The Eastern margin of the Yokote Basin Fault Zone (EYBFZ) is one of the seismogenic reverse faults that developed in Northeast Japan, generating the 1896 Rikuu Earthquake (M7.2). The EYBFZ is divided into range boundary fault (Kawaguchi Fault) and frontal fault. The major segment of the frontal fault consists of three faults : the Shiraiwa Fault, the Ota Fault, and the Senya Fault, at which coseismic surface ruptures with irregular and highly sinuous traces, gaps and echelon steps occurred during the 1896 Earthquake. In the study area, the irregular coseismic surface ruptures probably originate from differences in fault geometries and sedimentary conditions, which are associated with thrust-front migration in the Quaternary. We discuss the relationships among fault traces, geomorphic displacements, and fault geometries on the Senya Fault and the Kawaguchi Fault, based on data from high-resolution seismic reflection profiling, investigations into tectonic geomorphology and structural geology with the help of the balanced cross-section method.<BR>The seismic image obtained shows changes of geomorphic and geologic features around the Senya Hills expressed as differences in faulting geometries along the Senya Fault and the Kawaguchi Fault. In fact, the Senya Hills are situated as an unsymmetrical ordering.The disparity in the timing of thrust-front migration affects the development of fluvial surfaces, which are related to the appearance of the fault scarp on the surface. That is why, in the central portion of the Senya Hills, the Senya Fault is a typical emergent thrust with flat-ramp structures associated with the uplifting and the back-tilting of late Quaternary fluvial terraces. On the other hand, in the northern portion, the Senya Fault is a concealed thrust with step-like geometry and prominent folding and small faults in the hanging wall. The small faults infer the structure of the tectonic scarp and short wavelength deformation on the surface. This means that the Senya Fault at the central portion preceded the northern portion as an emergent fault. Moreover, fault traces show different dips between central and northern portions. The dip of the central portion is lower than that of the northern portion.<BR>By restoring the balanced cross-sections and assuming the uniform net-slip rate, the initial thrusting along the boundary faults is retroactive to 2.02.6 Ma at the central portion and 1.8 2.3 Ma at the northern portion. The initiation of thrust front migration from the boundary faults to the frontal faults is ca.1.6 Ma at the central portion and 0.6Ma at the northern portion.