完新世海成堆積物からみた相模湾沿岸地域の地形変動 Holocene Crustal Movements around Sagami Bay as Inferred from Holocene Marine Deposits

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相模トラフの北東側に位置する大磯丘陵や三浦半島は,完新世においても地殻変動の活発な隆起地域で,3~4段の完新世海成段丘が発達する.その中で,三浦半島南部に分布する段丘は,海食洞窟や考古学資料などから約6,000yrs BP,約4,000yrs BP,約3,000yrs BPの汀線や離水が明らかとなった.三浦海岸に対応する3回の間欠的な地殻変動のあったことが分かった.<br>相模湾北東沿岸は顕著な隆起を示し,その背後に「秦野-横浜線」の沈降帯が存在しているが,完新世ではその軸方向が南にやや下がり,秦野-大船-金沢八景を結んだ線で把えられる.<br>国府津-松田断層を挾んで大磯丘陵南西部一帯は,約6,500yrs BPに離水したことが貝類群集,<sup>14</sup>C年代や鬼界アカホヤ火山灰(K-Ah)によって確認された.この離水を起こした変動では,国府津-松田断層に変位が認められず,約6,500yrs BP以降における本断層の活動で大磯丘陵の著しい隆起,森戸川低地の緩やかな沈降となっている.足柄平野南西端の小田原は約4,500yrs BPには離水しており,海成層が+5mを超えて明瞭な隆起を示す.その上限高度は,東の国府津-松田断層が位置する森戸川低地に向かって序々に沈降する.<br>相模トラフの南西側に位置する伊豆半島では,約6,000yrs BPの旧汀線を示す証拠を陸上で見出だすことができず,縄文海進は半島北部で約4,000yrs BP,南部では約3,000~2,000yrs BPまで存続していて,その後に隆起に変わっている.<br>相模湾沿岸の溺れ谷低地の海成層中には,多くの津波堆積層の介在が確認され,その形成年代から沿岸に分布する海成段丘の離水時期を解明する手掛かりが得られた.今後は,この研究手法によって相模湾沿岸の詳しい地殻変動史を明らかにすることができよう.

This paper describes Holocene marine deposits around Sagami Bay and discusses Holocene crustal movement in relation to crustal deformation associated with great earthquakes in the Sagami Trough. Flights of Holocene marine terraces are distributed on the coasts along the Oiso Hills and Miura Peninsula, which are uplifted regions located on the northeastern side of the Sagami Trough. Marine terraces located in the southern part of Miura Peninsula indicate that the emergences occurred around 6, 000yrs BP, 4, 000yrs BP, and 3, 000yrs BP on the basis of the ages of sea caves and archaeological data.<br>Remarkable uplifted features in the Holocene are observed along the northeastern part of Sagami Bay; to the north of this uplifted zone a belt of subsidence in the Holocene is located on the line connecting Hadano to Kanazawa-Hakkei through Ofuna.<br>The emergence of the southwestern part of the Oiso Hills around 6, 500yrs BP is inferred from the fossil molluscan assemblage, the K-Ah (Kikai-Akahoya) volcanic ash, and related carbon-14 dates. The Kozu-Matsuda active fault which delineates the western margin of the Oiso Hills did not move at the time of the emergence around 6, 500yrs BP. The Oiso Hills to the east of the Kozu-Matsuda fault have uplifted and the Morito River Lowland to the west of the fault has subsided along with the slip of the Kozu-Matsuda fault since 6, 500yrs BP.<br>Marine sediments that emerged around 4, 500yrs BP are present over 5m above the present sea level in Odawara city, which is located in the southwestern margin of the Ashigara Lowland. The upper limit of Holocene marine deposits descends eastward to the Morito River Lowland.<br>No geological evidence of emergence around 6, 000yrs BP is found in Izu Peninsula to the southwestern side of the Sagami Trough. The post-glacial Jomon tranagression continued until 4, 000yrs BP in the northern part of Izu Peninsula and until 3, 000 to 2, 000yrs BP in the southern part of the peninsula. Izu Peninsula became a site of emergence after these periods in its various region.<br>Tsunami deposits have been discovered in marine sediments located in submerged drowned lowlands around Sagami Bay. The ages of emergence of marine terraces have been inferred from the ages of these tsunami deposits. The history of coseismic vertical movements at Sagami Bay may be clarified by further study of tsunami deposits.

収録刊行物

  • 第四紀研究  

    第四紀研究 38(6), 503-514, 1999-12-01 

    Japan Association for Quaternary Research

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各種コード

  • NII論文ID(NAID)
    10004721432
  • NII書誌ID(NCID)
    AN0034136X
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    04182642
  • NDL 記事登録ID
    4927064
  • NDL 雑誌分類
    ZM49(科学技術--地球科学--地質)
  • NDL 請求記号
    Z15-67
  • データ提供元
    CJP書誌  NDL  J-STAGE 
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