This study elucidates the possibility of CO₂ injection-induced formation of a complex cloud-fracture network (CFN) in granite, along with shearing (shear slip) of natural pre-existing fractures under conventional (~ 150–300 °C) and superhot (> ~ 400 °C) geothermal conditions, potentially providing additional connecting flow paths between the stimulated natural fractures. Here, we conduct a set of experiments under triaxial stress states at 150 °C and 450 °C on cylindrical granite samples containing a sawcut (representing a natural fracture) inclined 45° from the sample’s axis. CO₂ injection induced dynamic shear slip on the sawcut, with higher slip velocities at 150 °C owing to the higher elasticity of the rock than that at 450 °C. The lower viscosity of CO₂ also allowed it to more uniformly pressurize the sawcut, resulting in higher slip velocities and slip displacements compared with those based on water injection in the 150 °C experiments. This implies that under conventional geothermal conditions, CO₂ injection can stimulate the same volume of rock as that of water injection at lower injection-well pressures. The CFNs then formed via CO₂ injection at pressures similar to those predicted by the Griffith failure criterion as the sawcut shear slip progressed at both experimental temperatures. The experiments also revealed potential challenges associated with CO₂ injection in naturally fractured geothermal environments, such as narrow aperture CFNs, owing to decreasing differential stress during their formation; all these factors should be addressed in future research.

This work developed a model for predicting the volatilization flux from the unsaturated soil contaminated by volatile chemical substances (VCSs) such as mercury and benzene. The model considers a series of phenomena under the unsaturated condition such as multi-phase flow consisting of a non-aqueous phase liquid, water, and gases together with the permeation of rainfall into the surface soil, the volatilization/condensation of VCSs, and the adsorption/desorption of VCSs. On this basis, this work clarified a mechanism for the generation of a volatilization flux at the ground surface. In addition, the effects of various transport phenomena in the surface soil on the magnitude and seasonal changes in this flux due to variations in weather factors such as rainfall level, temperature, and air pressure were quantitatively evaluated. This newly developed prediction model can be utilized to estimate dynamic variations in the flux under real-environmental conditions.

From the perspective of disaster prevention, it is necessary to clarify the distribution and historical frequency of tsunami deposits. However, tsunami deposits are disturbed by earthquakes and tsunamis. Therefore, it is difficult to identify the layers of deposits. Layers deposited during disturbances are dated and grain-size distribution is measured. Using the results of geochemical data and principal component analyses with detailed elemental distributions, stratigraphic boundaries of past tsunami deposit layers and other layers left in the disturbed deposits are estimated. Samples collected with a geo-slicer consist of a 50-60 cm deep disturbed layer deposited by the Great East Japan Earthquake and underlying sand, peat, and marine deposits. 24 elemental distributions in a core measured using Itrax indicate that the disturbed layer is dominated by heavy metals. Changes in principal component analysis scores infer traces of a layer that could not be visually identified in the disturbed layer. Traces of this layer are consistent with traces that are deposits indicated from the isotopic analysis and the grain size distribution. Therefore, even if it is difficult to identify deposits due to disturbances caused by the tsunami, it may be possible to identify unidentified layers and estimate their thicknesses by compressing their dimensions and clarifying elemental relationships.