[S-CG48] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG48-07] Determination of sVLFE epicenters around the subducted Kyushu-Palau Ridge from coda interferometry

*Takashi Tonegawa¹, Yusuke Yamashita², Aki Ito¹, Masanao Shinohara³, Daisuke Suetsugu¹, Tsutomu Takahashi¹, Yasushi Ishihara¹, Shuichi Kodaira¹, Yoshiyuki Kaneda⁴ (1.JAMSTEC, 2.DPRI, Kyoto University, 3.ERI, The University of Tokyo, 4.Kagawa University)

Keywords:shallow very low frequency earthquake, Hyuga-nada region, Kyushu-Palau Ridge

Shallow very low frequency earthquakes (sVLFEs) occur at the shallow portion of the plate boundary. The Rayleigh waves at a frequency of 0.03–0.2 Hz radiated from their sources are observed in land and seafloor seismic records. Although sVLFE locations have been estimated from land records, there is a possibility to determine them more precisely if seafloor records near the epicenters are available. In the Hyuga-nada region and off Nansei Islands, ocean bottom seismometer (OBS) arrays were deployed 4 times during 4 years, each of which contained 5–13 OBSs and could observe at least one episode of sVLFE activity. In this study, we estimated the group velocity of the Rayleigh wave in the OBS arrays from coda interferometry, and applied it to the envelope correlation method (ECM) using seafloor seismic records for determining sVLFE epicenters.

As a result, the locations of sVLFE during 2014 and 2016 were determined at the northern edge of the Kyushu-Palau Ridge (KPR) (Yamamoto et al. 2013), while those on 2017 and 2018 were estimated to be the southern edge of the KPR. In particular, the sVLFEs on 2015 firstly occurred at the northern edge of the KPR, but occurred near the trough afterwards. Also, comparing the area of slow slip event (SSE) in the Hyuga-nada region (Ozawa 2017), sVLFE region at the northern part of KPR directly connects to the SSE region, indicating that slow earthquakes potentially occur from deep to shallow plate interface around the KPR.



Acknowledgement
This study was supported by “Research Project for Compound Disaster Mitigation on the Great Earthquakes and Tsunamis around the Nankai Trough Region” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. We thank Shinzaburo Ozawa and Yojiro Yamamoto for providing us SSE and KPR data.