During the 2008 Wenchuan earthquake, the seismic intensity in the old town of Beichuan reached Grade IX, causing extremely heavy casualties and catastrophic damage. Among the many factors affecting earthquake disasters, site conditions are among the most critical. As an important site condition, the soil layer structure exerts a significant influence on the amplitude and frequency spectrum characteristics of seismic ground motion. Therefore, it is essential to investigate the effect of soil layer structure on ground motion in severely damaged areas.

To explore the influence of soil layer structure on the earthquake damage in the old town of Beichuan, field drilling investigations were conducted in the region. The drilling program focused on the earthquake ruins of old Beichuan, covering three typical affected areas: the Jianhe River Valley, the old town of Beichuan, and the new town of Beichuan. A total of nine engineering geological boreholes were completed, with a cumulative drilling depth of 281.1 m. One borehole was drilled in front of the former Beichuan Tea Factory building in the old town area. In the new town area, two boreholes were placed in front of the eastern building of Beichuan Hotel and beside the ruins of the former County Committee Building, respectively. Two borehole rows, each consisting of three boreholes with generally consistent orientations （northern row: 90°, southern row: 87°）, were set up in the Jianhe River Valley. All boreholes were drilled to the bedrock surface, with an average depth of approximately 31.9 m and an average overburden thickness of 17.0 m. A detailed soil layer structure of the Beichuan old town site was obtained.

In the river valley area, the soil is dominated by sandy gravel with a relatively uniform structure. In the old town area, the surface layer consists of 1.50–3.00 m of yellow clay （alluvial deposits formed after debris flows）, underlain by sandy gravel extending to the bedrock. In the new town area, the overlying strata are mainly composed of gravelly soil and sandy gravel. All boreholes were implemented in accordance with the Code for Investigation of Geotechnical Engineering （GB 50021-2001, 2009 edition）, including soil sampling, standard penetration tests, shear wave velocity measurements, soil layer classification, and geotechnical property description. Engineering geological profiles were then constructed.

Based on the field investigation data, the study site was classified as a Class Ⅱ medium‑stiff site. Combined with a comprehensive review of relevant literature, dynamic soil parameters and input ground motions for different zones were determined. Calculation profiles for seismic soil response were established, and the one‑dimensional equivalent linearization method was used to perform seismic response analyses, from which the surface ground motion parameters at each borehole were obtained. The peak ground accelerations and response spectrum characteristics in different areas were analyzed and compared with the original design ground motion parameters.

The influence of the soil layer structure on the surface ground motion parameters in the old town of Beichuan was revealed. A comparative analysis with the actual earthquake damage observed during the Wenchuan earthquake shows that the numerically predicted vibration‑damage zones correspond well with the field damage distribution, demonstrating that the soil layer structure had a significant effect on the earthquake damage.

The main conclusions are as follows:

1） The site of old Beichuan is mainly composed of coarse‑grained soils such as gravelly soil and sandy gravel, with an average overburden thickness of approximately 17.0 m, which gradually thins toward the surrounding mountains. The site is classified as Class Ⅱ （medium‑stiff）.

2） The site presents obvious seismic ground motion amplification effects. The amplification factors in the river valley area range from 1.08 to 1.29, and the old town area has the highest peak ground acceleration and the most significant amplification. The river valley area amplifies both short‑period and long‑period ground motions, with a stronger effect on long‑period components. The new town area amplifies ground motions within a specific period range （0.04 s–0.4 s）, whereas the old town area shows more significant amplification of short‑period ground motions.

3） The characteristic period of the normalized response spectrum in the old town of Beichuan ranges from 0.35 s to 0.75 s, which exceeds the specified range for Class Ⅱ sites （0.4 s–0.45 s）with a 10% exceedance probability in 50 years according to the Code for Seismic Design of Buildings （GB 50011-2010, 2024 edition）. Meanwhile, the horizontal seismic influence coefficient exceeds the level for major earthquakes （2% exceedance probability in 50 years）, indicating that the actual seismic intensity experienced by the old town of Beichuan far exceeded the designed fortification level.

4） The amplification of seismic ground motions by the loose coarse‑grained site soils contributed significantly to the catastrophic earthquake damage in the old town of Beichuan.