稠密地震観測網を用いた地下構造推定

概要

令和 ４年度

京都大学化学研究所 スーパーコンピュータシステム 利用報告書
稠密地震観測網を用いた地下構造推定
Seismological structure beneath dense-array seismic network
京都大学大学院理学研究科地球惑星科学専攻

佐脇 泰典

研究成果概要
At Hyuga-nada in the western Nankai, 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) (e.g., Tonegawa et al., 2020; Yamashita et al., 2015, 2021).
Akuhara et al. (preprint) found a striking LVZ of shear waves at ~3–4 km beneath the seafloor
over the KPR through transdimensional inversion of teleseismic GFs (Akuhara et al., 2019) and
surface wave dispersion curve from a dense OBS array observation. The 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 their dense array
of ~2 km radius. Aiming 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 at
Hyuga-nada.
We detect successive GF phases of negative and positive amplitudes around 2-s lapse time,
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. 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). Our results might
reflect the lateral variation of physical properties such as frictional property, thermal structure,
and upper plate permeability, together with shallow slow earthquake activities.
Using the SCL supercomputing system, we were trying to test the transdimensional
Bayesian inversion for estimating one-dimensional shear-wave velocity structure (e.g., Akuhara
et al., 2020). However, we failed to obtain the final outputs due to several complicated
problems. ...