Changes in rainfall–groundwater level response associated with large displacement of a deep-seated landslide in Japan

Understanding the relationship between rainfall and groundwater level response is crucial for elucidating landslide mechanisms and for planning structural mitigation measures, such as groundwater drainage works, for deep-seated, slow-moving landslides. It is well known that elevated groundwater levels induce landslide displacement; however, such displacement often causes fracturing and deformation of the landslide mass. These processes can alter the internal hydrogeological structure of the landslide, potentially changing the rainfall–groundwater level response relationship. Although some previous studies have reported differences in this relationship before and after large landslide displacements, its linkage to fracturing and deformation remains unclear.

In this study, we observed groundwater level dynamics at four observation wells (depth: 1.3–7.4 m) within a deep-seated, slow-moving landslide in Biratori, Hokkaido, northern Japan. The slip surface was estimated to be located at a depth of approximately 8 m. In November 2023, the landslide experienced approximately 4 m of displacement over a two-week period. The observation period was divided into intervals before and after this large displacement, and covariance analysis was applied to evaluate changes in the rainfall–groundwater level response relationship. For each analysis period, an antecedent precipitation index (API) was calculated from daily rainfall data. The half-life (days) and lag time (days) were optimized to maximize the correlation coefficient with the observed groundwater levels, and these optimized parameters were used in the covariance analysis. Preliminary results indicate statistically significant changes at three of the four observation wells. Furthermore, comparison with topographic changes derived from UAV-LiDAR measurements (10-cm resolution DEMs acquired on November 9 and 28, 2023), suggests that changes in half-life reflect variations in landslide-mass permeability caused by compression and tension, whereas decreases in lag time indicate the formation of new seepage pathways associated with increased fracturing. These findings suggest that the rainfall–groundwater level response relationship is not stable in actively moving landslide masses. Further analyses will examine and discuss its linkage to topographic changes in greater detail.