[J06-P] Poster

[J06-P-13] Low-frequency tremor activity in the shallow part of Nankai Trough and Ryukyu Trench revealed by long-term ocean bottom observation

^○Yusuke Yamashita¹, Masanao Shinohara², Hiroshi Yakiwara³, Tomoaki Yamada², Kazuo Nakahigashi⁴, Hajime Shiobara², Kimihiro Mochizuki², Takuto Maeda², Kazushige Obara² (1.Kyoto University, Miyazaki, Japan, 2.The University of Tokyo, Tokyo, Japan, 3.Kagoshima University, Kagoshima, Japan, 4.Tokyo University of Marine Science and Technology, Tokyo, Japan)

Low-frequency tremor is one of the slow earthquakes occurred in the deep and shallow part of the subduction zone. The detail of shallow tremor was documented by Yamashita et al. (2015), they have carried out the ocean bottom observation in the Hyuga-nada, southwestern part of Nankai Trough, and succeeded in detecting shallow tremor as a complete episode lasting for one month exhibiting similar migration property of deep tremor for the first time. From 2014, we started long-term ocean bottom seismological and geodetic observation in the Hyuga-nada. In addition, we also started that in the north part of Ryukyu Trench from 2014, in the central part of Ryukyu Trench from 2015.

During these observations, we found some tremor activities in those areas. In the Hyuga-nada, we detected tremor activity in 2014, 2015, and 2016. The 2016 activity may be triggered by 2016 Kumamoto earthquake. In the Ryukyu Trench, although it is hard to estimate the epicenter of tremor source because the separation between the ocean bottom seismometer is large, we found the locality of the shallow tremor activity.

From 2017, we started a new project for observation of shallow slow earthquake. We installed ocean bottom seismometers and pressure gauges around the focal area of shallow tremor in the Hyuga-nada. This observation will continue until 2020. For this presentation, we will introduce the ocean bottom observations and discuss the property of shallow tremor activity along the Nankai Trough and Ryukyu Trench using preliminary result by analyzing the data which were recovered from the ocean bottom.
Acknowledgement: This study is supported by the research project for compound disaster mitigation on the great earthquakes and tsunamis around the Nankai Trough region, the MEXT of Japan, under its Earthquake and Volcano Hazards Observation and Research Program, and JSPS KAKENHI Grant Number JP16H06473 in Scientific Research on Innovative Areas “Science of Slow Earthquakes".