要　旨　関東山地北東縁部，比企丘陵中央部に分布する下部中新統七郷層中の小断層解析を行った．この結果，本地域の七郷層中に発達する共役断層は，七郷層が堆積・固結した後に，東北東一西南西ないし北東一南西方向の圧縮応力場で形成されたと推定される．この圧縮応力と，七郷層中の摺曲構造や横断断層の形成などとの関係が注目される．ただし，この種の小断層は，地震が起きた時に発生した地震波から二次的に生じた可能性もあるので，広域応力場の復元にあたっては，より解析地点を増やすなどして検討する必要がある．

キーワード：関東山地北東縁部，比企丘陵，下部中新統，七郷層，小断層解析

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An Analytical Study on the Minor Faults in the Miocene Nanasato Formation, in the Northeastern Periphery of the Kanto Mountains, Central Japan.

Takeshi HOMMA
Saitama Museum of Natural History. Nagatoro, Chichibu-gun, Saitama 369-1305

Abstract
　　An analytical study on the minor faults in the Miocene Nanasato Formation in the northeastern periphery of the Kanto Mountains, suggests that these minor faults were of a strike-slip type formed under the compressional stress field in the direction of ENE〜NE - WSW〜SW after the deposition and subsequent induration of the Nanasato Formation. It will be necessary to make a closer investigation on the relation between the compressional stress field and formation of folded structures or transverse faults in the Nanasato Formation.
　　However, such minor faults could be associated with the local seismic wave generated by an earthquake. So it is necessary to extend the analytical site for clarifying the regional stress field.