Deep-Seated Landslide Associated with the Typhoon Talas, 2011, detected by TerraSAR-X
Hashimoto, Manabu; Yamada, Masumi; Fukushima, Yo; Chigira, Masahiro; Matsushi, Yuki
DPRI, Kyoto University, JAPAN
A strong typhoon (tropical storm) named Talas attacked the southwestern part of Japan and caused severe floods and large-scale landslides in Kii peninsula in September 2011. The total precipitation exceeded 1,000 mm at most meteorological stations in this peninsula during September 1 - 4. The landslides resulted in 56 fatalities, with a further 32 deaths due to flooding or unspecified hazards in the Kii Mountains. The total loss is estimated as at least 124 billion yen (~ 1 billion euro). This is the second largest natural disaster after the Tohoku-Oki earthquake and tsunami in the year.
Some landslides ruptured basement rocks, which are considered to be deep-seated landslides. It is important to understand their mechanism from the viewpoints of geomorphology/geology and mitigation of disasters. Therefore, we analyzed 5 TerraSAR-X image pairs acquired before and after the disaster from descending tracks, identified the location of landslides, and extracted their features.
Kii peninsula has a steep topography and is covered with deep forest of cedar or Japanese cypress, which makes it difficult to detect changes on surface by interferometry. Therefore, we applied the color composite technique to intensity images to identify the location of landslides. Cyan and red colors were added in proportional to the intensity of back-scattered wave of pre- and post-typhoon images, respectively, and composed into one image. Due to foreshortening, it was difficult to detect changes on slopes facing east, but we could identify the location of landslides on other slopes by the changes of color. We observed more than 10 large-scale landslides, which are concordant with aerial photos or field surveys. We also identified several landslide dams where debris of landslides dammed up a river. By filtering composite color images, we could detect smaller scale landslides and buried portion of rivers.
In order to pursue the possibility to detect any changes before the disaster, we processed ALOS/PALSAR images acquired before September 2011. We examined an interferogram of the pair of 2 images acquired before the disaster from a descending orbit similar to TerraSAR-X images, but could not find any clear indication of slide. Therefore, we concluded that the deformation preceding to landslides should be, if any, less than several centimeters.
TerraSAR-X images were provided by the PASCO Corporation, Japan, under the program of the Research Consortium of Application of SAR Technology. The ownership of TerraSAR-X images belongs to Infoterrra GmbH. PALSAR level 1.0 data were obtained from JAXA (Japan Aerospace Exploration Agency) under the program of the Utilization of Land Observation Satellite for the Disaster Prevention. The ownership of PALSAR data belongs to METI (Ministry of Economy, Trade and Industry) and JAXA.