大分県中部火山地域の活断層系

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  • ACTIVE FAULT SYSTEMS OF THE QUATERNARY VOLCANIC REGION IN THE CENTRAL PART OF OITA PREFECTURE, KYUSHU DISTRICT, SOUTHWEST JAPAN
  • オオイタケン チュウブ カザン チイキ ノ カツダンソウケイ

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Recent studies have revealed regional variations in types and intensities of the active faulting in Japan. Normal faults are developed exceptionally well in Central Kyushu, while strike-slip or reverse faults are predominant in the main part of Japan (Matsuda, 1973). The focal mechanisms of several shallow earthquakes in Central . Kyushu showed normal fault type (Yamashina and Murai, 1975), such as the Oita earthquake of April 21, 1975 (M=6.4). The purpose of this paper is to present new data on the active fault system of the Quaternary volcanic region in the central part of Oita prefecture, Central Kyushu, where normal faults are densely developed, and tectonic implications of graben formations are also discussed. The results obtained are summarized as follows:<br> (1) Normal faults with short and irregular traces are densely developed in the central part of Oita prefecture, most of which are integrated into several grabens, such as the Haneyama graben and the Hayami graben (Fig. 3).<br> (2) Judging from the wide-spread distribution of Tsukushi lavas across the present rims of grabens, the Haneyama and Hayami grabens had not yet been formed before the eruptions of Tsukushi lavas. But the faulting had already taken place before the eruptions of Haneyama lavas in the Haneyama graben and before the deposition of Yabakei pyroclastic flow in the Hayami graben. Based on the absolute ages of these volcanic rocks, the faulting of these grabens is estimated to have begun during the period from 800, 000 to 500, 000 years B. P.<br> (3) Haneyama lavas erupted from inside the Haneyama graben and hornblende andesites of San'in volcanic series erupted from inside the Hayami graben. These volcanisms occurred along with faulting.<br> (4) The Hayami graben is developed on the crest of a broad up-warping as indicated by general relief and drainage pattern, and the altitude of its floor is considerably high in spite of local subsidence caused by the normal faulting. Such marginal uplifting is recognized also around the Haneyama graben, though it is less distinct.<br> (5) Among the faults near the northwest rim of the Hayami graben, the faults located on the north (Fault 1-5 in Fig. 6) had finished their activities before the formation of Tateishi-yama volcano. However, the faults located on the south (Fault 8 and 9 in Fig. 6) have been active after the formation of the volcano. This indicates that the location of faulting shifted from north to south, namely inward the graben. The distance of about 2 km between Fault 1 and Fault 8 represents the amount of shifting.<br> (6) The arrangement of volcanic vents inside the grabens suggests the existence of exten. sion fractures underneath the grabens. If we assume the existence of extension fractures, , it becomes considerably easier to understand the phenomena (4) and (5): marginal uplifting of the grabens can be explained as a deformation due to opening of extension fractures, and the opening causes extension of a graben, so that locations of faulting will shift relatively inward the graben (Fig. 9). Faulting itself seems to be a surface effect associated with opening of extension fractures.<br> (7) Surface displacements due to opening of a crack with various depth in an elastic half space are calculated (Fig. 10). By comparing the observed pattern of marginal uplifting with the calculated curves of surface displacements, the depth and the amount of opening of the extension fracture underneath the Hayami graben are estimated to be 8-12 km and 1.7-3.2 km, respectively. The latter value is in well-agreement with the extension of about 2 km inferred from the inward shift of faulting.<br> (8) Based on the arrangement of surface faults and volcanoes, a possible model of subsurface structure of the Hayami graben is presented (Fig. 11).

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  • 地理学評論

    地理学評論 52 (1), 10-29, 1979

    公益社団法人 日本地理学会

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