Site effect of unconsolidated soil hill based on seismic array records

Following the 2008 M_S8.0 Wenchuan earthquake, a topographical array including four digital seismograph stations was installed along Shangcheng hill, Wenxian, Gansu Province, which is a piedmont diluvial terrace about 50-m high and made up of Quaternary silty clay and gravelly soil with thin layer of coarse sand. One of the stations was installed at a rock cave in northeastern Shangcheng hill, and the other three stations were respectively installed at the foot of the hill, hill slope and hill top. From May 25 to August 7, 2008, a series of aftershocks of Wenchuan M_S8.0 earthquake occurred, but only twelve events (M_S≥4.0) were recorded at three of the four stations at least. In this study, seismic response of each station is evaluated as a function of frequency by reference site spectral ratio (RSSR) method and single-station horizontal to vertical spectral ratio (HVSR) based on all shear waves from the twelve earthquake events. The analyses lead to the conclusions that: ① with the elevation climbing and soil thickening, the PGAs of the twelve events increase and the broadband seismic spectra gradually narrow to the characteristic frequency band; ② dynamic amplification of Shangcheng hill occurs in both topographic characteristic frequency band (2--4 Hz) and shallow stratigraphic fundamental frequency band (7--9 Hz)in horizontal direction, while amplification caused by the soil stratigraphy near surface is comparatively more obvious than that by the topography in NS at hill top; ③ in medium-high frequency band, the spectral ratio from HVSR method is much lower than that from RSSR method due to the amplitude of seismic waves in vertical direction. The characteristics of seismic array records of the unconsolidated soil hill give expression to the combined effect of topography and soil stratigraphy, and reveal the reasons for the aggravation of structural damage and the concentration of seismic landslide on topographic site in strong ground motion zone.