Impact of topographic effect on ground motion characteristics in the extreme seismic region of Yangbi M_S6.4 earthquake on May 21，2021

Historical earthquake experience has shown that complex terrain in mountainous areas can exacerbate earthquake damage. Complex terrain can alter the duration, amplitude and frequency characteristics of seismic waves, causing unanticipated and severe damage to buildings located on them. On May 21, 2021, a M_S6.4 earthquake occurred in Yangbi, Yunnan Province. Near the epicenter of the earthquake, Xiuling village is located in a mountain range with a straight line distance of about 4.5 km from the top to the foot of the mountain, with an elevation difference of about 800 m. The investigation found that the damage to the houses on the top of the mountain was serious, generally manifested as wall collapse or serious damage; the damage to the houses on the mountainside was moderate, manifested as wall collapse and foundation settlement; the damage to the houses at the foot of the mountain was relatively light, without wall collapse, and was dominated by the tensile cracks in the contact surfaces of the structural columns and load-bearing walls. According to the difference in topographic seismic damage, four strong motion observation stations were set up at the top of the mountain, the mountainside and the foot of the mountain in Xiuling village, respectively, on June 8, and a reference station was set up in Huai’an village, which is located in a basin, with a total of five stations, and a total of 63 aftershocks with magnitudes ranging from 1.0 to 4.9 were recorded by the topographic mobile observation station array. According to the spatial distribution of mobile stations and aftershocks, the size of aftershocks and the mountain range orientation, three earthquakes with different magnitudes were selected for acceleration recording analysis, and the analysis results show that when the seismic waves are incident perpendicular to the mountain range orientation, there is anomalous amplification of the high frequency at the steep slopes of the mountainside, and the low-frequency portion of the Fourier amplitude spectrum at different elevations differs significantly along the mountain range orientation and the vertical orientation, at the same time the analysis results of the basin station under the three seismic effects also reflect the directional difference of the basin effect. A method is used to calculate the site effect term κ₀ of the high-frequency attenuation parameters, and the correlation between the horizontal and vertical components of κ₀ and the site softness and predominant frequency is analyzed, and the results show that, in the close range （＜30 km）, κ_H correlates with the softness of the local site conditions, which is determined by the predominant frequency and the peak amplification bandwidth of the site, and κ_V has a weaker correlation with local site softness, both of which have a decreasing trend with increasing local site predominant frequency, κ_V is especially obvious. Due to the relatively small number of observation stations, the relevant conclusions are only for the present study area. Considering the complexity of the actual mountainous terrain, more actual observational data are needed for validation. Through further verifying, it can provide a useful reference for the site adjustment estimated from ground motion impact on a small scale.