Groundwater, possibly originated from subducted sediments, in Joban and Hamadori areas, southern Tohoku, Japan

Abstract

We studied the origin of deep groundwater in the Joban and Hamadori areas in southern Tohoku, Japan, based on δD, δ 18 O, 129I/I, 36 Cl/Cl, and 3 H concentrations. Deep groundwater was collected from the basement rocks (Cretaceous granite) and from the margin of the Joban sedimentary basin (latest Cretaceous to Quaternary sedimentary rocks deposited on the basement rocks). We sampled groundwater pumped from depths ranging from 350 to 1,600 m in these areas. A hypothetical end-member of deep groundwater was estimated from the relationship between δ 18 O and Cl concentrations, and our data reveal a much higher iodine concentration and lower Br and Cl concentrations than found in seawater. The iodine ages inferred from 129 I/I are quite uniform and are about 40 Ma and 36 Cl/Cl almost reached the secular equilibrium. The relationship between iodine and Cl can be explained by mixing a hypothetical end-member with meteoric water or seawater. Moreover, the I/Cl ratio increases linearly with increasing water temperature. The water temperature was high in Joban, with a maximum of 78°C at a depth of 1,100 m. The geothermal gradient in the Joban basin is 18°C km, and the temperature even at a depth of 3 km in the basin was not high enough to supply thermal water to the sampling sites. Thus, sedimentary rocks in the Joban basin are unlikely to be the source of iodine in the deep groundwater. Several active faults such as the Futaba Fault are developed in and around the studied areas. The Iwaki earthquake occurred 1 month after the 2011 Tohoku-oki earthquake, and normal-fault type surface ruptures formed and discharged hot groundwater in Joban. The deep groundwater we studied probably came up through the basement rocks from greater depths. There are no sedimentary rocks younger than Tertiary age beneath the pre-Cretaceous basement rocks, and the subducted sediments in the Japan Trench are a possible source of iodine in the groundwater. The Joban and Hamadori areas may be an ideal window to look into the water circulation in the forearc of the Tohoku subduction zone.

Underestimation of Microearthquake Size by the Magnitude Scale of the Japan Meteorological Agency: Influence on Earthquake Statistics

“…For tectonic events, normal-faulting events often give higher b values, strikeslip ones moderate values, and reverse-faulting ones lower values (Gulia & Wiemer, 2010;Schorlemmer et al, 2005). Moreover, in the Fukushima Hamadori area after the Tohoku-oki earthquake, the groundwater continuously discharged to the ground surface (Togo et al, 2014); therefore, the fault may also be rich in groundwater. Note that the prior studies estimated b values using either the local magnitudes (b j or b L , b value for the local magnitude (M L )) or the moment magnitude and sometimes did not distinguish between these two types of b values.…”

“…In our study area, normal-faulting events are dominant. Moreover, in the Fukushima Hamadori area after the Tohoku-oki earthquake, the groundwater continuously discharged to the ground surface (Togo et al, 2014); therefore, the fault may also be rich in groundwater. These two reasons may produce the very large b w values.…”

Chemical Composition of Mantle Wedge Fluids

“…B) Schematic illustration showing separation of supercritical fluids into aqueous fluid and hydrous melt in subduction zones with relatively young oceanic plate such as Southwest Japan and Luzon arc (Kawamoto et al, 2012 . Dehydration reactions beneath the forearc region, serpentinization and S-free hot springs may not occur in subduction zones with relatively old plates such as Northeast Japan arc McCrory et al, 2014;Togo et al, 2014) . When a sediment-derived supercritical fluid enters the overlying mantle, the fluid reacts with the peridotite to become HMA-bearing supercritical fluid.…”