Evidence for coseismic slip along a geologic formation-bounding fault associated with the 2008 Iwate-Miyagi-Nairiku earthquake:

Exposure along the Ubusume River in Gembi Town, Ichinoseki City, Iwate Prefecture, northern Japan

Abstract

We performed geologic reconnaissance to find the surface-rupturing fault associated with the 2008 Iwate-Miyagi Nairiku earthquake in the Neogene-Pleistocene bedrocks. A bank of the Ubusume River at Okayama, Genbi Town, Ichinoseki City exposes the Miocene Monji Formation which thrusts up on the younger Plio-Pleistocene Himematsu Formation with a fault contact. Several other reverse faults without any broad plastic deformation, suggesting brittle faulting after complete consolidation of the strata, are also exposed on the hanging wall of sandstone and mudstone in the Monji Formation. Amount of exposed thickness of the Himematsu Formation on the footwall implies that the cumulative vertical throw due to faulting would be more than ~10 m. From the viewpoints of the spatial relations with the 2008 breaks and freshness of the fault zone, we speculate that this formation-bounding fault is an “active fault” and might also have moved at the time of the 2008 earthquake.