In this study, we evaluated temporal changes in fracture systems associated with the operation of a geothermal power plant by applying S-wave splitting analysis, a phenomenon that S-waves separate into fast and slow polarization components when passing through anisotropic media, to records from an existing seismic network. The analysis focused on the Wasabizawa geothermal area in Akita, using 32 seismic events recorded at the Hi-net Ogachi station (N.OGCH) between 2006 and 2022. By applying the cross-correlation method to the S-wave portion of velocity waveforms, we estimated the fast S-wave polarization direction and the lag time between the two split S-waves. The results revealed a distinct shift before and after the 2011 Tohoku-Oki Earthquake, where the fast S-wave polarization directions converged from two dominant directions into a single orientation (N70°E-N110°E) consistent with the regional stress field. Meanwhile, the slowness (lag time normalized by path length) for deep events (>10 km) remained stable throughout the entire period, suggesting that the massive earthquake induced a reorganization of fracture orientations in the deep crust rather than structural degradation. In the vicinity of the geothermal power plant, the average lag time decreased by approximately 40% following the start of operations in 2019, and the slowness estimated from shallow earthquakes exhibited greater variability compared to that of deep events. Although no decisive changes in the physical state of the reservoir were identified within the first three years of operation, disturbances in the fracture structure attributable to the geothermal reservoir were detected through comparison with deep seismic data.

　The amplitude and polarity of reflected waves provide important information for interpreting lithological variations and fluid distributions. In this study, we re-examine the reflection response of a low-velocity layer with a thickness comparable to the wavelength via one-dimensional numerical modeling and investigate the conditions under which such a layer can generate apparent positive-polarity reflections. For a model in which impedance gradually decreases from the top of the low-velocity layer and then sharply increases at the bottom to recover its original value, negative-polarity reflections become relatively weak, while positive-polarity reflections become prominent. In addition, we confirm that the waveform of the reflected wave varies with the incident-wave frequency due to the influence of negative-polarity components. These numerical modeling results suggest that prominent positive-polarity reflections do not preclude the presence of an interbedded low-velocity layer. For such layers, broadband data acquisition, including low-frequency components, and frequency-dependent waveform analysis are effective. It is then important to interpret such reflections by considering both velocity variations within the layer and the relationship between layer thickness and wavelength.