Mapping Aquifer Storage Properties Using S-Wave Velocity and InSAR-Derived Surface Displacement in the Kumamoto Area, Southwest Japan

We present a novel approach to mapping the storage coefficient (Sk) from InSAR-derived surface deformation and S-wave velocity (Vs). We first constructed a 3D Vs model in the Kumamoto area, southwest Japan, by applying 3D empirical Bayesian kriging to the 1D Vs profiles estimated by the surface-wave analysis at 676 measured points. We also used the time series of InSAR deformation and groundwater-level data at 13 well sites covering April 2016 and December 2018 and estimated the Sk of the confined aquifer. The Sk estimated from InSAR, and well data ranged from ~0.03 to 2 × 10−3, with an average of 7.23 × 10−3, values typical for semi-confined and confined conditions. We found a clear relationship between the Sk and Vs at well locations, indicating that the compressibility of an aquifer is related to the stiffness or Vs. By applying the relationship to the 3D Vs model, we succeeded in mapping the Sk in an extensive area. Furthermore, the estimated Sk distribution correlates well with the hydrogeological setting: semi-confined conditions are predicted in the Kumamoto alluvial plain with a high Sk. Our approach is thus effective for estimating aquifer storage properties from Vs, even where limited groundwater-level data are available. Furthermore, we can estimate groundwater-level variation from the geodetic data.

Quartz Microstructures from the Sambagawa Metamorphic Rocks, Southwest Japan: Indicators of Deformation Conditions during Exhumation

The Sambagawa metamorphic rocks in central Shikoku, southwest Japan consist of an inverted metamorphic sequence from the upper chlorite to oligoclase-biotite zones at the lower structural level (LSL), which is overlain by a normal metamorphic sequence consisting of the albite-biotite and garnet zones at the upper structural level (USL). These sequences form a large-scale recumbent fold called the Besshi nappe. To unravel the mechanism of recrystallization and physical conditions in quartz, and their relation to exhumation tectonics, microstructures of recrystallized quartz grains in quartz schist from the Asemi-Saruta-Dozan River traverse were analyzed. The recrystallized quartz grain size increases with increasing structural level from 40 µm in the upper chlorite zone to 160 µm in the garnet zone of the USL. Further, the mechanism of dynamic recrystallization of quartz changes from subgrain rotation to grain boundary migration with increasing structural level across the uppermost garnet zone of the LSL. From these data, the deformation temperatures in quartz schist are calculated to increase with increasing structural level within the range between 300 and 450 °C using paleopiezometers and experimental flow laws. It could be interpreted that a rapid cooling of the Besshi nappe from above is responsible for the deformation temperatures recorded in quartz schist.

Oxygen Isotope Equilibrium of the Shallow-Water Benthic Foraminifer Hanzawaia nipponica Asano in Tosa Bay, Southwest Japan

Oxygen isotopic compositions (δ18O) of benthic foraminifer tests are widely used for reconstructing paleoceanographic changes, such as global ice volumes during glacial–interglacial cycles. Although deep-sea benthic foraminifers have been well characterized and are considered reliable indicators, little attention has been paid to the geochemistry of shallow-water benthic foraminifers. In this study we evaluated δ18O in the shallow-water benthic foraminifer Hanzawaia nipponica Asano, which lives in surface sediments on continental shelves and upper slopes under the influence of two warm currents, the Kuroshio and Tsushima currents, in the East China Sea, northwest Pacific, and southwestern Japan Sea. To evaluate oxygen isotope equilibrium, we analyzed δ18O of H. nipponica and ambient seawater on the continental shelf in Tosa Bay, southwest Japan. Seawater δ18O and salinity in Tosa Bay are similar to those of surface and subsurface waters in the Kuroshio region in the Okinawa Trough and the northwest Pacific. Vertical profiles of seawater δ18O show no variation with water depth (0–200 m) in Tosa Bay. However, tests of living H. nipponica (as determined by staining with Rose Bengal) and fossil (non-stained) H. nipponica, picked from samples of the top centimeter of seafloor sediment, yielded carbonate δ18O values that clearly increase with water depth, suggesting a temperature-dependent relationship. A comparison of carbonate δ18O values in living H. nipponica and those predicted on the basis of seawater δ18O and annual mean bottom temperature shows that H. nipponica tests are in oxygen isotopic equilibrium with ambient seawater. We determined the linear equations of δ18O–temperature relationship, and the slope of −5.26 (0.19‰°C−1) for living and −4.50 (0.22‰°C−1) for the fossil H. nipponica, respectively. The carbon isotopic compositions (δ13C) of H. nipponica also closely match seawater δ13C. Thus, we propose that the carbonate δ18O and δ13C of H. nipponica are useful proxies to reconstruct shallow-water paleoenvironmental changes in the northwest Pacific and its marginal seas.