During the 2004 Mid-Niigata Prefecture earthquake, thousands of landslides were triggered, among them two large scale landslides (Higashi-Takezawa and Terano landslides) occurred within past landslide masses and dammed the river at the toe of the landslides. To understand the internal structure of the landslide dam and then provide more reliable parameters for the stability analysis of landslide dam, we used a multiple channel surface wave technique and microtremor array measurements to survey the S-wave velocity profile. The results showed that the integrated use of these methods is effective for the S-wave velocity structure of landslide dam, irrespective of the mountainous environment. Based on survey results, the landsliding mechanism and further research on elevating the reliability of landslide dam stability were discussed.
On October 16, 2013, Typhoon No.26 (Typhoon Wipha) passed through Izu-Oshima Island, approximately 100km south of Tokyo. On this day, the Motomachi district, on the northwestern slope of Mt. Mihara, experienced its heaviest rainfall in recorded history. The 24-h precipitation reached 824mm, inducing many shallow landslides and mudflows, particularly in the Okane-zawa river basin. Recent studies have shown that most of the landslides occurred along a boundary between high-permeability layers consisting of scoriaceous ash and low-permeability layers consisting of loess deposits. Many shallow landslides also occurred in the Omiyasawa river basin, which lies towards the south of the Okane-zawa river basin. However, the reason for these landslides and subsequent processes, such as mudflows, had not previously been clarified. In this study, we estimated the areas of the landslides and erosion and deposition sites from altitude changes determined using digital elevation models (DEMs) developed before and after the typhoon. Through field surveys, we also found that the topographies and processes of the shallow landslides in the upper Omiya-sawa basin differed from those in the Okane-zawa basin. The landslides in the Omiya-sawa basin occurred along a boundary between scoriaceous ash layers and low-permeability layers consisting of lava and lahar deposits.