# Study on the Seismic Effect of the Pebble Soil Site in the Zhongwei Basin
## 1 Introduction  
The Zhongwei Basin is located north of the Fragrant Mountains on the northeastern margin of the Tibetan Plateau and the southern margin of the Tengger Desert, where the Yellow River crosses from west to east. According to previous research results and on-site exploration, the pebble strata deposited in the basin have thicknesses ranging between 25 and 100 m, and the overlying silty clay layer ranges between 0 and 5 m. The stratigraphic structure is more unique than the general basin strata. There are several large-scale active faults around the basin. Historically, strong (large) earthquakes have occurredfrequently, causing serious disasters, such as the 1709 Zhongwei 71/2 \(M\)s earthquake. With the proposal of policy to support Ningxia's construction of the Yellow River Basin's ecological protection and high-quality development pilot zone, a series of infrastructure, industrial parks, housing reinforcement, and renovation projects are being conducted in the Zhongwei Basin, and new requirements for the seismic fortification of the project also entails new challenges. Therefore, research on the seismic effect of the pebble soil site in the Zhongwei Basin can provide a reference for earthquake-prevention and disaster-reduction work and seismic engineering design in the Zhongwei Basin. In 1928, [PERSON] [1] realized the importance of site conditions in the site seismic response when analyzing the seismic damage to buildings during the 1906 San Francisco earthquake. This finding has been shown by the following earthquakes: the 1923 Kanto earthquake in Japan, the 1967 Venezuela earthquake, the 1968 and 1970 Manila earthquakes in the Philippines, the 1975 Haicheng earthquake in China, the 1976 Tangshan earthquake, the 1985 Mexico earthquake, and the 2008 Wenchuan earthquake. In general, site conditions refer to the local geological conditions, such as the overlying soil layer, landforms, and fracture zone near the site's surface. With the development of strong earthquake observations and theoretical studies, numerous studies have shown that the seismic effect at the site is closely related to the soil properties, soil structures, overburden thickness, motion intensities, and other parameters. Essentially, the effect of site conditions on earthquake damage is the result of significant amplification or attenuation of the ground motion that originates in the bedrock. In recent years, the effect of near-surface soil conditions on ground motions has received increasing attention, and strong-motion observations demonstrate that the amplification effect of the surface ground motion is more evident in soft- to medium-hard clay and sandy soil grounds than in hard-soil conditions. In addition, the amplification effect has a large range of periods for soft- to medium-hard grounds, generally in the range of 0.3-1.1 s [1]. In contrast, the surface spectral acceleration for the hard soil site shows a significant amplification in the higher-frequency range. Using a one-dimensional equivalent linearization method, [PERSON] [2] investigated the influence of local site conditions on the peak values and the spectral characteristics of ground motion. [PERSON] [2] summarized some recent advances in seismic site effects and further analyzed the influence of different soil structures on the characteristic period of the surface-acceleration response spectrum based on several typical site profiles. [PERSON] et al. [3] conducted a numerical analysis on the influence of the burial depth and thickness of a soft soil layer on the ground motion at a deep site. The analysis showed that, with a special sediment thickness or input ground motion, the amplification effect decreases as the softness of the surface soil layer increases, and the variations in the peak ground accelerations and amplification ratios are not evident when a limit thickness is reached. Interestingly, the deamplification effect of ground motions can be observed when a very thick soft soil layer is located at the ground, acting as a natural damper. [PERSON] et al.