Three-dimensional Structure of the Omiya and the Iriyamase Faults and Its Formation Process, the Eastern Margin of the Fujikawa-kako Fault Zone, the Northwestern Border of the Izu Collision Zone, Central Japan
Outcomes of the Multi-line Integrated Seismic Exploration in the Hoshiyama Hills
Bibliographic Data
The Fujikawa-kako fault zone (FKFZ) is one of the most active fault zones constituting the northwest border of the Izu collision zone between the Honshu and Izu arcs. Its vertical slip rate is estimated to reach 6 to 7 m/ky along the eastern margin of FKFZ. In order to elucidate the causes of such high fault activities, the structure of this fault zone from the seismogenic zone to the surface is revealed. An integrated seismic exploration, which links several seismic exploration techniques, is employed. Thus, a two-stage integrated seismic exploration is designed and carried out. The first stage, named FIST2012, aims to reveal the shallow to deep structures of the upper surface of the Philippine Sea Plate (PHS), FKFZ and Itoigawa-Shizuoka Tectonic Line (Fujiwara et al., 2024). The second stage, named Multi-line Hoshiyama 2015 attempts to elucidate the three-dimensional structure of the Omiya fault (Omy) and the Iriyamase fault (Irs) from the seismogenic zone to the surface, both of which form the eastern margin of FKFZ. The main purpose is to present the outcomes of the second stage, and to discuss any remaining problems. The results are as follows. Omy consists of the Sole thrust (St) and the Roof thrust (Rt), which together form a N30°W trending wedge-thrust structure, whereas Irs mainly consists of two segments: N50°E-trending Fa and N48°E-trending Fd. Fa originates from St of Omy as a reverse fault. On the other hand, Fd plays a role in a tear fault of St. The three-dimensional structure of Omy and Irs shows that both faults form a single active fault system driven by the thrust motion of St, which connects with the branching fault from the upper surface of PHS. This means that the high fault activities of FKFZ directly reflect the motion of PHS. Although the outcomes are significant, major problems remain. Firstly, ages cannot be assigned to depth sections due to the lack of geological and geophysical data from boreholes, with the exception of two shallow boreholes. This prevents precise interpretations of depth sections from being completed. Thus, geological and geophysical data from boreholes are urgently needed. Secondly, the N60E-trending thrust motion on N30W-trending St seems to contradict the NW-trending PHS motion relative to the Honshu arc, even though St connects with the branching fault that originates from the upper surface of PHS. The structural relationship between the two motions between St and PHS is still unresolved. To address this issue, the three-dimensional geometry of the upper surface of PHS where the branching fault originates needs to be understood
Fault (geology) · Seismology · Stage (stratigraphy) · Tectonics · Thrust · Thrust fault · earthquake and tectonic studies · Engineering · Geological and Geochemical Analysis · Geology · High-pressure geophysics and materials · Paleontology
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