The MW 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.
Recent activity of the Chojahara fault of the Shinjo basin fault zone in Northeast Japan, is examined base on interpretation of aerial photographs, field survey, trenching survey and arrayed borings. At the Horiuchi site near the center of the basin, humus layers in the middle and upper parts of the deposits forming the L4 terrace were dated to 6.8-7.0 ka and 1-1.5 ka respectively by 14C dating. Upper part of the terrace deposits includes the Towada-a tephra (AD915). From these ages, the L4 terrace surface was formed nearly 1 ka. The vertical offset of the surface is about 1.6 m. The middle part of the L4 terrace deposits which was deformed (dragged) by reverse faulting, was overlain by the upper part of the deposits. At the Chojahara site, the L3 terrace surface was dated to 4.5-5.2 ka from 14C ages of the soil. Vertical offset of the L3 terrace surface is about 2.0 m. The latest event of the Chojahara fault is younger than 1 ka, because the fault dislocates the L4 terrace surface. The penultimate event was 4.5-5.2 ka (the age of L3 terrace surface) or older, and it was probably younger than 6.8-7.0 ka (age of the middle part of the L4 terrace deposits). The slip per event is estimated around 1.6-2.0 m from the vertical offsets of the L4 and L3 terrace surfaces.
Six active fault zones have been selected on the basis of the reports on the long-term evaluation of active faults published until 2008 by the Earthquake Research Committee, Headquarters of Earthquake Research Promotion (ERC/HERP); the paleoseismic activity data of these zones reveal three or more earthquake recurrence intervals. Using the maximum likelihood method, seven probability density functions of a renewal process model are compared in order to determine the function that best fit the paleoseismic activity data of these active fault zones. The exponential distribution model obtained by using the maximum likelihood method does not clearly reveal the earthquakes recurrence intervals. In contrast, the results obtained by using six other statistical models, i.e., Brownian passage time (BPT) distribution, lognormal distribution, gamma distribution, Weibull distribution, double-exponential distribution, and normal distribution, reveal the earthquake recurrence intervals. Thus, the new paleoseismic activity data of major active zones in Japan confirm the provisional conclusion of ERC/HERP, i.e., the exponential distribution does not clearly show the earthquake recurrence intervals. On the other hand, differences among the goodness of fit of the six models excluding the exponential distribution are small. In 2001, ERC/HERP stated that when renewal process model with the BPT distribution is applied to the data of the occurrence intervals of earthquakes in the inland active fault zones in Japan, the aperiodicity parameter of the distribution should be set to 0.24 as a value common to all active faults. The aperiodicity parameter obtained by applying the same method to the data of the six active fault zones is equal to 0.44. Although the aperiodicity parameters, obtained by using the maximum likelihood method, reported in the ERC/HERP’s report range between 0.17 and 0.29, those obtained in this study range between 0.09 and 0.66. Thus it is inappropriate to assume the same aperiodicity parameter for all the inland active fault zones in Japan.
We construct the subsurface fault structure using balanced cross section based on the observation of the stratal deformations around fault outcrops. These fault outcrops were found in the Shimosekita area on the active fault zone along the western margin of the Kitakami Lowland, northeast Japan. Pliocene-Pleistcene Mitusawagawa Formation thrusts over the terrace deposits beneath the small fault scarp on the lowland (F1 fault). Miocene to Pliocene strata thrust over Mitusawagawa Formation on the mountain front (F2 fault). Uplift width of the hanging wall of F1 fault was 30 meters. The hanging wall structure of F2 fault was kink fold with overturned forelimb. Our analysis with a balanced cross section suggests F1 fault with a detachment depth being 20 meters. On the basis of the comparison with the seismic reflection profile at the Ishidoriya area, we propose that a distance between the fault on the mountain front and the frontal fault on the terrace surface may be controlled by a depth to the detachment underneath.
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.