1998 Volume 9 Pages 57-72
Surface fault ruptures appeared at the 1995 Hyogo-ken Nanbu(Kobe) Earthquake at Ogura, Hokudan Town, northern Awaji Island, Hyogo Prefecture. A portion of the ruptures is preserved in the Nojima Fault Preservation Pavilion, which extends about 140 meters along the fault. At the southern end of the preserved earthquake fault, a trench was excavated to display the subsurface features of the fault. In this paper, we describe the detailed geology and structures on the trench walls and bottom, and present the results of pollen analysis of the sediments for age determination. The strata at the trench site are composed mainly of alternating beds of clay and sand. They are correlated to the Lowest to Lower Osaka Group deposited during 2 to 1.5 Ma, based on a pollen assemblage at the trench site and reported age of a volcanic ash layer north of the site .These beds are overlain by slope deposits and artifici aflill. In the trench, the earthquake fault steeply dips eastward, and is accompanied by a shear zone less than 1 meter wide within the Osaka Group. No geologic evidence for the right-lateral displacement along the fault is observable in the trench except for a remarkable difference in the thickness of the artifici aflil lon both sides of the fault .This differenc ecan be explained by the right-lateral displacement observed on the ground surface. Two secondary high-angle reverse faults dipping west appear in the hanging wall side of the main fault. A small secondary thrust exists in the foot wall side, in association with reverse faulting of the earthquake fault. Many sand dikes parallel to the earthquake fault can be seen on the trench bottom, indicating the paleo-liquefaction.