抄録
We conducted pit excavation study across the northernmost section of the surface rupture associated with the Mw=6.9 14 June 2008 Iwate-Miyagi Nairiku earthquake to test our hypothesis that the surface break occurred along a pre-existing fault based on topographic observation. The 2.2-km-long northernmost trace of the surface rupture, characterized by a curvilinear trace with N-S-striking section with a west-side up thrust component to the south and NW-SE-striking section having a southwest-side up with significant sinistral slip components to the north, occurred in the westward or southwestward sloping mountain slope, forming the distinct uphillfacing scarps. We dug two pits across the scarp on the NW-SE-trending section where oblique slip with 0.9 m of southwest-side-up vertical and 1.7 m of sinistral offset occurred during the 2008 earthquake. Both pits exposed the steeply dipping rupture zone associated with the 2008 earthquake, which is typical of the strike-slip faulting fracture zone. On the basis of structural and stratigraphic features, including residual stratal deformation after restoration of the 2008 slip, thick accumulation of colluvial and fluvial deposits on upstream side and presence of buried palaeosol mantling over paleo-scarp, we identified two possible events predates 2008, both of which accompanied with uphill-facing scarps similar to the 2008 earthquake. Results of tephra analysis and radiocarbon dating of the strata show that the growth of uphill-facing scarp occurred in the age before 9.5-9.3 ka and another movement might have occurred possibly in the age ranging from 9.5-9.3 ka to 6 ka. Significant erosion in the steep mountain slope during the last glacial period together with long recurrence interval of the earthquakes might result in weak topographic relief on the fault trace in our study area.
