3:30 PM - 4:45 PM
[SCG59-P14] 13 year continuous observation of ambient gamma ray at cold seepage site on deep seafloor with a multidisciplinary cabled observatory off Hatsushima Island in Sagami Bay
Keywords:NaI(Tl) scintillation detector, mudflow, multidisciplinary cabled observatory
After the off Tohoku earthquake occurred, rapid signal level increase of Bi-214 and K-40 was observed. This would not be associated with the off Tohoku Earthquake itself, but would be associated with the mudflow caused by the Eastern Shizuoka Prefecture Earthquake (M6.4) which occurred on March 15, 2011; 4 days after the off Tohoku Earthquake.
After this event, significant event was not recognized except the abrupt signal level fluctuation associated with the underwater work with remotely operated vehicle (ROV) near the observatory.
Since December in 2014, gradual signal level decrease of Bi-214 was observed, while the signal level decrease of K-40 started gradually shortly after the off Tohoku Earthquake, roughly in October 2011. The effect of Cs-137 associated with atomic plant accident on the signal level of Bi-214 should be considered.
Throughout the observation period, predominant event is the significant signal level increase of Bi-214 in October 2006, whose fluctuation pattern was different from the other fluctuations associated with the disturbance on seafloor caused by the sedimentation or the underwater work with the ROV. It would reflect the sub-bottom incident associated with tectonic deformation.
1) Kumagai, et al., Gamma Radiation (Intech, Croatia, 2012), DOI:10.5772/36392, p.64.
2) Iwase and Takahashi, JpGU meeting 2011, SCG059-P19 (2011).