Abstract
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.
Highlights
The 2018 northern Osaka earthquake occurred on June 18, 2018, and its epicenter was located at the junction of the Arima-Takatsuki fault zone and the Ikoma faultNishiwaki et al Earth, Planets and Space (2021) 73:86Earthquake Research Promotion of Ministry of Education (MEXT) 2001, 2004; Director General for Disaster Management 2006; Fig. 2).To mitigate earthquake damage, it is essential to predict ground motion caused by earthquakes that occur on active faults, and the attitude of the source fault in the seismogenic zone is one of the most important factors used to predict the ground motion
In the shallow part of the active faults with low dip angles, the fault plane is close to the ground surface and peak ground acceleration will be higher (e.g., Midorikawa 1993)
Concluding remarks In this study, to examine the geometries of the active 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 and surface sedimentation processes, and evaluate the relationship between the observed geological structures of the Quaternary sediments in the Osaka Plain and the model results
Summary
The 2018 northern Osaka earthquake occurred on June 18, 2018, and its epicenter was located at the junction of the Arima-Takatsuki fault zone and the Ikoma faultNishiwaki et al Earth, Planets and Space (2021) 73:86Earthquake Research Promotion of MEXT 2001, 2004; Director General for Disaster Management 2006; Fig. 2).To mitigate earthquake damage, it is essential to predict ground motion caused by earthquakes that occur on active faults, and the attitude of the source fault in the seismogenic zone is one of the most important factors used to predict the ground motion. Because geological and geophysical observations are restricted to shallow depths, the attitude of the active faults at the depth of the seismogenic zone is not well understood. A numerical analysis would be useful to estimate the attitude of the active faults at depth, based on a comparison between the results of numerical analyses and the geological and geophysical observations. In the Osaka Plain, the Kinki region, Japan (Fig. 1), to clarify the distribution of active faults, many geological and geophysical observations have been performed, and it is a good field for the evaluation of the attitude of the active faults at depth estimated by a comparison between the results of the numerical analysis and the geological and geophysical observations
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