鈴木 康弘

文部科学省が地震動予測地図を作成するに当たり、私もこれに加わってきたが、その過程で明らかになった活断層評価の問題点を提起した。地震規模を大きめに、活動間隔および地震発生確率を低めに誘導してしまう根本的な問題を提起した。この視点は先見性が高く、数年後には共通認識になり、次期改訂の大きな課題の一つとなり、改善に向けた作業にも中心的に関わることとなった。

A remarkable surface rupture appeared in the 1999 Chichi earthquake, in central Taiwan. The nature and location of the earthquake fault was studied in detail immediately after the earthquake (e.g., Central Geological Survey, Taiwan, 2000). Its location to the pre-existing active fault trace, however, was unknown. We wish to establish a location relationship between the earthquake fault and the pre-existing active faults which are mapped from photo interpretation at a scale of 1 : 20, 000, taken in 1970's, supplemented by field observation. The identified active faults are divided into four types from I to IV, depending on their certainty as active faults as well as their location accuracy. A Type I fault is where the active fault is definite and location is certain, II is also an active fault, but with a little uncertainty as to exact location due to subsequent erosion of the fault sacrp, and also because of sedimentation on the foot-wall, and III is a concealed fault beneath the younger sediment. Type IV appeared as a lineament without any clear evidence of deformed morphology. After mapping these active faults, we added the location of our observation to the 1999 surface rupture and GPS sites for measuring the earthquake fault using CGS map (2000).<BR>We present eight areas to show the exact relationship between active fault trace and earthquake fault trace and summarized them into Fig. 10. We concluded that most (ca. more than 80%) of the earthquake fault trace occurred exactly on the active fault of Type I and II. The earthqauke fault often appeard even on lineament of Type IV, implying that this lineament should be mapped for the acive fault map. On the young alluvial lowland where it is too young to record past faulting, the earthquake fault still appears on the probable extension of known active fault trace. The earthquake fault sometimes jumps from one fault to another where two or three active fault traces are recognized. Although we can not explain the reason for such a jumping, the earthquake fault still appears on one of the known faults. Therefore, repeated faulting activity during the late Quaternary on the same trace was confirmed for the Chelugmu Fault. This implies the detailed mapping of many other active faults in Taiwan, including Type III and IV, is essential for the understanding of future rupture locations.

While large-scale active fault maps are being published to present important informations for the earthquake disaster mitigation, it is necessary to estimate the accuracy of locality information shown on the maps. When the locations are digitized based on topographical maps of 1: 25,000, it is impossible to eliminate an error of about 20m which came from the accuracy of the based maps. In order to improve the accuracy of the data, we propose here a methodology for making digital locality information by use of photogrammetrical system.<BR>As it is necessary to observe original landforms deformed prior to land modifications, aerial photographs taken in 1940s' by the U. S. Armed Forces are often used for active fault mapping. However, the photographs have been much distorted, and the mapping by the photographs are often problematic and some countermeasures should be considered carefully. By utilizing the method presented here, the error of locality information can be minified within 5-7m horizontally.

活断層の情報として,最も基本的な情報である地表分布形態のうち,断層の分岐形態,横ずれ断層の縦ずれ変位分布パターン,断層線に沿った変位の累積過程は,断層の破壊過程に密接に関連し,地震動予測のために不可欠な情報源となることを予測する.また,これまで地震発生前には不可能であった破壊開始点や破壊伝播方向の予測やセグメント区分および活断層の発生に関して,新たな視点を提示する.

We have mapped a series of north-trending faults along the northeastern coast of Sakhalin, Russia, based on aerial photograph interpretation and field observations. These faults include the Upper Pil&#039;tun fault which ruptured during the 1995, M&lt;SUB&gt;w&lt;/SUB&gt; 7.0 Neftegorsk earthquake, and the Ekhabi-Pil&#039;tun and Garomay faults east of the surface ruptures. The coseismic displacement associated with the 1995 earthquake shows that the Upper Pil&#039;tun fault is a right-lateral strike-slip fault. Right-lateral stream offsets as much as 80m demonstrate that similar earthquakes have occurred repeatedly along the fault in the late Quaternary. The fault extends for 10 km further to the south beyond the southern termination of the 1995 surface breaks, which is consistent with the coseismic ground deformation detected by SAR interferometry. The Ekhabi-Pil&#039;tun fault trends N10&amp;deg; W to N10&amp;deg;E with a series of fault scarps down-to-the-west across low-relief hilly terrain eroded byperiglacial processes. Right-laterally offset streams and lake shores suggest thatthe Ekhabi-Pil&#039;tun fault is also a right-lateral strike-slip fault. A 3-m-deep trench across the fault north of the Sabo River contains geologic evidence of two episodes of pre -historic surface faulting. The most recent event occurred after 4000 yr B.P. and the penultimate earthquake occurred at about 6200 yr B.P. The interval between the events is greater than 2000 years, which is significantly longer than that obtained for the Upper Pil&#039;tunfault by previous trench excavations.

Trench excavation across the surface rupture of the 1995 Hyogoken-nambu Earthquake revealed at least two and possibly three surface-faulting events including the 1995 earthquake on the Nojima Fault during the past 2000 years. A pre-1995 event probably occurred about 400 years ago and this event may be correlated to the Keicho Earthquake of 1596. An event before the pre-1995 event occurred about 2000 years ago.

東北日本に分布する構造性盆地には,広い沖積面が分布するものと,盆地床の大部分が開析の進んだ丘陵地となっているものの2つのタイプがある.本研究では,両タイプの盆地が隣接する,新庄盆地から山形盆地にかけての地域を取り上げ,活構造の分布および変位の様式を明らかにし,盆地床の形態に相違が生ずる原因とメカニズムを考察した.<br> 新庄盆地は,鮮新世にすでに沈降域であったため,盆地内堆積物は非常に厚い.そのため,低角逆断層が山地寄りから盆地内へ次々に移動発生し,それらの上盤側は隆起域へと転じ丘陵地化した。それに対し,山形盆地では第四紀後半に初めて盆地域となったため,盆地内堆積物は未だ低角逆断層の移動発生にとって充分な厚さを有していない.そのため,断層運動は盆地の縁辺部に集中し,広い沖積面を形成している. 両盆地における断層運動様式の相違とその変遷から,低角逆断層の移動発生と共に盆地床の形態が変化していく盆地発達過程のモデルが提示された.