Recent global navigation satellite system (GNSS) data for the Japanese Islands have revealed a high-strain-rate region suggesting the existence of a region of broad-scale crustal deformation. The Niigata-Kobe Tectonic Zone (NKTZ), which is the high-strain-rate zone in central Japan, shows a short-term dextral strain rate of ~ 12 mm/year. The total slip rate of the Quaternary fault zones in the NKTZ has been estimated as ~ 6.7 mm/year, accounting for just over half the short-term strain rate of the zone. However, this slip rate underestimates the total slip rate on faults within the NKTZ owing to possible distributed deformation on minor faults. This study quantifies the slip rate attributable to these other faults in the southeastern-central NKTZ and reveals the unique deformation structure across the highstrain-rate zone, which comprises a Quaternary fault core, a Quaternary fault damage zone, an incipient brittle shear zone (active background), and an inactive background. The spatial characteristics of the incipient brittle shear zone can be explained in terms of fault density, which increases toward the central NKTZ. Minor faults located > 500 m from major Quaternary faults but within the NKTZ have sense of shear consistent with that of the major faults. In contrast, minor faults outside of the NKTZ show sense of shear that differ from the dextral displacement of the high-strain-rate zone and do not contribute to the slip rate of the zone. The total slip rate of minor faults in the southeastern-central NKTZ is estimated to be 0.46-2.88 mm/year (roughly equal to a major Quaternary fault in the zone), which implies 4-24% of crustal strain is stored in the active background.

No abstract

The eastern segment of the Inagoe Fault in the Furukawa Fault Zone, northern Gifu Prefecture, is thought to be active based on the discovery of active fault outcrops and geomorphologic evidence. Although the fault trace is thought to extend westward to the Kurigatanigawa area along the western segment of the fault, direct geologic evidence of fault activity there has not been reported. This study describes a newly discovered active fault outcrop along the western segment of the Inagoe fault and assesses fault activity on this segment. The outcrop is located on the trace of the Inagoe Fault and has a 5 m-wide fault gouge or breccia zone. The composite planar fabric in the fault gouge zone shows a dextral sense of shear, consistent with the sense of movement of the fault inferred from stream offsets. Unconsolidated sand and gravel layers, which were originally deposited above the fault zone, are also sandwiched within the fault zone. The major axes of gravel clasts adjacent to the fault gouge are vertical, and striations are found on their surfaces. Radiocarbon dating of a soil sample in the sand and gravel layer yields an age of AD 1521-1658, suggesting that an earthquake occurred on the western segment of the Inagoe Fault after this time. These observations suggest that (1) the fault outcrop corresponds to the core of the Inagoe Fault, (2) an overlying sand and gravel layer fell into crack and was subsequently deformed by movement on the fault, and (3) the western segment of the Inagoe Fault might have ruptured during one of the four historical earthquakes known to have occurred in northern Gifu Prefecture over the past few hundred years.

The NE-SW-trending Saigatao Tectonic Line with the length of 20 km long in central-southern part of Yamaguchi Prefecture, Southwest Japan, has been known as a geological boundary fault between Suo Metamorphic Rocks of the Triassic to Jurassic period and Ota Group in the Permian period. Tectonic reliefs and active fault outcrops concerning the tectonic line have not yet been described. Fault outcrop Loc. 1(Nagaono, Ube City)and Loc. 2(Mana, Mine City) near the Saigatao Tectonic Line are newly identified by topographic and geological investigations. In the fault outcrop Loc. 1, the fault is recognized between mudstone of Ota Group and pelitic schist of Suo Metamorphic Rocks and this fault is corresponding to the Saigatao Tectonic Line, but does not give Lower river terrace deposits the displacement and deformation. In the fault outcrop Loc. 2, the fault in the sandstone and mudstone belonging to Ota Group is recognized. This outcrop is located in the 40m westward to the Saigatao Tectonic Line and the fault concerning the Saigatao Tectonic Line, because the strikes of the fault in the outcrop and the Saigatao Tectonic Line have the same strike NE-SW. The boundary between the fractured zone and the host rock displaces the Middle river terrace deposits(after 85,000-90,000～130,000 y.b.p.). Therefore, it is possible that, at least, the part of the Saigatao Tectonic Line is an active fault and the latest movement age of the active part is after the Late Pleistocene.