長野市街地における善光寺地震断層の水準測量

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長野市街地を走る善光寺地震断層を横断する4本の路線からなる46点の稠密な水準網を張り,1986年3月と1987年3月の2回,精密水準測量を実施した.往復差の統計から,水準点の各区間の水準変動の偶然誤差(標準偏差)は0.3mmである.3つの環の閉合差から求めた累積誤差は,1点当り0.1mm以下であった.1年間の水温変動は水準網全体に対して,北下り南上りの傾動を示す.その大きさは,2.5～2.6×10-6rad/yearであって,松代群発地震直後のこの地域の傾向とは逆のセンスである.水準点近傍の環境や直下の地盤の影響と思われる局所的な異常を除けば,断層を境とした明瞭な西上り東落ちの変動は見い出せなかった.Precise leveling has been conducted across the Zenkoji Earthquake Fault in the central part of the city of Nagano in central Honshu, Japan. The bench marks, totaling 46, form four routes across the fault and three circuits. The first leveling was carried out in March, 1986 and the second in March, 1987. The standard deviation of accidental errors for the changes of relative heights is estimated to be 0.3 mm, with cummulative errors within 0.1 mm for each point. One year period leveling change shows tilting movement dipping to the north, with the slope of (2.5-2.6)×1O-6 rad/year. This region had once been in the field of tilt dipping nearly south in 1969-1977. The recent trend revealed by the leveling survey is nearly opposite. Any significant level changes across the fault cannot be recognized except for one route, where the eastern side of the fault has subsided up to 1.4 mm in comparison with the opposite side. Even along this route, the vertical displacement of 2-3 cm, with the eastern down, discovered on the pavement in front of the entrance of the prefectural office, just near the fault, cannot be attributed to crustal movement, because the level change was less than 0.4 mm per year for the span of 100 m across the fault, which would have yielded an offset of eight milimeters at most during the past 20 years.
東京大学地震研究所,Earthquake Research Institute, University of Tokyo,地震研究所の論文
1988-03-31

長野市街地における善光寺地震断層の水準測量

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