山地斜面におけるピット調査から推定された琵琶湖北方，駄口断層の活動履歴

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タイトル別名
  • Surface-faulting History of the Daguchi Fault, North of Lake Biwa, Southwest Japan, inferred from Pit Excavations on a Mountain Slope
  • サンチ シャメン ニ オケル ピット チョウサ カラ スイテイ サレタ ビワコ ホッポウ ダグチ ダンソウ ノ カツドウ リレキ

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Excavating a pit across a small up-hill facing fault scarp on a mountain slope could be a paleoseismological tool for intramontane active faults, which usually lack suitable sites for traditional trenching. We tested this method on the Daguchi fault, north of Lake Biwa, southwest Japan, a typical short strike-slip fault in a mountainous region. Our test excavation at the Kurokoyama site on the southern subsegment of the fault, together with a re-interpretation of our previous excavation at the Daguchi site on the northern subsegment, allow us to obtain some constraints on the surface-faulting history of the fault. The timing of the most recent event is estimated to be 450-1050 cal. y.B.P. (900-1500 A.D.), whereas that of the penultimate event is poorly constrained : 1500-7300 cal. y.B.P. or prior to 5000-7300 cal. y.B.P., depending on different interpretations of the infilled fissure exposed on the pit wall. For either interpretation, our result is consistent with estimates from a previous paleoseismic study by Sugiyama and Yoshioka (1999), but the presence of other stratigraphically unidentifiable events cannot be ruled out, given the lack of piecing layers across the main fault in our pits, as well as their trench. The principal reason for the poor age control is an unexpected lack of peaty sediments at the Kurokoyama site, which is probably related to a high sedimentation rate and an unusual sedimentation process due to rapid soil creep on the mountain slope. Although application of this method might be limited on a mountain slope with rapid soil creep, which is commonly implied by the downslope bending of tree trunks as it is at the Kurokoyama site, our test excavations suggest that pit excavation on a mountain slope basically works as a paleoseismological tool in mountainous regions.

収録刊行物

  • 地学雑誌

    地学雑誌 114 (5), 724-738, 2005

    公益社団法人 東京地学協会

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