Wide distribution of thermal brines along Median Tectonic Lines SW Japan: Their chemical nature and origin

Chemical and isotopic studies on groundwater and dissolved gases collected from wells and self-spouting springs in wide areas of SW Japan have been performed to investigate their nature, origin and chemical processes. We also try to classify the groundwater into 3 types; volcanic, non-volcanic but from deep-seated sources, and fossil seawater, and discuss genesis of deep-seated water comparing the spatial distribution of groundwater data with geological and tectonic settings and seismicity. According to the spatial variation of hydrogen and oxygen isotopic ratios of groundwater, oxygen-shifted water is found in wide area in SW Japan not only close to quaternary volcanoes but along the tectonic lines where low frequency seismic events occur. We found many oxygen-shifted waters along 1000 km length of Median Tectonic Line originally related to the water previously categorized as Arima-type thermal water. Arima-type water has more than two times Cl concentration than the present seawater and amounts of free CO2 gas. The isotopic composition of Arima-type water is very similar to that of magmatic gases from island arcs, suggesting that this water occur from dehydration of the subducting slab. To give the other supporting evidence, the flow rate of Arima-type water in various places has been hydrologically obtained and compared with the water circulation balance in the solid earth. The difference between the source and sink of water budget in the solid earth is estimated as 1.3-11x1014 g/yr (Ito et al., 1984), and we calculated that flow rate of 40-400 kg/sec in SW Japan area (1000 km wide) is missing. As of now, our observational results show 10 kg/sec in a limited area which is a summary of only 6 local places. So far we believe these flow rate values are in good agreement to assess the origin of Arima-type water. Co-occurrence of low-Q seismic events and brine water ascent may give another evidence of origin of low-Q quakes.