Abstract
Ground penetrating radar (GPR) and high‐resolution shallow reflection seismic surveying were carried out to investigate the subsurface geology in and around the Uemachi Fault zone in the Yamato River area, Osaka, Japan. Shallow drilling in the area showed a major displacement event during the middle Pleistocene. The main Uemachi Fault plane could be clearly imaged on the seismic section, except for the most shallow 200 m. Several shallow normal fault planes with less displacement could be detected on both sides of the fault plane. GPR profiles confirmed the presence of several shallow normal faults within the area near the fault zone. These shallow faults could be followed in all of the GPR profiles crossing the fault zone. The integration of seismic section, GPR profiles and drilling data led to a conceptual model that explains the evolution of the Uemachi Fault system. The proposed model suggests the occurrence of several cycles of small vertical displacement along the deep part of the fault plane caused by the regional east–west compressional stress. The ductile nature of the shallow sedimentary cover and the absence of confining pressure in the shallow part allow for a considerable amount of plastic bending before failing in the shallow sedimentary layers. This bending generates stretching force within the shallow sedimentary cover, which in time, along with gravitational force, gives rise to the formation of the swarm of normal faults within the shallow layers near the fault zone. Some of the detected faults extend to a depth of less than 3 m below the ground surface, suggesting that the last tectonic activity along the fault plane may have occurred recently.