抄録
The Neodani Fault extending approximately 100 km NW-SE in central Honshu is one of the well-known active faults in Japan. While under road construction at a site of Neo-Osso, Motosu City, Gifu Prefecture, part of an outcrop of the fault was discovered, where a clear vertical displacement was also identified. On the NE side of the outcrop, a horizontal boundary between a basement and terrace deposits is clearly visible at a height of 6.5 m. On the SW side of the crop, however, thick terrace deposits being only visible cause the corresponding boundary to be hardly confirmed.
This study tried to explore a boundary hidden by the terrace deposits, for which a spatial autocorrelation (SPAC) method, a kind of microtremor survey methods, was applied as an exploration technique suitable for this case. Following the procedure of the SPAC method, observations of microtremors were made at four sites selected around the outcrop. For each site phase velocities of the fundamental mode Rayleigh wave were derived as a function of frequency and inverted to a subsurface structure, i.e., an S-wave velocity structure. To see a general view of the studied area, all the S-wave velocity structures derived were put together in a 2-dimensional cross-section along the outcrop and also referred to geological situations observed at ground surfaces. Consequently the depth of the boundary hidden by terrace deposits in question could be estimated at about 4.5 m at a site just near the SW side of the fault.
Examining the hidden boundary estimated on the SW side together with the corresponding boundary observed on the NE side, the vertical displacement of the fault was approximately estimated at 11 m in total, which could be taken as an accumulated displacement along the fault after the formation of the late Pleistocene terrace. The vertical displacement thus estimated can be also converted to an average vertical slipping rate, which is one digit smaller than an average lateral slipping rate of approximately 2 m in every 1000 years.
