上町断層帯の最新活動と河内平野の地形環境変化

抄録

 The Uemachi fault zone extends north to northeast for about 51 km across metropolitan Osaka, southwest Japan. We conducted paleseismological surveys along the fault zone to estimate the timing of recent faulting events. We identified two reverse faulting events in the northern section of the fault zone, including the latest event, which occurred after ca. 2400 yr BP. In the southern section, the timing of the most recent event is estimated to be ca. 2200–2300 yr BP or later. The timing of the latest events in the northern and southern sections is coincident, implying that the most recent event was a multi-segment rupture through the whole Uemachi fault zone. We also examined geomorphic changes around the fault zone resulting from the most recent faulting event, based on previously reported archaeological and geological studies. As a result, we found: 1) coastal uplift occurred in the Osaka Castle Town archaeological site on the hanging wall side of the fault zone at the end of the middle Yayoi period; and, 2) numerous archaeological sites around the old Lake Kawachi were abruptly submerged at the end of the middle Yayoi period, and the submerged area had not been in residence again until around the final stage of the Yayoi period. These major, simultaneous geomorphic changes can be explained only by crustal movements resulting from a faulting event in the Uemachi fault zone. Thus, the timing of the most recent faulting event is estimated to be at the end of the middle Yayoi period. To evaluate the total risk due to active faulting, we have to understand not only tectonic landforms and coseismic surface ruptures, but also related regional abrupt geomorphic changes such as coseismic broader submergence in the Kawachi Plain from the perspectives of tectonic geomorphology and paleoseismology.