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Sanga, Tomoji*

Crustal deformation until the present in the Northern Hokkaido including Horonobe Town, concomitant changes of geological environments and arising earthquake have been nvestigated as the Horonobe underground research laboratory project. As a part of this project, two surveys were performed to elucidate fracture systems in an area including the Horonobe Town:(1)}Interpretation of lineament, Filed survey for fault and fracture(2)}Investigation for eruption history by the RIPL method (This survey is intended for the terrace deposit distributing around the Omagari Fault.)The lineaments, which was recognized by the interpretation and automatically extracted, have common characteristics on the point of direction and density. In the Koetoi and Wakkanai Formation, the peculiar characteristic of fracture is similar. But, the density of lineament in the latter was almost twice as much as that in the former. In the drill hole near the Omagari Fault, there is a group of fractures crossing the bedding plane. By the comparison with the data of this group, lineaments show similar characteristics on the point of direction, density and dip. So, it is considered that the lineaments reflect this group of actures. When the changes on the peculiar characteristic of lineaments and fractures in drill hole are examined, the density of fractures in the hanging side is higher than that in the footwall. In the west side of the fault, the direction of lineament greatly changes on the border almost 400m away from the fault. So, it is estimated that the range of fluence of the fault is up to this border.The investigation of eruption history was performed at the seven points. As a result of this investigation, it became clear that MIS5c and MIS5e in the Atras of Quaternary Marine Terraces (2001) correspond MIS7 and MIS9 respectively and the velocity of uplift decreases to 1/3 compared with that onsidered so far.