房総半島南部保田低地の完新世海岸段丘と地震性地殻変動 Holocene Marine Terraces and Seismic Crustal Movements in Hota Lowland in the Southern Part of Boso Peninsule, Central Japan
房総半島南部の保田低地には,高位から保田I面,II面,III面,IV面の4面の完新世海岸段丘が発達する.詳細な地形・地質調査の結果,従来,元禄地震時(1703年)の地震隆起に伴い形成されたと考えられていた最低位の保田IV面は,段丘面上の泥炭の年代,歴史的遺物の証拠から元禄地震以前に離水していたことが明らかになった.さらに,地形的証拠と古文書・古絵図の記載から判断すると,保田低地は元禄地震時に沈降したと考えられる.また,保田I面,保田II面は4,350yrs BPより前,保田III面は2,200yrs BPより前に離水しており,いわゆる元禄型地震によって離水した沼面群とは対比できない可能性がある.これは,元禄型地震のたびに保田低地が沈降していることを示唆する.保田面群の成因を大正型地震によるものと考えれば,保田I面の旧汀線高度から,大正型地震の平均再来周期は670年以内であると推測される.
The Boso Peninsula of South Kanto, central Japan, is a seismotectonically active region which has experienced two historical large earthquakes, the 1703 Genroku earthquake (M=8.2) and the 1923 Kanto earthquake (Taisho earthquake; M=7.9), due to the complex convergence of three plates: the Pacific, North American, and Philippine Sea plates.<br>The Hota lowland in the southern part of the Boso Peninsula developed Holocene marine terraces which are classified into four levels, here named Hota I, Hota II, Hota III, and Hota IV terraces. The Hota IV terrace has been considered to have been created by crustal uplift that accompanied the 1703 Genroku earthquake. However, the <sup>14</sup>C age of peat (830yrs BP) and historical remains indicate that the Hota IV terrace emerged before 1703. Further, historical documents and old maps showing the coastline between before and after Genroku earthquake show that the Hota lowland was subsided at the time of the 1703 seismic event. The amount of subsidence was inferred to be about 0-0.9m on the basis of geomorphic data.<br>Hota I and Hota II terraces were formed before 4, 350yrs BP, and Hota III was formed before 2, 200yrs BP, as dated by <sup>14</sup>C ages from peat and tree roots. These terraces are not correlated to a series of Numa terraces in the southernmost part of the Peninsula that were generated by Genroku-type earthquakes (Nakata <i>et al.</i>, 1980). This fact indicates that the Hota lowland has subsided whenever Genroku-type earthquakes occurred. Therefore, it is considered that a series of Hota terraces have been formed by crustal uplift accompanying Taisho-type earthquakes. The mean recurrence time of Taisho-type earthquakes is deduced to be less than 670 years based on the height of the former shoreline of Hota I (14m asl.).
第四紀研究 38(1), 17-28, 1999-02-01
Japan Association for Quaternary Research