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| | Abstract | | In the southern part of focal region of the 2008 Iwate-Miyagi Nairiku Earthquake,Mj=7.2, serious landslides occurred by its strong motion. Especially, Aratozawa Canyon in Kurihara city of Miyagi prefecture was collapsed by the biggest landslides with some 1400mX800m area and 140m cliff fall. At the same time, maximum 300m horizontal sliding in the young sedimentary layers below the surface is reported. The geology in this area is so much complicated due to the past, early-mid-Miocene, volcanic activity and quaternary volcanic ash and flow which is supplied from Mt. Kurikoma located in 10km NNW may have filled the Aratozawa Canyon forming the layered sediments sorted by water stream. We tried a preliminary shallow seismic survey with P/S wave in order to define the structure of the Miocene basement and seismic velocity contrast between overlaying sediments. One of the two short seismic lines across the boundary between the outcrop of Miocene and layered sediments which is horizontally touching is showing clear velocity contrast. The basement has higher P velocity from 3.0 to 4.5km/s and upper sediments shows 1.0 to 2.0km/s based on the first arrival analysis of seismic data and the processed section shows clear reflection corresponding to this velocity contrast. More detailed analysis of S wave and data of another line are still underway but further application of seismic refraction and reflection method will be effective to solve the mechanism of such large landslides combining with other geological, seismological and geotechnical data. |
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