This paper aims to clarity the relationship between fault segments and fault geometry at the southern end of the Uwandaira fault group convexing along the western margin of the Kitakami lowlands, based on fault parameters and fault history. Neogene strata thrust over the terrace deposits along the high-angle fault plane dipping more than 48 degrees. Several angular unconformities in the fault outcrop and the displacement of the post-last glacial L2 terrace yield at least four paleoseismic events since the last glacial stage. The average vertical slip rate is calculated to be ca. 0.3±0.1 m/ky based on the displacements and ages of terrace deposits. The net displacement per event was estimated to be 2.4–3.4 m based on the plunge of slickenlines. The values of the average vertical slip rate and the net event displacement are equivalent to those of the central part of the Uwandaira fault group. These results imply that a geometric form of active fault trace does not always correspond to a fault segment.
The Usu Volcano erupted and the destructive pyroclastic flow and surge struck the Abuta settlement in 1822. The number who perished has not been confirmed, and there is some doubt about why only some of the settlement dwellers died. The purpose of this study was to clarify the number that perished and investigate why many settlement dwellers survived in spite of the eruption of the Usu Volcano in Hokkaido, northern Japan, in 1822. The authors confirmed the deceased whose names and/or kin were certified through historical records. Six of those who perished were Wajin (ancient Japanese), and 72 were Ainu. The genuine death rate (number of dead/number of those who encountered the pyroclastic flow and surge) was 85.7% (six of seven) for the Wajin and 98.6% (72 of 73) for the Ainu. The population of Ainu at the Abuta settlement was about 338 in 1822. Only 73 of those 338 were estimated to have been at the Abuta settlement and encountered the pyroclastic flow and surge of the eruption on March 23, 1822. Many of the Ainu were thought to be in the upper drainage of the Shiribetsu River for their annual seasonal migration to fish for salmon, a staple food from autumn to spring. The upper Shiribetsu River area was about 30–50 km from the Abuta settlement. Many of the Ainu thus survived because of that seasonal migration, which is thought to have been an independent, autonomous activity, even though the destructive pyroclastic flow and surge from the Usu Volcano struck the Abuta settlement.