Abstract:In recent years, several studies have been conducted on the effect of far-fault ground motions on deep soft soil sites; however, relevant research works for thick loess sites are still seldom. The Loess Plateau is seated on the upper and middle stream of the Yellow River in North China, covering an area of 440 000 km2, and the thickness of loess is up to above 500 meters. More than 1.4 million people have been killed by the earthquakes in this region, among which is the Wenchuan MS8.0 earthquake of 2008, which led to collapse of infrastructures, tremendous casualties, and economic losses. However, according to field investigations, observations, and analyses, the collapse and damage of buildings were not only caused by poor seismic performance or landslides, but also by amplified effects of site conditions, topography, and thickness of loess deposit on ground motion. In this paper, to explore the relationship between amplification factors and site conditions in terms of thickness of loess deposit, we chose a typical loess site for numerical analysis. Based on the drilling data of actual engineering sites, this paper selects 45 typical site profiles. Using the one-dimensional equivalent linear fluctuation method, the ground surface acceleration responses of each site were calculated under the input of ground motions with three different intensities. The results indicate the following: (1) Under the far-field ground motion, the PGA of loess site is large, and the platform of response spectrum is wide, mostly in 0.2~0.6 s; (2) the PGA decreases with increase in the loess thickness, but the earthquake damage on structures with natural periods above a certain range is aggravated; (3) for thick loess areas without topography, with increase of the loess thickness (20~100 m), the value of the descending segment of response spectrum amplifies 1.1~1.4 times, and the characteristic period amplifies 1.1~1.25 times. The numerical result is basically consistent with the field investigation; therefore, it is of great scientific and practical importance in earthquake engineering for seismic fortification in loess regions.