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[SSS32-09] Vertical displacement in Naruko Volcano area after the 2011 Tohoku earthquake deduced from precise leveling survey
Keywords:Great East Japan Earthquake, Naruko caldera, precise leveling survey, subsidence
To detect the subsequent displacement, we made the third precise leveling survey on 27-31 August 2015 on the same leveling route (benchmark number 047-064, 066, 068, 070, 072, 074 from east-west; hereafter indicated as BM64, BM66, etc.). We used bar-code leveling rods (Leica GPCL3) and an electronic digital level (Leica DNA03). We conducted round-trip survey between each benchmarks, and all residual errors fell within the acceptable range of the first-order leveling.
Relative to August 2013, BM66, BM68, BM72, and BM74 subsided 7.6 mm, 14.6 mm, 31.8 mm and 36.2 mm, respectively, against the eastern end BM64. These indicate westward-growing subsidence has continued (or probably is still continuing) along the survey route after August 2013, although the deformation rates have decelerated. The only exception is BM70; subsidence had changed into 4.6 mm uplift. This is the only uplift we have detected on this survey route since the 2011 Great Japan Earthquake.
Postseismic vertical displacements detected by GPS array, equipped semi-parallel to National Route 47 by Tohoku University, indicate that the surveyed area corresponds the transition zone from eastern uplift to western subsidence. Relative to the GPS station 0174 whose longitude is close to that of BM64, the next two western GPS stations collateral to the survey area have been subsiding, which is in harmony with our survey results. On the contrary the uplift such as seen at BM70 is undetected.
Leveling surveys in 1969 and 2009 by the Geographical Survey Institute indicate that BM70 subsided against BM68 while BM72 and 74 uplifted during this period. According to Prima and Yoshida (2010) and Ogawa et al. (2014), the eastern edge of the rim of Naruko caldera crosses between BM68 and BM70. The unique behavior of BM70 may be caused by such local geologic structure.