Basins represent a special type of site condition. Their complex assemblage of sedimentary soils and basin basement structure can produce substantial seismic amplification, known as basin effect. At the same time, the complex shape of the basin basement and the loose sediments within the basin can lead to ground motion focusing effects and the basin-edge effects, accompanied by a substantial increase in earthquake duration. Consequently, buildings located within basins often suffer severe damage during earthquakes. However, many large and medium-sized cities worldwide are situated in basins, such as Beijing, Xi’an, Yinchuan, Kunming, Chengdu, Fuzhou in China, Tokyo and Osaka in Japan, Los Angeles in the United State, Mexico city in Mexico, Umbria in Italy. Many scholars at home and abroad have emphasized that the study on earthquake damage mechanism of basins should be the focus in urban earthquake prevention and disaster reduction.

This paper analyzed the basin effect on ground motion in Dayao basin using microtremor tests and numerical simulations. The study aims to reveal ground motion characteristics of small-scale basins and provide a theoretical basis for rational determination of ground motion parameters of engineering structures in Dayao basin. In the ground pulsation test, data denoising was performed using baseline correction, digital filtering, superposition average and other methods. The standard spectral ratio method （H_S/H_R） was adopted to analyze the spectral ratio of the ground pulsation records after processing at each observation point on soil sites within Dayao basin, from which the predominant frequency and site spectral ratio amplification coefficient at each observation point were obtained. In the numerical simulation, the 2D model of Dayao basin was analyzed using the dynamic finite element method, and the dynamic amplification coefficients and spectral characteristics at each characteristic point were obtained.

In the ground pulsation observation test, the standard spectral ratio results for each observation point show that the predominant frequencies of measurement points SP1−SP3 are mainly concentrated in the high frequency band 6−9 Hz, and amplification coefficients in E-W direction are greater than those in N-S direction, with E-W values ranging from 2.5 to 4.2 and N-S values from 1.1 to 1.3. Measurement points SP4–SP6 exhibit similar characteristics: their predominant frequency are also concentrated in the high frequency band of 6−9 Hz, and the E-W amplification coefficient （2.5–4.2） is greater than the N-S coefficients （1.1−1.3）. For SP7 and SP8, the predominant frequencies are mainly distributed in the low frequency band of 1−3 Hz, and the E-W and N-S amplification coefficients are relatively close. In addition, the magnification coefficients differ somewhat between the E-W and N-S directions, and the spectral ratio curves show multiple peaks.

In the numerical simulation analysis, the spectral ratio characteristics at each observation point indicate the dominant frequency of 1−3 Hz and 6−9 Hz, with the most prominent peak occurring at 6−9 Hz. The amplification coefficients vary among different observation points, with a maximum of 20.8. The magnification coefficients also differ between the two subbasins, with softer soil yielding larger values. Furthermore, the amplification coefficient of the steeper basin margin is greater than that at the gentler basin margin.

The spectral ratio curves of each soil layer in Dayao basin show multi-peak characteristics, indicating that the site does not consist of a single soil layer. The predominant frequencies at most observation points within the basin are concentrated in 6−9 Hz, and certain differences exist among these frequencies, reflecting the variations in site stiffness at different observation points. The amplification coefficient showed a certain difference between the E-W and N-S directions, with a maximum of 4.2, indicating the anisotropy of the site. In the numerical simulation analysis, the amplification coefficients reveal a distinct basin-edge effect and the ground-motion focusing effect. The different amplification coefficients of the two small basins reflect the significant influence of the soil layer structure on ground motion, and the amplification coefficient is larger than that of the soft soil layer. The amplification coefficient at the steeper basin margin is greater than that at the gentler margin, demonstrating that the slope gradient of the basin edge has a significant effect on the ground motion near the basin boundary. The results indicate that the H_S/H_R method has good reliability in basin effect analysis, and the combined application of the microtremor observation and numerical simulation can reveal the mechanism of basin effect more effectively.

The different amplification coefficients between the two small basins reflect the significant influence of soil layer structure on ground motion, with thicker or softer soil layers corresponding to larger amplification.