Internal structure and present-day activity of deep-seated gravitational slope deformation (Chingjing, Taiwan)

Deep-seated gravitational slope deformation (DSGSD) is a rock mass wasting process of high mountain slopes, featuring slow movement rate. Although DSGSD movement is slow, it can continue for a long period, producing large cumulative displacements and could transform to catastrophic rockslides. In Taiwan, DSGSD has often been reported in the slate belt of the Taiwan’s backbone Range because of the inherent cleavage characteristic. When the slate slope undergoing DSGSD, the geometry of cleavage structures will interact with topographic slope and manifest by different internal structures such as toppling features and flexural folding. This study investigates how the DSGSD influences the internal structures and present-day activity of slate slopes in the Chingjing region, Taiwan. We focus on where the cleavage dip direction is parallel to the topographic downslope direction. To describe the relationship between cleavage structure and DSGSD movement, we present 2D numerical simulation of simplified slopes using the distinct element modeling approach. The slope topography and cleavage geometry are based on the typical values of slate slope in the study area. The simulation shows that the rotation of the cleavage dip angle has correlated with the slope deformation mechanics at different locations. The toppling structure appears to the slope toe, and the cleavage remains the same dip angle at the crest. Three hinge lines can be identified at different depths of the slope, which suggests the location of potential basal shear bands within the slope. We also observe the distribution of the shear bands emerging at higher elevation as the deformation velocity decreases. Parametric study shows that deformation of internal structures can exist at depths of 60 m and more as a result of slope height, slope steepness and cleavage dip angle. On the other hand, this study retrieves slope kinematics by performing 2D decomposition of PS-InSAR products derived from Sentinel-1 data acquired in ascending and descending orbits. The result shows that surface displacement ranges in 5 - 10 mm/year in the period of 2015 - 2017, and the displacement rate increases to 10 - 30 mm/year in the period of 2018 - 2020. By detecting velocity change and identifying deformation dip vector, we explain the present-day activity of DSGSD and driving mechanisms in the study cases. Overall, based on mechanical modeling, our analyses demonstrate that a cataclinal slate slope can exhibit different internal structure patterns in different sectors during DSGSD. We also highlight the need for InSAR-assisted monitoring in the region lack of surface displacement data for deeper understanding of this long-term process and interactions between slope activity and potential driving force.