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

利用JAMSTEC潜水器和ROV获得了大量的海底伽马数据，并讨论了与地质背景的关系。然而，随潜水器或潜水器附接的γ射线谱仪受到相对较短的测量时间和在着陆和起飞期间的海底扰动的影响。我们开发了一个独立的伽马射线测量系统，名为GRAMS（伽马射线监测系统），并在相模湾进行了测试。γ射线总强度和放射性核素含量监测约两天，相对稳定，没有周期性变化。利用GRAMS方法，对南海中部增生棱体中的逆冲断层进行了伽马射线沿着的空间分布研究。结果表明，冷泉区的γ射线总强度和Th系、U系放射性核素含量明显高于正常海底。22天的GRAMS γ射线监测记录显示，渗漏点的铀系放射性核素强度很高，异常度很高。然而，所有值均未显示出随时间的波动。这表明，伽马射线是不敏感的渗漏本身，但归因于积累的放射性核素，可能来自破坏岩石的断裂带，在表层沉积物。

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.