Subsurface Study of Isehara Fault, Kanagawa Prefecture, Detected by Drilling : Depositional Environments during the Last 7000 Years and Fault Displacement Associated with the Gangyou Earthquake in A. D. 878
伊勢原断層の両側で試錐調査を行い,試料中の火山灰,14C年代,貝・有孔虫・珪藻・渦鞭毛藻などを調査した.調査地域は,約6000年前頃,内湾性の海域から低湿な陸域に変わった.その海成層の上限の高さ(当時の海抜0m)は,現在標高およそO～-2mにあるが,断層の東側の方が1.6±0.6m高い.この高度差は伊勢原断層の変位によると考えられる.この変位が生じた年代は,地層の厚さの比較から,延暦・貞観年間のテフラ層堆積以後で,宝永スコリア堆積以前である.この""伊勢原地震""の規模は,その変位量などから考えて,M7.0～7.5程度である.また,約6000年前の海成層の上下変位量と約1100年前までのテフラ層の標高差との間に有意の差がないことから,伊勢原地震の再来間隔は約5000年以上である.このような伊勢原地震に最もよく適合する歴史地震は,元慶2年(878年)の相模・武蔵の地震(理科年表M7.4)である.
The Isehara fault has been suspected to be a causal fault of the Gangyou earthquake of A.D. 878. We drilled at four locations, W, X, Y and Z, on a line crossing the Isehara fault, loc. Y being on the western side and others on the eastern side, and obtained columnar samples of Holocene deposits down to about 40 m from the surface. The shallow marine deposits were found from the lower part of Y and Z. The depositional environment and stratigraphic position of the transition surface from marine to freshwater deposits were examined by analyzing molluscs, foraminifers, diatoms and dinoflagellates in the samples. The twenty carbonaceous samples from Y and Z cores were dated by the 14C method. The marine beds become shallower upward and grade into brackish to freshwater deposits consisting of peaty silt with thin beds of fluvial sand, gravel and volcanic scoria. The lithology and thickness of the peaty deposits are similar on the western and eastern sides of the fault, suggesting that the area has been generally a low, swampy plain since about 6000 years B. P. The tephra layers in the peaty deposits were examined and correlated with each other among localities and with the Younger Fuji Tephras (YFT) described previously in neiboring areas.
雑誌名
東京大學地震研究所彙報 = Bulletin of the Earthquake Research Institute, University of Tokyo
巻
63
号
2
ページ
145 - 182
発行年
1988-11-11
ISSN
00408972
書誌レコードID
AN00162258
フォーマット
application/pdf
日本十進分類法
453
出版者
東京大学地震研究所
出版者別名
Earthquake Research Institute, University of Tokyo
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伊勢原断層(神奈川県)の試錐による地下調査 : 過去約7000年間の堆積環境と元慶2年地震の変位
ji0632006.pdf
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