Post-earthquake airborne LiDAR DEM and field investigation reveal the details of the northernmost trace of previously reported surface rupture zone associated with the 2008 Iwate-Miyagi Nairiku earthquake, which displaced the southern slopes of the densely forested Mt. Kunimi, Oshu City, Iwate Prefecture. The LiDAR DEM allows us to unveil the distribution and slip sense of the fresh ruptures that occurred on the mountainous area. Field observation and in-situ digital surveying show that the rupture is characterized by i) curvilinear traces composed of the N-S-trending east-facing scarp with no lateral offset to the south and the NW-SE-trending northeast-facing scarp with sinistral offset to the north, ii) the uphill-facing flexure scarps that move up the southwestward-sloping mountain block toward the upslope direction with ＜1.0 m vertical offsets, iii) shortening deformation manifested by intrusion of trees on downthrown side into the upthrown side, and iv) continuous trace extending over 2 km in length. These features suggest that the rupture is a west dipping reverse fault accommodating W-E to WNW-ESE trending compression, which is consistent with several proposed source fault models inferred from seismological and geodetical studies. The location of the surface rupture corresponds to the zone of abrupt change in displacement gradient illuminated by SAR analyses. Furthermore, the large slippage at the northernmost part of the 2008 rupture zone well corresponds to the location of one of large slip patches on the causative fault as derived from waveform inversion. Together with the detail topographic measurements and trench excavations, we identified evidence for the past faulting behavior suggestive of the same slip sense along the rupture zone. We thus conclude that this portion of the surface rupture is tectonic origin reflecting the westdipping seismogenic reverse faulting.
In this paper, we describe distribution and activity of the Nanshozan active fault group (F1, F2 fault) in detail, based on geomorphic feature and surface geology (Plio-Pleistocene Siwa Formation), including the active reverse fault passing through the first outcrop of the flexure (F3 fault). The Miocene strata have been thrust over the Siwa Formation along the mountain front (F1 fault). The F2 fault deforms fluvial terraces in the footwall of the F1 fault. Deformations of fluvial terraces and arrangement of valley spread of alluvial terraces (L2 surface) provide that the F3 fault runs through east side of the hills on the footwall of the F2 fault. Distribution of fluvial terraces indicates that the F1 fault has been inactive since the middle Pleistocene (H surface). Structure of the Siwa Formation indicates that the F2 fault has been formed since the middle Pleistocene (upper Siwa Formation). Studies of the surface folding and faulting of the fluvial terraces indicate a late Pleistocene vertical offset rate of 0.3 m/k.y. at the F2 fault and 0.05 m/k.y. at the F3 fault. Comparison of vertical offset rates of fluvial terraces provides that the primary fault activity is on the F2 fault.
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.