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

[SCG45-44] Influence of the subducted Paleo-Zenisu ridge on source characteristics of shallow slow earthquakes

*Shunsuke Takemura¹, Suguru Yabe², Kentaro Emoto³, Satoru Baba⁴ (1.Earthquake Research Institute, the University of Tokyo, 2.National Institute of Advanced Industrial Science and Technology, 3.Kyoto University, 4.Japan Agency for Marine-Earth Science and Technology)

Keywords:Slow earthquake, Nankai Trough, Shallow plate boundary

We investigate source characteristics of shallow very low frequency earthquakes (VLFEs) southeast off the Kii Peninsula in the Nankai subduction zone. Shallow VLFEs are a kind of shallow slow earthquakes and are clearly observed at frequencies of 0.01-0.1 Hz. A non-linear inversion technique for moment rate function estimation (Takemura et al., 2022) and the permanent ocean-bottom seismometer network (DONET) provided us with precise locations and detailed kinematic source characteristics of shallow VLFEs. Activity change in the along-strike direction is similar to previous studies (e.g., Takemura et al., 2022; Yamamoto et al., 2022). Shallow VLFEs actively occurred around the western edge of the Paleo-Zenisu ridge, which is a subducted seamount beneath this region (e.g., Park et al., 2004).
We newly found a notable trend change in the along-dip dependency of shallow VLFE moment rates. Along the profile west side of the Paleo-Zenisu ridge (beneath KMC-KMD nodes of DONET), moment rates of shallow VLFEs increase with reaching the megathrust zone. Small-scale topographic fluctuations of the subducted oceanic plate exist along this profile, but large-scale seamount subduction has not been identified even from dense seismic surveys. Similar tendencies have been reported in deep tectonic tremors in the Nankai and Cascadia subduction zones (e.g., Yabe & Ide 2014; Wech 2020). On the other hand, the opposite trend appeared along the profile with the Paleo-Zenisu ridge (beneath KMB node of DONET). Shallow VLFEs with lower moment rates were dominant near the summit of the Paleo-Zenisu ridge. Fracture networks or stress fields due to seamount subduction (e.g., Wang & Bilek, 2014; Ruh et al., 2016) possibly impede large shallow VLFEs around the subducted seamount. The large-scale topography of the subducted oceanic plate could control source characteristics of shallow slow earthquakes.