Seismic Reflection Profiling Across the Central of the Itoigawa-Shizuoka Tectonic line in Fujimi, Central Japan

The Itoigawa-Shizuoka tectonic line (ISTL) active fault system displays one of the largest slip rates in the Japanese islands. In the Fujimi area, the southern part of the ISTL active fault system, there are two faults, trending NW-SE, parallel to each other: the Aoyagi fault to the east and the Wakamiya fault to the west. The distance between the two faults is 1-2km. The Aoyagi fault is a west-dipping reverse fault and the Wakamiya fault is east-dipping sinistral fault. To clarify the subsurface geometry and their connectivity, we carried out high-resolution shallow seismic reflection profiling across the two faults. The length of the seismic line is 3.6km, and the receiver and shot point intervals are 10m. Common mid-point seismic reflection data were acquired using a 144-channel recording system and a mini-vibrator. Based on the obtained seismic section, we interpreted that the deeper extension of the Aoyagi fault shows a west-dipping fault surface at a moderate angle, and that the Walkamiya fault merges with the Aoyagi fault. In other words, the oblique slip on the ISTL is partitioned into the dip slip on the Aoyagi fault and the left lateral slip on the Wakamiya fault in shallow fluvial deposits

Seismic Reflection Profiling Across the North of the Itoigawa-Shizuoka Tectonic Line in Omachi, Central Japan

The Itoigawa-Shizuoka tectonic line (ISTL) active fault system poses the highest seismic risk, and shows one of the largest slip rates among active faults on Honshu Island. In the Omachi area, the ISTL active fault system comprises the eastern Matsumoto basin (EMB) fault to the west and the Otari-Nakayama fault to the east. To understand the structural relation between these two faults, we conducted a seismic reflection survey across them. The length of the seismic line is 6.4km, and the receiver and shot point intervals are 10m. We used a mini-vibrator as a seismic source with a 180-channel seismic recording system. We can image the structure to less than about 1km in depth. The seismic depth section displays the EMB fault as a boundary between east-dipping reflectors to the east and horizontal reflectors in the Masumoto basin. The EMB fault is interpreted as an east vergent, emergent thrust, and its deeper extension merges with the Otari-Nakayama fault. The Omine Formation, which was laid down on an alluvial fan in the late Pliocene and the early Pleistocene, is distributed between EMB and Otari-Nakayama fault. It is evident that the thrust front migrated from the Otari-Nakaya fault to the EMB fault in Quaternary, and then the Omine Formation located at the foreland basin was uplifted

High-Resolution P-wave Seismic Reflection Imaging of the Kuwana Active Thrust : 2000 Inabe River Profile

We present new, high-resolution seismic reflection data (2000 Inabe River profile) acquired across the forelimb and backlimb of Kuwana anticline to further image its subsurface geometry. A seismic source (mini-vibrator) and 150-channel digital telemetry recording system were used in an off-end configuration with the nearest receiver adjacent to the source to record seismic waves from deeper reflection points. A 15-m source and geophone spacing give a 7.5-m CMP (common midpoint) spacing on the final section. The nominal CMP stacking fold had 75 traces. We also suppressed coherent noise by repeating source points 5-10 times. After data processing including surface-consistent statics, velocity analysis, normal moveout (NMO) correction, residual statics, CMP stack, and migration, the section was finally depth-converted using stacking velocities. Reflectors imaged on the seismic profile illuminate that fluvial terraces folded across the forelimb and backlimb are consistent with the subsurface structure of the fold imaged in a seismic section, providing insights into the kinematic solution of the underlying active wedge thrust

P- and S-Wave Seismic Reflection Profiling Across the Kamishiro Fault, Itoigawa-Shizuoka Tectonic Line Active Fault System, Central Japan

The Kamishiro fault forms the northern-most part of the Itoigawa-Shizuoka Tectonic Line (ISTL) active fault system. This fault displaced young lake deposits in the Kamishiro basin, and the shallow (less than 20m) structure of the fault and an average long-term vertical slip rate of 2.2－2.7m/kyr were revealed by drilling and geomorphological investigations. To reveal the subsurface structure of this fault system and estimate long-term net slip rate, we carried out S- and P-wave high-resolution shallow seismic reflection profiling across the Kamishiro fault. The S-wave seismic line is 200m, the receiver point intervals are 1m, and the shot point intervals are 2m. We used a 192-channel seismic system with a mini-vibrator to image the depth range of 5 to 70m. The P-wave seismic line is 2.1km, the receiver point intervals are 10m, and the shot point intervals are 10m. We used a 176-channel seismic system with a mini-vibrator to image the depth range of 50 to 800m. From these experiments, it is clarified that the Kamishiro fault cut and fold the lacustrine deposit. The deformation is associated with fault development near the surface. Given the dip angle of the Kamishiro fault determined by S-wave seismic profiling to be approximately 30°, the average dip slip-rate is calculated to be 4.4-5.4m/kyr. And, the Kamishiro fault corresponds to cutting across the anticline rather than the deformation front on the P-wave seismic profile. The fold is related to blind thrust fault, fold propagation fold, or drag structure