新潟県小千谷地域の活褶曲 : 約30年間の水準測量結果

概要

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Ojiya city and its surrounding regions in Niigata Prefecture, central Japan, are located in an active tectonic zone along the eastern margin of the Japan Sea, and where Quaternary folding activity is at the highest level in Japan. To elucidate the present-time mode of folding and its activity, levelling surveys were conducted at intervals of two years across the active fold zone, from 1968 until 1998. The data on vertical movements obtained by this levelling study revealed very active folding even at the present day. The uplifting rate was 6mm/yr at maximum during the period from 1978 to 1986. The rate decreased to 1～3mm/yr during the other period. The vertical movement profile shows steep folding. With this in mind, a pure reverse creep faulting model is applied instead of folding. An optimal fault model is obtained by a grid search technique for various values of fault parameters. The final solution is as follows: depth of the top of the fault is 1km, dip angle 65 degrees, width of the fault 2.5km, fault length 4km, and dislocation displacement 1.89m over 28 years. The east-west horizontal shortning rate is 2.6cm/yr on average during 28 year period, which is roughly comparable with the horizontal ground movements obtained by GPS measurements.
2008-12-22