Abstract
A big earthquake of JMA scale magnitude 7.2 occurred at 8:43 on June 14th in 2008. The hypocenter of the quake was 8 kilometers underground of the mountainous area of the Iwate-Miyagi prefectural border. The strong ground motion (maximum acceleration of 1G at 260 meters depth) and the significant surface displacement (over 2 meters rise in elevation) were observed close to the epicenter. Many large scale landslides and many natural dams are striking geological phenomena caused by this earthquake. To estimate these distinctive slope failures, we should consider the relationships among the geology, the geological structure, and the engineering properties of the ground under the strong ground motion.
Japan Society of Engineering Geology (JSEG) dispatched the research commission to the stricken area on September 14th to 15th. Characteristically, these slope failures mainly occurred at several spots underlain by Neogene to Pleistocene volcanic and pyroclastic rocks with occasional soft thin beds of lacustrine sediment layers. The temporary results of the research were reported at the JSEG annual congress held in Yokohama, October 31st, and the images of them was also uploaded onto the website of JSEG.
This report presents the results of the above mentioned fact-finding research and additional field surveys. Geological conditions of the researched area are described, and the processes and the mechanisms of these slope failures are discussed in this paper.
