Hydrological changes due to the M7.0 earthquake at Iwaki, Fukushima induced by the 2011 Tohoku-oki earthquake, Japan

After a month of the 2011 Tohoku-oki earthquake (Mw9.0) in Japan, two normal faults located 300 km southwest of the epicenter moved with several earthquakes (Max Mw7.0, 11 April 2011). Significant hydrological anomalies occurred around the faults after the movements. We have surveyed spatial distribution of the hydrological anomalies within 10 km of the faults, and analyzed chemical composition of some hot spring waters including the spouting waters which began to discharge just after the M7.0 earthquake. Increases of groundwater level were observed on the lower block of the normal faults. Especially water level of a hot spring well increased about 10 m in the Iwaki Yumoto area, located 3 km away from the faults. The GPS observation by the Geospatial Information Authority of Japan (GSI) showed that contraction of 16 cm between the two GPS stations 9.6 km away and uplift of 6 cm occurred simultaneously in the Iwaki Yumoto area. The observation results for the crustal deformation indicated that the increases of groundwater level were caused by contraction strain change due to the fault movement. On the other hand, decreases of groundwater level were observed on the upper block of the faults, which was thought to be caused by dilatation strain change due to the fault movement. In the Iwaki Yumoto area, a large amount of hot water with higher temperature (58 degree-C) has flowed out from the old coal mine after the M7.0 earthquake. The flow rate was up to about 10,000 m3/day on May, 2011. The chemical composition of this hot water has gradually changed to that of 1000m-deep-groundwater from boreholes drilled to basement rocks in this area. The analytical result indicates that the outflow occurred by discharging of the hot groundwater stored in the fractures of the basement rocks to the roadway of the old coal mine by contraction strain changes of the basement rocks due to the M7.0 earthquake.