13:45 〜 14:00
[SCG45-32] Towards monitoring physical properties of plate boundary in the Nankai subduction zone using a vertical array with borehole and seafloor sensors
★Invited Papers
キーワード：モニタリング、反射波、プレート境界
Temporal variations of seismic velocity and heterogeneity have been investigated using seafloor sensors in the Nankai subduction zone (e.g., Ikeda and Tsuji, 2018; Tonegawa et al. 2022). However, because those studies examined surface waves, the obtained temporal variations mainly reflect the seismological structure at shallower depths. In this study, we use ambient noise records obtained by a vertical array at C0002 site in the Kumano basin of the Nankai subduction zone (Kimura et al. 2019,SSJ). The vertical array contains a geophone deployed at the seafloor and a borehole broadband sensor at a depth of 900 m from the seafloor. If reflections from deep interfaces, e.g., plate boundary, are detected persistently using the vertical array, we can monitor the physical properties of plate boundary with the amplitudes and phases of the reflections.
The geophone is 4.5-Hz short-period sensor. We removed only the amplitude of the instrumental response to use long period components up to ~3 s. The analyzed period is from 2015 to the beginning of 2021. We used the two horizontal components, and rotated them with an increment of 1º. To retrieve S reflections for every polarization direction from below, we employed the deconvolution using ambient noise records for every polarization direction at the seafloor and borehole, in which the reference site was the seafloor. The bandpass filter is 0.3–3.0 Hz.
Our results show S reflections from the top of high velocity body above the megasplay fault (Shiraishi et al. 2019; 2020), the megasplay fault, and the top of the subducting oceanic crust. The arrival time of the S reflection for a polarization direction of N70ºE was faster, indicating that the direction is the fast polarization direction. We also investigated the temporal variation of the amplitudes of the S reflections at a slow polarization direction (N160ºE), and obtained scattered amplitudes of the S reflections from the top of the high velocity body and megasplay fault for 1 year after 2016 off-Mie earthquake. On the other hand, such variations cannot be observed in the amplitudes of S reflection polarized into the fast polarization direction.
Acknowledgement
We thank T. Kimura for deploying the geophone at the seafloor at C0002 site. This work was supported by JSPS KAKENHI Grant No. 21H05202 in Scientific Research on Tranformative Research Areas “Science of Slow-to-Fast earthquakes”.
The geophone is 4.5-Hz short-period sensor. We removed only the amplitude of the instrumental response to use long period components up to ~3 s. The analyzed period is from 2015 to the beginning of 2021. We used the two horizontal components, and rotated them with an increment of 1º. To retrieve S reflections for every polarization direction from below, we employed the deconvolution using ambient noise records for every polarization direction at the seafloor and borehole, in which the reference site was the seafloor. The bandpass filter is 0.3–3.0 Hz.
Our results show S reflections from the top of high velocity body above the megasplay fault (Shiraishi et al. 2019; 2020), the megasplay fault, and the top of the subducting oceanic crust. The arrival time of the S reflection for a polarization direction of N70ºE was faster, indicating that the direction is the fast polarization direction. We also investigated the temporal variation of the amplitudes of the S reflections at a slow polarization direction (N160ºE), and obtained scattered amplitudes of the S reflections from the top of the high velocity body and megasplay fault for 1 year after 2016 off-Mie earthquake. On the other hand, such variations cannot be observed in the amplitudes of S reflection polarized into the fast polarization direction.
Acknowledgement
We thank T. Kimura for deploying the geophone at the seafloor at C0002 site. This work was supported by JSPS KAKENHI Grant No. 21H05202 in Scientific Research on Tranformative Research Areas “Science of Slow-to-Fast earthquakes”.