[S-CG45] Science of slow-to-fast earthquakes

[SCG45-P34] Seismic Imaging of the Subducted Seamount in a High-Tremor Seismicity Region in the Hyuga-nada, Nankai Trough

*Yanxue Ma¹, Rie Nakata¹, Kimihiro Mochizuki¹, Yoshitaka Hashimoto², Yohei Hamada³, Ryuta Arai⁴, Seiichi Miura⁴, Yasuyuki Nakamura⁴, Gou Fujie⁴, Yuka Kaiho⁴, Shuichi Kodaira⁴ (1.Earthquake Research Institute, University of Tokyo, 2.Kochi University, 3.Japan Agency for Marine-Earth Science and Technology Kochi Institute for Core Sample Research, 4.Japan Agency for Marine-Earth Science and Technology)

Keywords:seismic imaging, seismic processing, subducted seamount, slow earthquakes, Nankai Trough, traveltime tomography

Hyuga-nada is located offshore southwestern Japan. The Kyushu Palau Ridge (KPR) separates the Ryukyu Trench and the Nankai Trough. Shallow slow earthquakes, such as tremors and very low-frequency earthquakes, have been detected by broadband ocean bottom seismometers around the subducted seamount located in the northwestern part of the KPR. Some studies suggest that subducted seamounts under tectonic plates may increase friction, acting as inter-seismic locking asperities. This can result in an increase of stress or strain energy in the surrounding region. Others propose that subducted seamounts act as barriers that prevent earthquake rupture propagation by creating areas of low stress.

To better understand the relationship between seamount subduction and seismic activity, we analyzed the KR0114-8 reflection line that intersects the KPR and nearly perpendicular to the subduction direction.Our high-resolution seismic image revealed various clear lithological boundaries, including various clear lithological boundaries, such as bottom-simulating reflectors, faults, the accretionary prism, the décollement, and the basement (Figure 1b and c). By comparing the distribution of shallow tectonic tremors with the reflection section image, we found that the tremors were predominantly located in the Ryukyu Trench and Nankai Trough zones (Figure 1a). On the contrary, few tremors occurred above the subducted seamount, suggesting a stress shadow. Within the Nankai Trough zone, a larger number of tremors occurred close to the trough where the underthrust sediments between the décollement and the basement are thick, and where the in-sequence thrusts develop in the accretionary prism. A smaller number of tremors occurred where the décollement and basement topography is rough. The décollement is indicated by a strong reflector with a negative polarity, and its larger amplitude near the trough suggests a larger impedance contrast between the accretionary prism and the underthrust sediments when compared to that close to the KPR. If we assume that velocity of the underthrust sediment is constant across the Nankai Trough zone, the velocity above the décollement is slower near the trough than near the KPR. The rich tremor distribution may indicate that pore pressure is elevated in the apparent lower velocity area.

To further constrain the model of seamount subduction and geological features, we are analyzing the HYU02 line that is parallel to the subducting direction and intersects with the KR0114-8. We apply travel time inversion analysis to the HYU02 refraction data. We will present a preliminary result and discuss geological interpretation. Our result will provide a new case example of seamount subduction, and contribute to a more complete understanding of the relationship between subducted seamounts and seismicity, and how these processes impact the stress state and fluid pressure distribution.