| 講演要旨(和文) | | 平成20年、岩手・宮城内陸地震による強い地震動により、宮城県栗原市荒砥沢地区においても、大規模な地滑りが発生した。この地域の北西約10kmには、第四紀の火山である栗駒山があり、この地域の地表は、火山噴出物によって覆われており、その下部には、湖成の水平な堆積層が分布している。さらに、その下位には第三紀の堆積層や火山岩類が分布しており、地すべりの面は、ほぼ水平で軟弱な湖成層の中にあると考えられている。この面を検出する一つの方法として、P/S波による反射法及び屈折法地震探査を試みた。調査は、複雑な地形・地質の地域であり、現地復旧工事の最中でもあり、周辺の短い測線上で調査を試みたが、第三紀層基盤面は明瞭に捉えられ、その上部の堆積層の詳細分布についても、把握できる可能性が示唆され、今後の本格的な調査の立案に有意であった。 |
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| | 講演要旨(英文) | | 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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