11:00 〜 13:00
[SCG44-P13] Correlations between geological structures and distribution of tectonic tremors in Hyuga-nada, Nankai Trough
キーワード：seismic image、slow earthquakes、tremors、 Nankai Trough、 Décollement、Hyuga nada
There are different kinds of slow earthquakes ranging from slow slip events (SSEs) to very low frequency earthquakes (VLFEs) and tectonic tremors occurring around the plate boundary in the Hyuga-nada, Nankai Trough. VLFEs and tremors have been observed around a subducting seamount of the Kyushu Palau Ridge (KPR, Yamashita et al., 2017 and 2020; Tonegawa et al., 2020; Nakata et al., 2020).
A number of seismic surveys have been conducted in this study region. We reprocessed the multichannel seismic reflection profile of KR0114-8 (Park et al., 2008) to investigate the relationship between the tremor distribution, the plate-boundary morphology and seismic structures. The KR0114-8 line cuts across KPR spanning both Nankai Trough and Ryukyu Trench and is parallel to the Nankai Trough on its eastern side. We applied the broadband processing to the reflection data to recover a wide signature spectrum. After applying grid-based reflection tomography to update a migration velocity model, we performed Kirchhoff pre-stack depth migration to image complex subsurface structures.
The reflection image is in a much higher resolution than previous post-stack/pre-stack migrations. We identify various clear lithological boundaries including bottom-simulating reflectors (BSR), faults, accretionary prisms, décollement and top of the oceanic crust. We observe BSRs and many basin structures in the shallow portion of the Ryukyu-Trench side, the western part of the line. At the middle, a subducted seamount of KPR is clearly imaged as a rough positive reflector. Many splay faults branch from the seamount, which are likely created by the subducted seamount. The undethrusted sediments are clearly seen between the décollement and the basement. Accretionary prisms develop and show a little influence of the seamount subduction on west of the KPR on the Nankai-Trough side. The thickness of the sediments between the décollement and the basement varies significantly. The sediments package is thinner near the KPR and suddenly becomes thicker and layered towards the deformation front. At the seaward of the trough, we observe some normal faults.
Along the KR0114 line, tremors occurred mostly on the Ryukyu-Trench side and Nankai-Trough side, but few occurred above the subducting seamount. A lack of tremors on the seamount may suggest that the area right above the seamount is in the stress shadow (e.g., Sun et al., 2020). On the Nankai Trough side, the tremors more frequently occurred where the underthrusted sediments between the décollement and the basement are thick. Thus, we suggest that the occurrence of the tectonic tremors may be controlled by the basement topography and the thickness of the underthrusted sediments.
A number of seismic surveys have been conducted in this study region. We reprocessed the multichannel seismic reflection profile of KR0114-8 (Park et al., 2008) to investigate the relationship between the tremor distribution, the plate-boundary morphology and seismic structures. The KR0114-8 line cuts across KPR spanning both Nankai Trough and Ryukyu Trench and is parallel to the Nankai Trough on its eastern side. We applied the broadband processing to the reflection data to recover a wide signature spectrum. After applying grid-based reflection tomography to update a migration velocity model, we performed Kirchhoff pre-stack depth migration to image complex subsurface structures.
The reflection image is in a much higher resolution than previous post-stack/pre-stack migrations. We identify various clear lithological boundaries including bottom-simulating reflectors (BSR), faults, accretionary prisms, décollement and top of the oceanic crust. We observe BSRs and many basin structures in the shallow portion of the Ryukyu-Trench side, the western part of the line. At the middle, a subducted seamount of KPR is clearly imaged as a rough positive reflector. Many splay faults branch from the seamount, which are likely created by the subducted seamount. The undethrusted sediments are clearly seen between the décollement and the basement. Accretionary prisms develop and show a little influence of the seamount subduction on west of the KPR on the Nankai-Trough side. The thickness of the sediments between the décollement and the basement varies significantly. The sediments package is thinner near the KPR and suddenly becomes thicker and layered towards the deformation front. At the seaward of the trough, we observe some normal faults.
Along the KR0114 line, tremors occurred mostly on the Ryukyu-Trench side and Nankai-Trough side, but few occurred above the subducting seamount. A lack of tremors on the seamount may suggest that the area right above the seamount is in the stress shadow (e.g., Sun et al., 2020). On the Nankai Trough side, the tremors more frequently occurred where the underthrusted sediments between the décollement and the basement are thick. Thus, we suggest that the occurrence of the tectonic tremors may be controlled by the basement topography and the thickness of the underthrusted sediments.