Ground Motion Model Incorporating Azimuthal Dependence on Topographic Effects in Japan

Jangsoo Lee , Junyoung Lee, Byungmin Kim 

Conventional topographic terms in ground motion models (GMMs) typically characterize topographic effects using site-specific parameters such as relative elevation, curvature, or slope. This study demonstrates that topographic response is influenced by source-to-site geometry, specifically the incidence direction with respect to the slope aspect. Utilizing 15,189 records from 641 Kiban–Kyoshin network (KiK-net) stations in Japan, we analyze the corrected within-event residuals from which path, source, and site effects are eliminated. We introduce the azimuthal difference (α) between slope aspect and epicentral direction alongside the relative elevation (hr) to capture the directional influences of incoming seismic waves on topographic response. Our empirical findings reveal that topographic effects (amplification or deamplification) are determined by the interplay between hr and α, where facing slopes (α = 0°) at ridges exhibit maximum amplification, while back slopes (α = 180°) can induce deamplification. These responses exhibit a period-dependence, appearing prominently at short-to-intermediate periods (T ≤ 0.3 s) where the seismic wavelength is comparable to the topographic scale, but diminishing at longer periods as the wavelength exceeds the topographic scale. By parameterizing this azimuthal dependence into a topographic term (ftopo), the model effectively reduces the standard deviation of the residuals.