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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.

Development of a notch at the base of a cliff reduces cliff stability and often induces a collapse. Pleistocene limestone coastal cliffs of elevation 5 m in Kuro-shima, Ryukyu Islands, have a prominent notch with a depth of 3-4 m at their bases. Around these coastal cliffs, collapses different from previous studies of cliff collapses in the Ryukyu Islands were found; collapses in Kuro-shima have a horizontal failure surface. The horizontal failure surface, situated at the height of the failure surface corresponding to the retreat point of the notch, is bounded by vertical joints cutting the whole cliff and the reef fl at in front of the cliff. Two types of horizontal failure surface were found, triangular and quadrangular; the distinction appears to depend on the angle between the vertical joints and the front face of the cliff. Prior to collapse, these cliffs appear to have been separated from the adjacent cliffs by the development of vertical joints. Consequently, a cliff that will collapse can be identifi ed in advance; cliff instability is strongly dependent on the development of a notch. To study the effect of notch development on cliff collapse, the notch depth at which collapse occurs was calculated using stability analysis. Instability of a cliff increases with notch depth; collapse occurs at the horizontal failure surface when the ratio of the notch depth to the seaward length of the cliff is approximately 0·5-0·7 for a triangular failure surface, and 0·7-0·9 for a quadrangular failure surface.

Many boulders are located around the coastal cliffs with height of below 5 m made of coral limestone at Kuroshima, Okinawa, Japan. The origin of the boulders appears to be coral limestone cliffs which show developed notches. We undertook stability analysis, involving the wave pressure due to tsunamis, of wave-induced collapse of the cliffs. We fi nd that extreme waves are capable of inducing cliff collapse, as observed in circumstances where gravity is insuffi cient.

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