To reveal the geologic structure and repeating behavior of the source fault associated with the 2008 Mw=6.9 Iwate-Miyagi Nairiku earthquake, we excavated a 51-m-deep borehole on the up-thrown side of the surface rupture at Okayama, Genbi Town, Ichinoseki City. The borehole penetrated 2.5-m thick young sediment (surface soil, loam, and gravel), and a sequence of horizontal or sub-horizontal sandstone, siltstone, conglomerate, and tuff breccia of the Miocene Monji Formation. The borehole sample extracted from a depth of 17 m exposed the 30°-dipping distinctive fault contact juxtaposing hanging wall tuff breccia and footwall siltstone. Furthermore, we found the identical stratigraphic sequence at the shallower part (2.5-5.6 m) and at the deeper part (38.5-41.9 m) bounded by the fault, which suggests 36 m of vertical separation of the same strata. Together with the data showing other parallel-running fault strands and widely deformed late Pleistocene terraces, our estimates would suggest a minimum cumulative slip of the entire fault zone along eastern bounds of the Ou Backbone Range.
