The geometries (i.e., dip angles) of active faults from the surface to the seismogenic zone are the most important factors used to evaluate earthquake ground motion, which is crucial for seismic hazard assessments in urban areas. In Osaka, a metropolitan city in Japan, there are several active faults (e.g., the Uemachi and Ikoma faults), which are inferred from the topography, the attitude of active faults in surface trenches, the seismic reflection profile at shallow depths (less than 2 km), and the three-dimensional distribution of the Quaternary sedimentary layers. The Uemachi and Ikoma faults are N–S-striking fault systems with total lengths of 42 km and 38 km, respectively, with the former being located ~ 12 km west of the latter; however, the geometries of each of the active faults within the seismogenic zone are not clear. In this study, to examine the geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone, we analyze the development of the geological structures of sedimentary layers based on numerical simulations of a two-dimensional visco-elasto-plastic body under a horizontal compressive stress field, including preexisting high-strained weak zones (i.e., faults) and surface sedimentation processes, and evaluate the relationship between the observed geological structures of the Quaternary sediments (i.e., the Osaka Group) in the Osaka Plain and the model results. As a result, we propose geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone. When the friction coefficient of the faults is ~ 0.5, the dip angles of the Uemachi and Ikoma faults near the surface are ~ 30°–40° and the Uemachi fault has a downward convex curve at the bottom of the seismogenic zone, but does not converge to the Ikoma fault. Based on the analysis in this study, the dip angle of the Uemachi fault zone is estimated to be approximately 30°–40°, which is lower than that estimated in the previous studies. If the active fault has a low angle, the width of the fault plane is long, and thus the estimated seismic moment will be large.

从地表到地震发生带的活动断层的几何形状（即倾角）是用于评估地震地震动的最重要因素，这对于城市地区的地震危险性评估至关重要。在日本的大都市大阪，有几个活动断层（例如上町断层和生驹断层），这些断层是从地形，地表沟槽中的活动断层的姿态，浅深度的地震反射剖面（小于2 km），以及第四纪沉积层的三维分布。上町断层和生驹断层是N向冲断层，总长度分别为42 km和38 km，前者位于后者以西约12 km。然而，在地震发生带内每个活动断层的几何形状尚不清楚。在这项研究中，为了检查上町断层和生驹断层从地表到震源区的几何形状，我们基于水平压应力场下二维粘弹塑性体的数值模拟，分析了沉积层地质结构的发展。包括预先存在的高应变弱区（即断层）和地表沉积过程，并评估了在大阪平原发现的第四纪沉积物（即大阪群）的地质结构与模型结果之间的关系。因此，我们提出了从地表到地震发生带的上町断层和生驹断层的几何形状。当断层的摩擦系数约为0.5时，地表附近的上町断层和生驹断层的倾角约为30°–40°，且上町断层在震源区的底部具有向下的凸曲线，但没有收敛到生驹断层。根据这项研究的分析，上町断层带的倾角估计约为30°–40°，这比以前的研究估计的要低。如果活动断层的倾角较小，则断层平面的宽度较长，因此估算的地震矩将很大。