Study on Cumulative Activities of Passively Ruptured Faults through a Trenching Survey at the Matoishi Bokujo I Fault, Northwest Side of the Aso Caldera, Southwestern Japan

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  • トレンチ調査による阿蘇外輪山北西域の的石牧場I断層の変位の累積性の検討
  • トレンチ チョウサ ニ ヨル アソガイリンザン キタセイイキ ノ テキセキ ボクジョウ I ダンソウ ノ ヘンイ ノ ルイセキセイ ノ ケントウ

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Abstract

<p>Paleoseismic trenching surveys have helped estimate earthquake hazards based on the characteristic earthquake model, which does not consider passive rupturing. In this study, a trenching survey was carried out at the foot of the Matoishi Bokujo I fault scarp. The survey revealed cumulative vertical displacements of a passively ruptured fault reactivated by the 2016 Kumamoto earthquake (the 2016 earthquake; Mj 7.3). The Matoishi Bokujo I fault is located on the northwest outer rim area of the Aso caldera in Kumamoto prefecture, southwestern Japan, approximately 7 km from the Futagawa fault, which is the seismogenic fault of the 2016 earthquake. On the Matoishi Bokujo I fault where a fault scarp is clearly identified, a phase discontinuity caused by the 2016 earthquake was detected via displacement analysis using synthetic aperture radar interferometry of the Advanced Land Observing Satellite (ALOS)-2 data; the displacement measured 10-15 cm along the phase discontinuities. As aftershocks were not observed along the Matoishi Bokujo I fault, the phase discontinuities suggest that passive rupturing was induced by stress changes and/or strong shaking along the main Futagawa fault. On the trench wall of the uplift side of the fault scarp, an Aso-4 pyroclastic flow deposit ca. 1 m below the ground surface was covered with black soil; these deposits were deformed by normal faults. A clear displacement caused by the 2016 earthquake was not identified in the trench because it is assumed that the 10-15-cm displacement was separated into small displacements along the entire fault scarp. However, in the trench, two preceding faulting events at 3,430-2,890 cal BP and after 2,810 cal BP were identified with a displacement of up to 50 cm. This indicated the possibility that after 2,810 cal BP, the Matoishi Bokujo I fault could have passively ruptured simultaneously with the preceding earthquake on the Futagawa fault. In contrast, another earthquake triggered a passive or independent rupturing at 3,430-2,890 cal BP on the Matoishi Bokujo I fault. These results show that a fault scarp can be formed through different types of rupturing events. Although earthquake hazards related to active faults are typically evaluated based on the long-term slip rate estimated using tectonic landforms and recurrence intervals chronicled by trenching surveys, passively ruptured faults such as the Matoishi Bokujo I fault demonstrate the necessity of restructuring this evaluation method. To improve the method, distributions and characteristics of passively ruptured faults should be investigated in detail.</p>

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