Distributed fiber optic sensing with Rayleigh backscattering, which has been recognized as a novel technique for measuring differences in temperature or strain, was adopted in a borehole to a depth of 16 m in an actual landslide to detect a vertical profile of strain changes. Strain changes were measured every with a spatial resolution of 10 cm and strain resolution of 1.87 με. The measurements provided a clear-cut vertical profile of the strain changes caused by rainfalls that cannot be detected by conventional methods. The results show that there are two types of deformation in the landslide mass: (1) sliding at the boundary between tuff and mudstone and (2) creep in mudstone layers. Activation of deeper sections of the landslide by heavy rainfalls has also been detected.Plain Language Summary This paper reveals in great detail the deformation process of a landslide corresponding to rainfall, which has never been detected in previous research, through the monitoring of the vertical distribution of strain changes with distributed fiber optic sensing analyzing Rayleigh backscattering. The measurements were completed on 18 October 2017. Therefore, this paper needs to rapidly report the results. This paper visualizes the unknown deformation of a mudstone layer below a slip plane that has been gradually activated by heavy rainfalls. The detection of the deformation of deeper zones leading to a larger landslide is a significant advance in disaster prevention and mitigation. The applicability of this technique to strain measurements for landslides shown in this paper suggests possible strain measurements for rocks in many research fields, including in studies of the deformations of active faults and volcanos. The results shown in this paper may also be useful for researchers working in optics, photonics, and physical as well as geophysical research fields where they are trying to develop new technologies.
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