Evaluation of dip angles of active faults beneath the Osaka Plain, inferred from a 2-D numerical analysis of viscoelastic-plastic models

Abstract:

The geometry (i.e., dip angle) of active faults from surface to the seismogenic zone is one of the most important factors to evaluate earthquake ground motion which is crucial for seismic hazard assessments in urban areas. In Osaka, a metropolitan city in Japan, there are some active faults (e.g., the Uemachi fault and the Ikoma fault), 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 sedimentary layers. Although the Uemachi fault and the Ikoma fault are N–S-striking fault system with a total length of 42 km and 38 km, respectively, and the former is located ~12 km west from the latter, the geometry of each active fault at the seismogenic zone is not clear. In this study, to clarify the geometry from surface to the seismogenic zone of the Uemachi fault and the Ikoma fault, we analyze that the development of geological structures of sedimentary layers based on numerical simulations of a two-dimensional visco-elasto-plastic body under horizontal compressive stress field, including preexisting linear weak zones (i.e., faults) and process of surface sedimentation, and evaluate the relationship between geological structures of the Quaternary sediments (i.e., Osaka Group) and topography in the Osaka Plain and model results. Based on the comparison between simulation results and geological structures (e.g., lateral variation of the thickness of surface sediments, sedimentation rates at some locations and vertical displacement rate along the Uemachi and Ikoma faults), we propose the geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone. When frictional coefficient for the faults is about ~0.4, 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 is not converged to Ikoma fault. Based on the analysis of this study, the dip angle of the Uemachi fault zone is estimated to be around 30°–40°, and then the downward extension of the Uemachi fault zone almost coincides with the epicenter of the 2018 northern Osaka earthquake.