There are several marine terrace surfaces in the southeastern part of Shimokita peninsula, northeast Japan. They are classified into H1, H2, M1, M2 and M3 surfaces in descending order. The M1 and M2 surfaces are correlated with those formed in MIS 5e and in MIS 5a, respectively. The Rokkasho fault merging into the extensive submarine fault along shelf edge in the north has successively deformed these terrace surfaces, resulting in a 1-2 km wide flexural scarp tilting to the east on the M1 and M2 surfaces.Vertical offset of the M1 surface is over 30 m in the north and less than 20 m in the south. The flexural scarp extends at least 15 km to the south of the Takahoko Lake.The Detoseiho fault is a subsidiary branch fault of the Rokkasho fault developed in the Rokkasho flexural scarp.Because the M1 surface is bending toward the east (toward the coast), the sand layer composing of the M2 surface abutted on the monocline slope. Following movements of the Rokkasho fault have deformed the M2 surface in the same direction as the M1 surface, and consequently the M1 and M2 surfaces converged upon in the flexural scarp.This makes it very difficult to distinguish one from the other.Such geomorphic development should be strongly controlled by the activity of active fault differentiating tectonic relief and the width of flexural scarp reflecting the depth of tip of fault plane.Further examinations on active faults extending parallel to the coast lines are required to solve the problems.
The Fujikawa Valley located at South Fossa Magna is filled by a thick sedimentary pile of Middle Miocene to Pleistocene age deformed by several large-scale faults. However, there is no report on the existence of active faults in the area between the Fujikawa-kako fault zone and Itoigawa-Shizuoka Tectonic Line active fault system. Based on aerial photograph interpretation and field observations, we find displaced landforms in the middle part of the Fujikawa Valley associated with the recent activities of the Minobu fault which has been identified as a pre-Quaternary fault by previous works. Geomorphic features such as systematically offset stream channels and scarps on terraces along the fault display that the Minobu fault is a left-lateral fault, trending NNW to SSE for a length of about 20 km. The horizontal slip rate during the late Pleistocene is more than 0.7 mm/yr. Our study suggests the Minobu fault, together with the Fujikawa-kako fault zone and the Itoigawa-Shizuoka Tectonic Line active fault system constitutes a major active tectonic zone in the western margin of the Fossa Magna.
We developed the high-resolution three-dimensional seismic survey system for shallow water and carried out the survey of the active fault in the nearshore waters of the northern Suruga Bay using the system. The system is designed based on the following concepts, 1) usable system in nearshore waters using a small boat, and 2) acquirable seismic data in small bin size (2 m × 2 m) to image sub-seafloor geological structures in high-resolution. The system consists of onboard data acquisition instruments and towing equipment such as one boomer sound source and four receiver cables. Each receiver cable has 8 channel hydrophone arrays arranged at 2.5 m interval, and is towed behind the survey boat arranged at 8 m interval in the rectangular direction to the survey line.
The boomer was shot at intervals of 1.25 m under ship speeds between 3 and 4 knots.
The three-dimensional seismic survey area is 1,400 m long in the east-west and 300 m wide in the northsouth. The seismic stratigraphy is mainly divided into three units, Unit A, Unit B and Unit C in descending order. A prominent fault called the Kanbara Fault, which splits the survey area in two parts, is a reverse type extending from northeast to southwest. The deformation in Unit A seems to be extended near the seafloor. The amount of vertical displacement is 22 to 26 m based on the upper surface of Unit B. As the upper surface is interpreted to be approximately 11,000 years old, the average vertical slip rates on the fault is 2.0 to 2.4 m/kｙ calculated by the above-mentioned interpretations.
We obtained some remarkable stripe-like patterns on the time-slice sections related to the uplifting western part of the survey area. If a strike-slip fault crosses above-mentioned stripe-like patterns, it is expected that the amount of lateral displacement caused by the strike-slip movement will be estimated on the time-slice sections.