Short-term variation and history of cold seep activity along active faults off Tokai

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A large number of sea bottom gamma ray data have been obtained using JAMSTEC submersibles and ROVs, and have been discussed about relations to geological background. However, γ -ray spectrometers attached with the submersible or vehicle are affected by relatively short measurement time and seafloor disturbance during landing and taking-off. We developed a stand-alone system of gamma ray measurement, named GRAMS (Gamma Ray Monitoring System) and tested it in the Sagami Bay. Total intensity of gamma ray and radionuclide contents monitored for about two days are relatively constant and show no periodic variation. We carried out GRAMS to know spatial distribution of gamma ray along the out-of-sequence thrust in the central Nankai accretionary prism. It is evident that total intensity of gamma ray and radionuclide contents of Th-series and U-series at cold seep sites are higher than those at normal seafloor. Twenty-two day's records of gamma ray monitoring by GRAMS show very high intensity and high anomaly of U-series radionuclide at the seep site. However, all values show no fluctuation with time. This suggests that gamma ray is not sensitive to seep itself but attributed to accumulation of radionuclide, probably derived from damaged rock of the fault zone, within surface sediments.