Matsurube-Ohashi Rockslide Triggered by the 2008 Iwate-Miyagi Nairiku Earthquake and its Geological Precursors

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  • 2008年岩手・宮城内陸地震による祭畤大橋右岸の初生岩盤地すべりと前駆現象
  • 2008年岩手・宮城内陸地震による祭時大橋右岸の初生岩盤地すべりと前駆現象
  • 2008ネン イワテ ・ ミヤギ ナイリク ジシン ニ ヨル サイジ オオハシ ウガン ノ ショセイ ガンバン ジスベリ ト ゼンク ゲンショウ

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Abstract

The 2008 Iwate-Miyagi Nairiku Earthquake recorded extremely-high accelerated velocity of resultant component 4,022 gal and unprecedented vertical surface acceleration of nearly four times gravity, simultaneously. On the occasion of this earthquake, some earthquake-surface-faults discontinuously emerged in the NE direction, and large number and various types of landslide including big rockslides extensively occurred. Especially, collapse of the R342 Bridge Matsurube-ohashi due to the rock-slide attracted a good deal of public attention, and become a big issue among specialists related to geotechnical and civil engineering, because both of the abutment rock looked intact even after the detail geotechnical investigations for clearing up the cause. Civil engineering department of Iwate Prefectural Office, therefore, made some detail investigations including drilling tests, and organized the investigation committee constituent of learned specialists both of bridge designing and civil engineering. JSEG also organized the investigation party for the earthquake disaster, including one of the authors Nozaki, and have reported the result including this case to the journal. After that, the authors who were members of “the 1st JSEG research subcommittee for initiation of landslides and their assessment” made again more detail field reconnaissance. Based on all of these data and informations, we conclude that this case was due to the primary rockslide, which was almost laterally dislocated, triggered by the earthquake with “trampoline effect” (Aoi et al., 2008), and support the idea of “kami-sumo model” proposed by Takami et al. (2010). Our more important conclusion is, however, the slid rock mass had have some “scars” before this event, that is, it had been already in “a transitional stage of landsliding”.

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