Distribution of shallow landslides and related mass movement processes induced by the heavy rain in July 2018 on Gogoshima Island, Ehime Prefecture, Japan

A heavy rain event in July 2018 caused many natural disasters such as landslides and floods in western Japan. Many shallow landslides were induced by heavy rain between the night of July 6 and the morning of July 7 in Gogoshima Island, which is located offshore of Matsuyama City, Ehime Prefecture. The main industry of the island is the cultivation of oranges. However, the orange farming infrastructure was devastated by shallow landslides across a large portion of the island. We created a spatial distribution map of the shallow landslides, based on interpretation of the SPOT－7 satellite imagery, to understand their characteristics. Moreover, the mass movement processes were clarified using the results of geomorphological and geological field surveys. The analysis revealed the following : 1) The shallow landslides induced by the heavy rain event were distributed across the island, and the total number of landslides was 207. 2) Relatively elongate, shallow landslides are distributed on the slopes of Mt. Kofuji. The shallow landslide materials changed to debris flows. 3) The field survey revealed that the heads of shallow landslides are located on the geological boundary between granodiorite and andesite. The landslide material was formed of loose, highly weathered granodiorite, called “masa” in Japanese, and thermally metamorphosed granodiorite. 4) The previous debris flow deposits that filled the gullies were eroded by the new debris flows and flash floods.