Abstract
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. Although FKFZ has the highest vertical slip-rate in Japan, its structures in the seismogenic zone have not been sufficiently elucidated. In order to clarify the detailed structures, we carry out two stages of integrated seismic exploration. The first stage named FIST2012 is for revealing shallow to deep structures from the Philippine Sea Plate (PHS) across FKFZ to the Itoigawa-Shizuoka Tectonic Line (ISTL) at the northwest border of the Izu collision zone. A gravity anomaly analysis is also performed. The second stage aims to elucidate the overall structure of the Omiya and Iriyamase faults from the surface to the bottom for presenting the outcomes of the first stage. The results of the first stage confirm that the upper surface of PHS, from a depth of 2,000 m to 20,000 m in the FIST2012 depth section, dips at about several degrees westward from a depth of 2,000 to 2,500 m, and about 25 degrees below 2,500 m. A branching fault is generated from the seismogenic zone at a depth of approximately 2,500 m from the upper surface of PHS, and reaches an area a few hundred meters below the surface trace of Omiya fault, which is one of the eastern marginal faults of FKFZ. Although this suggests a close relationship between the branching fault and the Omiya fault, it is not revealed clearly because the specifications of FIST2012 are not set for such very shallow targets. ISTL dislocates P-wave velocity contours in a high angled west-dipping reverse fault sense. It shifts the P-wave 5 km/s contour from 3,000 m below sea level to the east of ISTL up to nearly sea level. The relationship between ISTL and the upper surface of PHS is not revealed, because of sparce shot and receiver points along the FIST2012 seismic line across the Jizo Pass where ISTL runs. Volcanic bodies composed of the middle Pleistocene Iwabuchi volcanic rocks of the Ihara group are recognized as Bouguer anomaly highs in FKFZ. They are considered to control the traces of both the eastern and the western marginal faults of FKFZ.
