During the 2008 Iwate�]Miyagi Nairiku earthquake, many slope failures occurred in mountainous areas in southwestern parts of Iwate Prefecture and northwestern parts of Miyagi Prefecture. In this study, in order to fully understand the mechanism of slope failures occurring in the Koei district, Kurihara City, we carried out drilling survey at the investigation site and some soil tests on the drilled cores and soil samples obtained from the ground surface. Various aspects of soil characteristics were multilaterally investigated including physical and chemical properties, mineral compositions, and mechanical properties under both static and dynamic conditions. As a result, depth distribution of clay minerals and N�]values of the drilling sites revealed that weathering and rock degradation of the soft pumiceous tuff are significant just under the strata boundary. This strongly indicates the role of underlying weak weathered layer and its destruction during the earthquake possibly triggering slope failures of the steep cliff locating near the flank of the caprock layer. Furthermore, Long�]traveling flow�]type failures occurring on the gentle slope could be caused by excess pore pressure generation by loading of collapsed materials on the top part of the slope. In addition, results of dynamic shear tests on the slip surface soils also suggest the possibility of liquefaction of previously collapsed and deposited materials which are composed of loosely sedimented volcanic ash soils.

Key words�Fearthquake, slope failure, weathering, caprock structure, liquefaction