[S-CG63_2PM1] 断層帯の化学

[SCG63-05] 地下水中Rn-222とCl-による立川断層帯の地殻化学的調査

*下舘 知也¹、角森 史昭²、安原 正也³、林 武司⁴ (1.国際基督教大学、2.東京大学、3.産業技術総合研究所、4.秋田大学)

キーワード：ラドン, 塩化物イオン, 立川断層帯, 温泉水

The 2011 off the Pacific coast of Tohoku Earthquake enhanced the probability of earthquake in Tachikawa Fault Zone. To evaluate the physical condition of Tachikawa fault zone, we researched Rn-222 concentration in groundwater, which is known as a precursory anomaly of earthquake. Additionally, we focused on the connection between the concentration of chloride ion in groundwater and fault damage zone and measured chloride ion in groundwater as well. Our purpose in this paper is to detect suitable groundwater for continuous measurement: (1) The depth of aquifer is deep enough to reach the bedrock. (2) Water contains much enough radon to monitor. (3)Water contains chloride ion whose concentration is controlled by the condition of fault damage zone. Radon concentration in confined aquifer is supplied by alpha decay of radium in the surface of the grain. If the specific surface area of the grain changes according to physical condition of fault, radon concentration in the groundwater would change. The upper edge of bedrocks, main source of radon, has a depth of under 2,000 m around Tachikawa Fault Zone and it deepen in the west.We focus on the distribution of chloride ion around Ayasegawa Fault is similar to one around Tachikawa Fault Zone. Low concentration of chloride ion in groundwater around Ayasegawa Fault is affected by fault damage zone, and we expect that groundwater around Tachikawa Fault Zone also shows low concentration of chloride ion due to fault damage zone. We use RTM1688(SARAD) for radon measurement, and ion chromatography for chloride ion measurement.As a result, samples around the fault show high concentration of radon and low chloride ion. This suggests that the well around fault reach the bedrock and contain low concentration of chloride ion due to fault damage zone. These samples would show concentration change according to earthquake.