[S-CG39] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG39-P24] Moment tensor inversion of shallow very low-frequency earthquakes in the northern Ryukyu subduction zone and the Hyuga-nada region

*Aki Ito¹, Hiroko Sugioka², Masaru Nakano¹, Yusuke Yamashita³, Masanao Shinohara⁴, Tsutomu Takahashi¹, Daisuke Suetsugu¹, Takashi Tonegawa¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Graduate School of Science, Kobe University, 3.Miyazaki Observatory, Disaster Prevention Research Institute, Kyoto University, 4.Earthquake Research Institute, University of Tokyo)

Keywords:shallow very low-frequency earthquakes , nothern Ryukyu subduction zone, Hyuga-nada region

Shallow very low-frequency earthquakes (sVLFEs) have occurred in the northern Ryukyu subduction zone and the Nankai subduction zone (e.g., Ito and Obara 2006; Asano et al. 2015; Asano 2020). Observations using long-term ocean-bottom seismometers equipped with 1Hz seismometer have been conducted since 2014 in the Hyuga-nada region to investigate the shallow slow earthquake activities. Studies from the offshore data revealed the spatiotemporal distribution and migration of shallow low-frequency tremors (Yamashita et al. 2015; Yamashita et al. this session). Although the shallow low-frequency tremors and sVLEFs are considered as a coupled phenomenon, the sVLFE activities have not been studied in detail mainly because only a few broadband ocean bottom seismometers (BBOBSs) have been deployed in the northern Ryukyu subduction zone and the Hyuga-nada region before 2017. Use of offshore observation data from BBOBS is crucial to reveal the sVLFE activities.

We conducted four consecutive temporary seismic observations using BBOBSs off the east of Tanegashima Island and the Hyuga-nada region from August 2017 to August 2020 in order to study sVLFE activities. The BBOBSs are equipped with either a Guralp CMG-3T broadband sensor (Kanazawa et al. 2009) or a Trillium Compact 120s broadband sensor (Shinohara et al. 2018). We applied the centroid moment tensor (CMT) inversion method (SWIFT system; Nakano et al. 2008) to the sVLFE waveforms observed by the BBOBSs. The waveforms were band-pass filtered between 0.03 and 0.05 Hz in the analysis. The sVLFE signals were detected from the continuous records according to sVLFE catalog from land-based observations (Asano 2020), which includes the date and time of sVLFEs and their epicenters. We assumed the one-dimensional structure model by referring to the following two velocity structures off the east of Tanegashima Island for computations of the Green’s functions. For depths shallower than 10 km, we referred to the S-wave velocity structure obtained by the method of Tonegawa et al. (2017). For depths deeper than 10 km, we referred to the three-dimensional P- and S- wave velocity structures estimated by seismic tomography (Yamamoto et al. 2020).

We determined the CMT solutions of approximately 15 sVLEFs so far. The preliminary results showed that the focal mechanisms of sVLFEs were mostly reverse faulting type, dipping to the direction of plate subduction. Moment magnitude were between 3.6 and 5.4. Although most sVLFEs occurred along the plate interface between subducting Philippine Sea plate and landward plate obtained by previous studies (Nakanishi et al. 2018; Yamamoto et al. 2020), some events seem to occur within the Philippine Sea plate. Increasing the event number is necessary to obtain further details of the characteristics of sVLFEs in this region. We will analyze more sVLEFs in the future and discuss the source processes of shallow slow activities.

Acknowledgments: This study is supported by the JSPS KAKENHI Grant Number JP16H06471 and the research project for compound disaster mitigation on the great earthquakes and tsunami around the Nankai trough region, the MEXT of Japan. We also use the data obtained as part of collaborative researches with Association for the Development of Earthquake Prediction.

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