報告書番号

U88052

タイトル（和文）

根尾谷断層の活動性－岐阜県金原における断層の分布・性状と活動様式の解析－

タイトル（英文）

ACTIVITY OF THE NEODANI FAULT -CHARACTERISTICS AND ANALYSIS OF FAULT MOVEMENT AT KINBARA,GIFU PREFECTURE-

概要 （図表や脚注は「報告書全文」に掲載しております）

根尾谷断層について，主としてトレンチ調査により，その性状，活動性を明らかにした。調査地域の岐阜県本巣町金原周辺には中古生層が広く分布する。この基盤岩中の約５００ｍ区間内に，根尾谷断層を構成すると考えられる硝砕帯を数条確認した。トレンチユーは幅約１２ｍの断層破砕帯が確認され，その最も東側の粘土化帯に沿って，上位の堆積層に連なる断層が発生しており，東側の基盤岩が正断層のセンスで平均２４５ｃｍ低下している。基盤岩を覆う堆積層には断層による累積変位が認められ，その変位の状態から，堆積後少なくとも４回以上の活動が読みとれる。堆積層の１４年代からその発生間隔は１回につき平均５０００年以下とみつもられる。さらに堆積層基底の変位量と年代から垂直変位量は１２ｃｍ／１０３年，水平変位は変位の各段階を通じて一定ではないとみられるが，最近２００００年間では垂直変位の数１０番の値を示すと考えられる。

概要 （英文）

THE NEODANI FAULT WHICH RUPTURED DURING THE 1891 NOBI EARTHQUAKE (M=8),HAS BEEN INVESTIGATED BY THE GEOLOGICAL SURVEY,USING BORING AND TRENCHING METHODS AROUND OF KINBARA,MOTOSU TOWN,GIFU PREFECTURE. THIS AREA IS COMPOSED OF MESOZOIC AND PALEOZOIC SEDIMENTARY ROCKS,OF THE MINO GROUP. THE NEODANI FAULT IS COMPOSED OF SEVERAL FRACTURE ZONES 0.1 15M IN WIDTH. IT IS DISTRIBUTED WITHIN AN AREA SEVERALHUNDRED METERS WIDE OF THESE BASEMENT ROCK. SOUND ROCK IS DISTRIBUTED OUTSIDE OF THIS AREA. TRENCHES WERE EXCAVATED ACROSS THE NEODANI FAULT.IN THESE TRENCHES,THE FAULT IS APPARENTLY A NORMAL FAULT WHICH WAS ASSOCIATED WITH THE FRACTURE ZONE OF ABOUT 12 M IN WIDTH. FAULTING WHICH EXTENDS INTO OVERBURDEN,HAS OCCURRED ALONG THE MOST EASTERN CLAYEY ZONE OF THE FRACTURE ZONE. THE FAULT PLANE DIPS EAST AT A HIGH ANGLE AND THETOP OF THE BASEMENT ROCKS IS DISLOCATED 245M,ON THE AVERAGE,ALONG THE FAULT PLANE. FIVE FAULTS ARE OBSERVED IN THE OVERBURDEN, INCLUDING THE FAULT EXTENDING FROM THE BASEMENT ROCK. THESE FAULTS IN THE OVERBURDEN SEEM TO HAVE BEEN GENERATED BY LOCAL SUBSIDENCE AND SWELLING CAUSED BY A REPEATED LEFT LATERAL SLIP ALONG WITH A SMALL VERTICAL SLIP. THE OVERBURDEN IS COMPOSED OF A DOUBLE PAIR OF PADDY SOILS(2 4,5 7),WAS ACHIEVED BY 14C THE DATING METHOD. THE NEODANI FAULT HAS BEEN REVIVED BY AT LEAST 4 MOVEMENTS IACLUDING THE NOBI EARTHQUAKE,SINCE THE FLUVIAL DEPOSITS WERE LAID. THE LOWEST FLUVIAL DEPOSITS HAVE NO EVIDENCE FOR DECIDING THEIR RADIOACTIVE AGE. THE AGE OF THE LOWEST FLUVIAL DEPOSITS ARE ESTIMATED TO BE 20,000 YEARS JUDGING FROM THEIR SEDIMENTARY ENVIRONMENT. CONSEQUENTLY, THE RECURRENCE INTERVALS ARE ESTIMATED TO BE LESS THAN 5,000YEARS. DURING THE PAST TENS OF THOUSANDS OF YEARS,FAULT MOVEMENTS HAVEOCCURRED AT THE SAME POSITION AND IN THE SAME MANNER. THE VERTICAL DISPLACEMENT OF THE NEODANI FAULT AT KINBARA IS ESTIMATED TO BE 12 CM EVERY 1,000 YEAR,BECAUSE THE VERTICAL DISPLACEMENT OF FLUVIAL DEPOSITS IS 245 CM EVERY 2,000YEAR AND THE REL

報告者

キーワード