise tomography, Interplate earthquake, Tremor

Tectonic tremors and regular earthquakes exhibit complementary spatial distributions in the northern Japan Trench. Tectonic tremors are distributed along the trench, with a noticeable gap zone observed from 40°N to 41°N. Remarkably, this gap zone coincides with the hypocenters of the 1994 Sanriku-oki and 1968 Tokachi-oki earthquakes. To understand the mechanisms behind the spatial heterogeneity of interplate slip behavior, we estimate the three-dimensional S-wave velocity structure using ambient noise surface wave tomography with a dense ocean-bottom seismic (OBS) network. This seismic network comprises 41 OBS stations with a denser interstation separation of approximately 20 km compared to S-net, and the observation data spans from October 2006 to June 2007. By analyzing multicomponent cross-correlation functions of ambient noise, we extracted fundamental-mode and first-overtone Rayleigh waves, as well as fundamental-mode Love waves, within the frequency range of 0.05–0.1 Hz. Array-based phase velocity measurement and subsequent phase velocity tomography revealed a clear phase velocity contrast between the tremor and gap zones, with the tremor zone exhibiting approximately 20% lower phase velocity than the gap zone. Given that surface waves within the frequency range used are sensitive to the S-wave velocity structure around the plate interface and in the upper plate, the observed phase velocity contrast suggests structural heterogeneity influences the slip behavior on the plate interface. Estimating a three-dimensional S-wave velocity structure and comparing it with regular and slow earthquakes will deepen our understanding of the relationship between heterogeneous structures and slip phenomena at the plate boundary and the mechanisms behind it.