Geomorphic History of the Hakodate Plain South Hoddaisdo, Japan with Special Reference to Active Faulting.

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  • 函館平野とその周辺の地形　とくに西縁の活断層に関連して
  • ハコダテ ヘイヤ ト ソノ シュウヘン ノ チケイ トクニ セイエン ノ カツ
  • とくに西縁の活断層に関連して

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Fluvial and marine terraces are distributed on the western and eastern sides of the Hakodate Plain, southern Hakkaido, and are divided into 5 levels, from terrace 1 to terrace 5 in descending order. Most terraces are of fluvial origin; however, terraces 1 and 2 on the southern part of the study area are marine terraces. Terrace 2, covered by tephras which include Toya volcanic ash of ca. 90-100ka, represents the last interglacial maximum of oxygen isotope stage 5e. The former shoreline height of terrace 2 at the west of the Hakodate Plain is ca. 80m, while it is ca. 50m at the eastern side. The lowest aluvial plain is underlain by marine deposits at least 25m thick. The upper limit of Holocene marine deposits, dated at ca. 6ka, is 0.6-2.1m above sea level on the eastern part of the Hakodate Plain and -0.8m along the coast, implying that the Hakodate Plain has been relatively stable during the last 6ka.<br>On the western margin of the Hakodate Plain, two prominent fault groups run nearly north-south: Oshima-Ono Fault on the north and Tomikawa Fault on the south. No fault is recognized on the east, however. These two fault groups are boundary faults between the hills underlain by early Pleistocene marine deposits and middle to late Pleistocene terraces on the west, and Holocene lowland on the east. These faults are represented as flexural scarps on terrace surfaces associated with reverse faulting, and are accompanied by rangefacing fault scarplets as antithetic faults. Progressive activity with a slip-rate of 0.2-0.9m/ka is observed on the main flexural scarps. Even range-facing scarps with a low slip-rate of 0.05-0.1m/ka show progressive deformation. The presence of thicker early Pleistocene marine deposits (ca. 600m) on the west of the plain, in comparison with those only 180m thick on the east, indicates that the Hakodate Plain has been under a compressive stress field and has been formed as an active fault-angle depression since the middle Quaternary, and that the western side of the fault has been absolutely uplifted.

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