The M_W 6.9 2008 Iwate-Miyagi Nairiku earthquake resulted in discontinuous surface ruptures with a total length of ca. 20 km along the eastern margin of aftershock zone. These ruptures occurred where no active faults were previously mapped. They were generally accompanied with small displacement (throw ≤ 50 cm), so only visible in cultural features such as rice paddies and roads. Exceptionally large, meter-order-slipping breaks were found in the densely forested mountainous terrain at the southern part of the entire rupture zone. We performed microtopographic examination using airborne LiDAR-derived high-accuracy and high-resolution topographic data acquired after the earthquake, in order to assess whether the surface ruptures with such small slip and those in densely vegetated terrain could be detected and to search active fault traces along the ruptures. One-metergridded digital elevation model (DEM) interpolated from the LiDAR data that removed effects of vegetation and buildings displays highly-detailed bare earth surface expression. By artificially enhancing colors, changing illumination angles and vertical exaggerations of the DEM data, we successfully extract fine-scale topographic features associated with the surface rupture and those with the pre-existing active fault traces along the 2008 ruptures at several locations. The LiDAR-derived high-resolution topography will expand our capability for identifying active faults with weak geomorphic expression and those hidden by dense vegetation cover.

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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.

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