Masayuki Sakagami

Catastrophic Landslides of Pyroclastics Induced by the 2011 off the Pacific Coast of Tohoku Earthquake

The 2011 off the Pacific coast of Tohoku Earthquake induced 4 soilslide-avalanches of pyroclastic fall deposits with volumes from 500 m3 to 30000 m3 in Shirakawa and Nakagawa, where 6+ and 6− seismic intensities of JMA scale were recorded, respectively. These landslides occurred on rather gentle slopes with 13° to 23° with sliding surfaces in the depths of 3 m to 9 m but had a high mobility, which is shown by apparent friction angles ranging from 10° to 16°. Trees on slopes have been transported on debris in a standing position. The causal factors of these landslides were slope-parallel bedding of pyroclastics that include very weak paleosol, in which a sliding surface was made, and that the beds that slid had been undercut to have lost the support from lower slope. The paleosols were rich in halloysite, which is a weathering product of volcanic ash and pumice. Soil slide-avalanches like those described above have been induced also by previous earthquakes, including 1949 Imaichi, 1968 Tokachioki, and 1978 Izu-Oshima-Kinkai earthquakes. Among these, the landslide numbers of the Tohoku Earthquake were much less than those of the other earthquakes, which could be related to less amounts of preceding rainfall before the Tohoku Earthquake than the others. The Tohoku Earthquake occurred during a dry season in Japan and had only 94 mm during 60 days before the earthquake, while the other earthquakes had 250–400 mm antecedent rainfalls. Pyroclastic fall deposits generally are distributed widely, so if much more rainfall had preceded the earthquake, more landslides could have been induced.

Continuous monitoring of the ground surface change in Sakurajima volcano using RADARSAT-2 data

We applied C-band SAR data acquired by RADARSAT-2 to examine ground surface changes in the active volcanic field. In the active volcano area, the observation of secondary disaster with rainfall is also important as well as direct influence by the volcanic eruption. By continuous monitoring, we generated a change detection image from the Spotlight mode data (1m resolution). By using the change detection images and also by field survey, sand movements with debris flow and ground surface change by eruption were extracted. We concluded that continuous monitoring using the SAR is effective for preventing volcanic disaster.

Method of Estimating Slope Movement Area Using DInSAR Analysis

For prevention/reduction of damage caused by large-scale sediment disaster, it is important to conduct field surveys and preventive measures by detecting the areas of minor slope movement as a sign of such sediment disaster at an early stage. As a technology for wide-area and routine monitoring of slope movement sites, Differential Interferometric SAR (DInSAR) analysis using L-band SAR satellite is effective in Japan. Accordingly, we studied a method of estimating the areas of slope movement representing a high possibility of landslide with interference fringes detected by DInSAR Analysis. This study compared actual slope movement area obtained from LiDAR data with the area estimated considering interference fringes and micro-topography and evaluated the consistency between both areas. As areas for estimating slope movement, we set up a total of four patters—two patterns of the area of interference fringes according to the frequency of appearance when interference fringe was detected at two or more timings, one pattern of the area considering the surrounding microtopography, and one pattern of the area considering the whole slope. As the result, it was found that the area of slope movement close to that of actual slope movement can be estimated by estimating as slope movement area an area that is around the area with a high appearance frequency of interference fringe and has micro-topography shaped according to the estimated direction of slope movement.

Soil-Slide Avalanches of Pyroclastic Fall Deposits Induced by the 2011 Off the Pacific Coast of Tohoku Earthquake

The 2011 Off the Pacific Coast of Tohoku Earthquake set off four soil-slide avalanches of pyroclastic fall deposits within Shirakawa (Fukushima Prefecture) and Nakagawa (Tochigi Prefecture). At Shirakawa, a seismic intensity of 6+ on the JMA scale was recorded and at Nakagawa, 6−. Predominant factors behind these slides were (1) a bedding of pyroclastic fall deposits parallel to the original slope and (2) a sliding surface of weak paleosol that, having been undercut by erosion at the bottom of the slope, lost its ability to support the soil above it. These landslides were highly mobile, as suggested by apparent friction angles of 10–16°. Many trees on them, supported by root systems, remained standing within their root–soil plate; that is, they were transported and deposited upon the underlying debris in an upright position. Similar slides have been observed to occur in numerous other earthquakes, which suggests that some pyroclastic fall deposits are highly susceptible to seismic shocks. Here, to contribute to efforts toward mitigating the earthquake-induced hazards presented by such deposits, we identify some of the characteristics of soil-slide avalanches induced by the 2011 Off the Pacific Coast of Tohoku Earthquake.