[J07-P] Poster

[J07-P-11] Monitoring of the shallow tremors around the source areas of the Nankai and Tonankai earthquakes by ocean bottom observations

^○Kensuke Suzuki¹, Eiichiro Araki¹, Toshinori Kimura¹, Yuya Machida¹, Demian Saffer², Narumi Takahashi^1,3, Shuichi Kodaira¹ (1.JAMSTEC, 2.The Pennsylvania State University, 3.NIED)

Japan Agency for Marine-Earth Science and Technology (JAMSTEC) installed two ocean bottom observation systems in the source area of the Nankai and Tonankai earthquakes along the Nankai trough, southwest Japan (e.g., Kaneda et al, 2015; Kawaguchi et al., 2015; Kopf et al., 2011); one is the cabled observation system called Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET), the other is the borehole observation system called Long-Term Borehole Monitoring System (LTBMS). LTBMS and DONET stations have several kinds of the sensors such as a broadband seismometer, a quartz-type pressure gauge and a pore pressure gauge on and under the seafloor for the seismic and geodetic observations. The digitized data recorded at stations, which are distributed between coast and trough axis, are continuously transferred to our laboratory in real-time. By using these data, we can continuously monitor the tremors on the plate interface shallower than the source areas of the mega-thrust earthquakes along the Nankai trough at where the past studies have little detected tremors. In this study we purpose to monitor the non-volcanic tremors around the LTBMS and DONET stations, especially shallow ones.
By applying the envelope correlation method (Ide, 2010; Ide, 2012) to records of the broadband seismometers between Jan. 2011 and Dec. 2016, we tried to detect non-volcanic tremors around the source areas of the mega-thrust earthquakes. We detected both of the stable and episodic tremor activities. Tremors have stably occurred at the shallower part than the source area of the Tonankai earthquake, which almost corresponds to the aftershock area of the 2004 off Kii peninsula earthquake. Episodic tremors occurred either alone or with the large earthquakes (e.g., the off Mie earthquake on 1 Apr. 2016). Some long-term deviations of the pore pressure, which might indicate the slow slips, were detected at almost same time with the occurrences of the tremors.