We performed detailed mapping of the Uemachi fault zone, Osaka, central Japan. The Uemachi fault zone is composed of east-dipping reverse faults, and it extends for about 51 km with discontinuous flexural scarps trending NS to NNE-SSW strike in almost central part of the Osaka Quaternary basin. The northern section of the fault zone cuts through the city central of Osaka, therefore, the fault zone is well-known as one of the most hazardous active fault zones in Japan. The fault zone around the city central district had been considered as blind faults covered with thick Holocene sediments. In order to re-examine the detailed location of the fault zone, we applied extensive 2-m-DEM analyses combining with air-photo interpretation along the entire fault zone, in particular, where the geomorphological surface is obscured by densely-populated buildings and constructions. As a result, we identified the flexure scarp on the Holocene geomorphic surfaces such as fan delta, alluvial lowland and fluvial terrace distributed near the fault zone, in contrast with the pre-existing consideration of concealed faults. Re-interpretation of seismic reflection surveys and geologic cross sections in previous reports support the existence of flexure scarps as tectonic geomorphological evidence. At the west of the Uemachi upland, that is uplifted terrace surface at the hanging-wall side of the northern fault zone, we identified that the Sakuragawa flexure forms flexure scarp near the ground surface. In addition, the surface distribution and orientation of the Sakuragawa flexure exhibit into an arch connecting to the Suminoe flexure to the south. This fault geometry suggests that the Sakuragawa flexure and the Suminoe flexure are continuously linked with each other, and they are merged to the main Uemachi fault at depth. Furthermore, we re-examined south western continuation of the Uemachi fault zone mostly along the coast line of the Osaka Bay. The south-western termination of the fault is located near Hakotsukuri in Hannan district. Along this fault section, flexure scarp on alluvial fan surface, backtilting and warping on terrace surface and uplifted beach ridge are recognized as affected by active tectonic movement. The pre-existing seismic reflection survey and geologic section revealed by bore hole data support the existence of the fault section near the coast line. In summary, these data on detailed surface geometry of the Uemachi fault zone suggest that the fault zone is composed of two geometric segments that are able to produce individual earthquakes and multi-segment earthquake. More data on paleoseismic behavior, slip rate on the fault and fault geometry at depth are necessary to be assessed for seismic hazard and strong ground motion prediction.
A distinct element method analysis was carried out to examine the deformation of unconsolidated overburden subjected to a high dipping oblique normal basement fault displacement. About 4 million spherical particles were used to model the overburden overlying a basement fault which has a dip angle of 60 degrees. Effect of normal fault component and strike slip fault component on the deformation of the overburden was investigated. The deformation was affected by both normal fault and strike slip fault component of the basement fault. Very steep high strain zones developed in the middle and lower part of the overburden and while they were formed on the footwall side of the basement fault in the case of a large normal fault component, they were formed on the hanging wall side of the basement fault in the case of a large strike slip fault component. Near the surface, in the case of a large normal fault component, large deformation mainly due to the horizontal compression was observed on the hanging wall side and it was followed by high strain zones on the foot wall side caused mainly by the horizontal extension. En echelon high strain zones were observed on the hanging wall side of the basement fault in the case where a strike slip fault component was large mainly because of the strike slip fault movement and partly because of the horizontal compression caused by the normal fault movement of the basement fault. When a normal fault component and a strike slip fault component were of the same magnitude, high strain zones almost parallel to the basement fault developed on both sides of the basement fault trace and between them high strain zones striking obliquely to the basement fault trace were formed. The angle at which high strain zones in the overburden strike to the basement fault trace was very small in the case of a large normal fault component and the result showed the possibility of a right step pattern if normal fault displacement with a slight right lateral strike slip fault component was applied to the overburden.
Reverse and conjugate strike-slip faults are densely and complicatedly distributed around the Tsuruga Plain, a high strain rate region in central Japan. In this study, we revealed the existence of the Ikemi fault distributed in the northeastern part of the Tsuruga Plain and estimated its property and activity using drilling and percussion cores excavated across the estimated surface trace of the Uchi-ikemi lineament (total extension is over 3 km) in a waste-filled valley called the Uchi-ikemi. We analyzed the sedimentary facies, tephras and radiocarbon dating for ages and established stratigraphic correlation among the cores. As a result, we clarified the cumulative folded structure in the sediments filling the Uchi-ikemi. Although no clear fracture zone was recognized, we consider the folded structure as the flexure due to movement of the Ikemi fault because the structure was recognized in the Uchi-ikemi and the Naka-ikemi along the Ikemi lineament. Based on eruption ages of tephras and radiocarbon ages, at least four times of activity (>95, 38-30, 19-7, <7 ka) are estimated for the Ikemi fault. Using the eruptive age and amounts of offset of correlation layers, the average vertical slip rate of the Ikemi fault is estimated to be 0.21 mm/yr.
Digital elevation model (DEM) have been indispensable data for identifying fault traces and measuring the amount of recent faulting. However, some of DEM produced by Air-borne / Terrestrial Light Detection and Ranging (LiDAR) are difficult to observe the tectonic geomorphology because of artificial modification, or because its density is too sparse. In this paper, we attempt to construct DSM by using SfM (Structure from Motion) - MVS (Multi-Video Stereo) with aerial photographs. We used old aerial photographs before artificially modified in Awa city, as well as aerial photographs by the 7-m-high pole camera (Hi-view) in Shikokuchuo city along the Median Tectonic Line active fault system in Shikoku, Southeast Japan.
As a result, the 0.5m-mesh DSM and 0.05m-mesh DSM were generated from old aerial photographs scaled 1 to 8,000 and Hi-view photographs by compact digital camera, respectively. The relative height of the fault scarp based on the 0.5m-mesh DSM is almost same as that based on the 5m-mesh DEM of Geospatial Information Authority of Japan and 1m-mesh DEM of aerial photograph survey company. On the other hand, the shape of dense points based on the 0.05m-mesh DSM along the line is quite similar to the topographic profile based on the measurement by conventional total station method. These cases illustrate that SfM- MVS photogrammetry with Old / Hi-view aerial photography is quite useful new method for studying active tectonic geomorphology.