鈴木 康弘

2004年新潟県中越地震は，1990年代以降に存在が明らかになった小平尾断層と六日町盆地西縁断層の北部が震源となり，断層線上に地表地震断層が出現した．しかし，活断層分布の情報が十分周知されていなかったことと，地表地震断層の変位量が大きくなかったことから，このような認識が徹底されていない．本稿では，地表地震断層の認定根拠を変動地形学の立場から明確にし，地表地震断層の出現が確実であることを最新の研究成果に基づいて解説する．防災上，「地震はどこでも起きる」ことを念頭に置く必要がある反面，地域ハザードを適切に評価する必要があり，変動地形学はこのことに大きく貢献している．

The Ulsan fault extends for 50 km along the NNW-SSE direction in the southeastern part of the Korean Peninsula; this is one of the most important active faults in Korea. Its paleoseismicity has recently attracted considerable attention. With the support of KOSEF (Korean Science and Engineering Foundation), excavation studies of this fault were conducted in 1999 as a part of the Korea-Japan cooperative research at Kalgok-ri in Kyongju city. The results obtained are summarized as follows. (1) The Ulsan fault plane has an eastward dip of approximately 30 degrees and exhibits typical reverse faulting. (2) It was reactivated three times in the past 30,000 years, in particular, twice after the age of AT tephra (approximately 25,000 years BP). (3) A vertical displacement of approximately 0.8 m occurred during the fault event, and the amount of net slip along the fault plane is calculated to be 1.6 m.

奇しくも阪神・淡路大震災から10年目に起きた新潟県中越地震は、大規模な山地災害を伴い、地方都市や山村において複雑な大災害を引き起こした。この震災を教訓にして今後の地震防災を模索するためには、地域の地震ハザードや自然環境、産業構造などを俯瞰的・地理学的に考察する視点が重要であることを、各分野の地理学関係者の提言から明らかにしたい。

This paper describes the results of a trenching study at the Tsautun area across the Chelungpu Fault that reptured during the 1999 Chichi earthquake. The deformation by the 1999 earthquake is recorded on both north and south walls of the trench as an eastern-side-up flexural scarp, which is 1.5 m in vertical slip, associated with an overriding of hanging wall on foot wall, 1-1.5 m. Complex folding is appeared on the south wall, where fine overbank deposit is thicker than the north wall. Faulting prior to the 1999 event is not found at this site, but is estimated to be older than 300 to 500 yr BP, judging from the ¹⁴C ages of the oldest age of deformed sediment.

The Ulsan fault system situated in the southeastern part of Korean Peninsula marks the western range front of the Mt. Tohan (745.1 m) to the Sandae Peak (629.1 m) with the remarkable Ulsan fault scarp. This system extends with a N-S trend for a distance of more than 40 km. The NNE-SSW trending 200 km-long Yangsan fault and this Ulsan fault are recognized as the most important active faults on the Korean Peninsula.<BR>From interpretations of aerial photographs and detailed topographic maps, and field surveys along the central part of the Ulsan fault system, the major results are summarized as follows : <BR>1. The Ulsan fault system extends in the direction of NNW-SSW to N-S and the trace is slightly sinuous (Fig. 1). The middle to higher river terrace surfaces on this system are displaced up thrown to the east side (Fig. 2). The streams and the ridges across this fault trace are not accompanied by systematic lateral displacement. The Ulsan fault system is a typical example of reverse faulting, considered from the behavior of fault outcrops (Figs. 3, 4, 5).<BR>2. The vertical displacements are about 5 m on the middle terrace, and about 15 m on the higher terrace from measurements of topographical cross-section across the fault scarplets. The cumulative vertical displacement is recognized during and after the formation of the terraces. The sense of vertical displacement of fluvial terraces coincides with the magnificent fault scarp. This mode of mountain-building movement has continued repeatedly at least since the middle of Quaternary.<BR>3. The terrace surfaces originated from the dissected fan surfaces in the late Quaternary. From geomorphic correlation, facies analysis, development of soil covering the terraces, and results of dating of humic material, etc., the ages of terrace surfaces are roughly estimated and the average slip rates of vertical displacement are measured to be about 0.1-0.08 mm/y.<BR>4. Fault outcrops are clearly observed along the Ulsan fault in this area. The shattered granite rocks thrust over the gravel deposits composing the middle terrace at the north bank of Sagok Pond, Malbang-ri, Wedong-eop, Kyeongju-gun, Kyeongsang-bukdo (Fig. 3-A, B). The fault strikes in the direction of N-S and dips eastward at an angle of 20-30°E. The shattered granite is in contact with the slightly weathered and deformed gravel at the northeast of Kaegok-ri settlement (Fig. 5).<BR>5. Liquefaction phenomena are observed within the upper horizon of the middle terrace deposits at the northwest of the Sagok Pond (Fig. 6).<BR>6. As the NNW-SSE (or N-S) trending Ulsan fault system is predominantly a reverse fault and NNE-SSW trending Yangsan fault system has been dislocated with a predominant right-lateral slip in the late Quaternary, it is estimated that this area is situated under the maximum horizontal stress field in the ENE-WSW (or E-W) direction.