今週のうなぎセミナーについてお知らせいたします。

Here is information of the Unagi-seminar(October, 6).

************** Seminar on Seismology IV B, D /地震学ゼミナールIV B, D (Unagi Seminar) **************

科目：地震学ゼミナールIV B, D / Seminar on Seismology IV B, D（修士・博士）
日時：2022年 10月 6日 (木) 14:00~
場所：京都大学 防災研究所 本館E-232D または オンライン(Zoom)

Date and Time：2022-10-06, 14:00~
Place：Uji Campus Main Building E232D or Zoom (Hybrid)

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Presenter: Yasunori Sawaki（佐脇 泰典）

Title: Seismic Impedance in the Nankai Subduction Zone

Abstract: In this presentation, I will give a brief overview of the preparing PhD dissertation.

In the Nankai subduction zone, slow earthquakes take place in deeper and shallower portions of assumed megathrust seismogenic zones. As the source area of deep tectonic tremor spatially correlates with the 30-km depth contour of the subducted Philippine Sea plate, pressure-temperature conditions and the related hydrous state around the plate interface are likely to control slow-to-fast earthquake activities because the hydrated slab dehydrates as it subducts. Since the hydrous slab interface is likely to exhibit negative seismic impedance (i.e., low-Vs slab top), I performed common-conversion-point (CCP) stacking of both teleseismic and regional receiver functions (RF) to investigate depth variations of seismic impedance in relation to slow earthquake activities in the northeastern Kii area (Sawaki et al., 2021). The CCP image showed the depth variation of seismic impedance for plate interface corresponding to the difference in typical durations of tectonic tremor, and the results suggested that upper plate lithology controls the depth-dependent hydrous state. I also computed RFs and blindly deconvolved Green’s functions (GFs; Akuhara et al., 2019) using ocean bottom seismograph (OBS) records at Hyuga-nada, where vigorous shallow slow earthquakes have been observed (Tonegawa et al., 2020; Yamashita et al., 2015, 2021). Recently, Akuhara et al. (preprint) found a prominent low-velocity zone (LVZ) beneath ~3-4 km thick sediment over the Kyushu-Palau Ridge (KPR) by joint inversion of teleseismic GFs and a surface wave dispersion curve from a very dense (radius of ~2 km) array observation. Our RF/GF traces showed significant phases with successive negative and positive amplitudes around 2-s lapse time. Assuming that RF/GF phases were attributed to P-to-S conversions beneath an OBS station, these successive phases showed a distinctive negative velocity impedance above a positive velocity impedance. This suggests the presence of the LVZ above the KPR (Akuhara et al., preprint) with ~100 km width around the source region of shallow slow earthquakes. We also found the lateral depth variation of the LVZ; the LVZ likely deepens towards the plate interface at the eastern end of the area with tectonic tremor activities in May 2015 (Yamashita et al., 2021). We will further discuss the spatial relationship between the LVZ and shallow slow earthquake events and related fluid processes.

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