群列ボーリング調査と地中レーダ探査による伏在活断層の活動履歴の検討一大分平野西部の府内断層の例一

摘要

中央構造線活断層系の西端に位置する別府湾は，およそ5Maから沈降場にある.その南縁の府内断層は同堆積盆を構成する主要な正断層である.その活動性を評価するために.断層を挟んで掘削された7本のボーリングコア試料を用いて，堆積相解析，珪藻化石の群集組成解析.合計17試料の陸源植物片と4試料の海生炭酸塩の放射性炭素年代測定を実施した.それらの解析結果を基にして，デルタフロント相.デルタプレーン相，人工盛土相を認定した.これらの堆積構造と変形構造は地中レーダ探査によって可視化された.これらの結果.デルタプレーン相の最上部の泥層の堆積年代と珪藻化石群集を基にして800~400calBPの最新活動を認定した.デルタフロント相の泥層の上下変位量を基にして2.100calBP頃の活動も認定した.これらからおよそ 1,700年の再来間隔が計算できる.%Beppu Bay, occupying the western portion of an arc-bisecting dextral fault system, is a tectonic depression that has existed since ca. 5 Ma. Funai Fault is a major normal fault along the southern margin of the tectonic basin. In order to estimate the activity of this fault, sedimentary fades, diatom assemblages, and radiocarbon ages of 17 terrestrial plant fragments and 4 marine carbonate samples were determined from seven drill cores. Based on the analysis, three sedimentary facies, namely delta front, delta plain, and artificial soil, were identified. These accumulation ana defor-mation structures were imaged by ground penetrating radar profiling. Based on the results' the latest seismic event was identified at 800 to 400 cal BP based on the depositional ages and diatom assemblages from the uppermost mud layers in the delta plain sediment The previous event was also identified at ca 2,100 cal BP on the basis of the vertical displacement of the lower mud layer in the delta front sediment. The recurrence interval was calculated at ca 1,700 years.