黄土丘陵河谷场地地震动特征研究

研究黄土丘陵河谷场地在地震作用下强地面运动特征的变化情况,可以揭示强震对该类场地上震害的触发机理。结合黄土高原的地貌特征,建立具有代表性的动力数值分析模型,通过输入不同幅值、频谱特性和持续时间的地震波,对起伏地形和覆盖黄土层共同影响下的黄土河谷场地进行地震反应分析。结果表明:黄土层和地形耦合作用控制了地表的PGA变化,使其趋于复杂,在同一输入波不同振幅作用下,与基岩河谷各测点相比,黄土覆盖河谷场地的地震动频谱幅值均有所增加,并且频谱主峰均向高频移动。在不同地震波输入下,场地不同部位的固有频率受地形高程和土层影响;而地震动大小和频谱幅值不仅与场地的基本频谱和地形起伏有关,也与输入地震波的频谱成分相关。输入波PGA与地震频谱特征都不变时,同一场地输出的地震频谱形状具有相似的特征,随着地震持时增长,能量向场地基本频率附近集中,从而可能导致场地上相应频率建筑物震动幅值增加,造成累积破坏。

Study on Ground Motion Characteristics in Loess Hill Valley Sites

The trigger for seismic damage may be determined by researching the characteristics of strong ground motion in loess hill valleys under earthquake action. In this paper, considering the geomorphic characteristics of the Loess Plateau, typical models of dynamic numerical analysis are established. In a valley site influenced by rugged terrain and a loess covering layer, the seismic response was analyzed by inputting seismic waves with different amplitudes, frequency spectrum characteristics, and duration. The results show that the coupling action of the soil layer and terrain controls the PGA change on the ground surface and that the change trend is complex. Under the input of the same wave with a different amplitude and compared with monitoring points in the bedrock valley, the spectrum amplitude of the ground motion at the loess-covered valley points increased, and the peak moved to a high frequency. Under the input of different seismic waves, the natural frequencies of different parts of the site were influenced by the terrain and the soil layers; the size and amplitude of the ground motion frequency spectrum was not only related to the natural frequency spectrum of the site and topographic change, but also to the spectral components of the input seismic wave. At the same site, with the same PGA and seismic spectrum characteristics of the input wave, the output seismic spectrum shape showed similar characteristics. As the seismic duration increased, the earthquake energy was concentrated in the field near the natural frequency, which may lead to an increase in the vibration amplitude of buildings with a corresponding frequency.