抄録
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.
