[S-SS04] Seismological advances in the ocean

[SSS04-01] Heterogeneous seismic structure at Hyuga-nada from receiver-side Green's functions

★Invited Papers

*佐脇 泰典¹、山下 裕亮²、大柳 修慧¹、Garcia Emmanuel Soliman²、伊藤 亜妃³、杉岡 裕子⁴、高橋 努³、篠原 雅尚⁵、伊藤 喜宏² (1.京都大学大学院理学研究科、2.京都大学防災研究所、3.海洋研究開発機構、4.神戸大学理学研究科、5.東京大学地震研究所)

キーワード：インピーダンス、レシーバ関数、スロー地震、日向灘

We investigate strong negative impedance contrasts at Hyuga-nada in the western Nankai subduction zone, revealed by receiver-side responses to teleseismic or regional deep-focus earthquake signals. Over the past two decades, seafloor seismic observations, including ocean bottom seismograph (OBS), have contributed to significant findings of shallow slow earthquakes in the Nankai subduction zone. Vigorous high activity of shallow tectonic tremors and very-low-frequency earthquakes has been noted in the vicinity of the subducted Kyushu–Palau Ridge (KPR) at Hyuga-nada (e.g., Tonegawa et al., 2020; Yamashita et al., 2015, 2021). In addition to P-wave travel-time tomography revealing low-velocity zones above the KPR (e.g., Nakanishi et al., 2018; Nishizawa et al., 2009), a striking low-velocity zone (LVZ) of shear waves was found ~3–4 km beneath the seafloor over the KPR through transdimensional inversion of teleseismic Green’s functions (GFs; Akuhara et al., 2019) and surface wave dispersion curve from a dense OBS array observation (Akuhara et al., preprint). This LVZ structure implies that dehydrated fluid originating from either the subducted KPR, hydrous sediment, or oceanic crust may then ascend to become stagnant in the hanging wall due to recurrent shallow slow earthquakes, however, the spatial distribution of LVZ was not constrained by the dense OBS array of ~2 km radius (Akuhara et al., preprint). To constrain the lateral and depth distribution of the LVZ around the KPR, we compute GFs from teleseismic and regional deep-focus events using widely distributed campaign networks with broadband and short-period OBSs starting from 2014 (e.g., Shinohara et al., 2021; Yamashita et al., 2021).
We detect successive GF phases of negative and positive amplitudes around 2-s lapse time. The successive phases are characterized as the LVZ and are in keeping with the presence of the LVZ above the KPR from the dense-array observation (Akuhara et al., preprint). High-frequency GF images show the lateral and depth variation of the LVZ. Around the subducted KPR with active shallow tectonic tremor (Yamashita et al., 2021), the negative GF phases for the top of LVZ are distributed ~3 km above the plate interface model, without any significant negative impedance contrasts along the plate interface. Also, the along-strike image corroborates the wide presence of LVZ over 100 km width around the source region of shallow slow earthquakes. This might suggest that a higher-permeability plate interface encourages frequent upward fluid migration in relation to shallow slow earthquakes. In contrast, the GF phases with negative impedance likely deepen towards the plate interface at the northeastern Hyuga-nada, where there are fewer tectonic tremor activities (Yamashita et al., 2021). We need to discuss the lateral variations of physical properties such as frictional property, thermal structure, and upper plate permeability, together with shallow slow earthquake activities.