汶川M_S8.0地震河谷地形对汉源县城高烈度异常的影响

汉源县城位于汶川M_S8.0特大地震六度区内唯一的八度异常区,为典型且罕见的远震高烈度异常区.汉源县城处于流沙河的河流阶地之上,河谷地形对地震震害有显著的影响.为定量分析河谷地形对汶川大地震汉源县高烈度异常的影响,基于汉源县城震害科学考察和场地勘察获取的资料,根据震害分布特征和流沙河谷地形的特点,选取1条实测得到的横切汉源县城并垂直流沙河河谷方向的典型剖面作为计算模型,以脉冲作为基底输入,采用有限差分方法研究了该剖面的场地放大效应,分析了地形对高烈度异常的影响.计算结果表明:汉源县城场地对地震动放大效应的显著频段是1.0~10Hz,且这一频段老县城场地的放大效应比新县城场地显著;汉源老县城场地对汶川M_S8.0地震主震的地震动有显著的放大效应,地表峰值加速度大大超过了抗震设计规范的规定值;汉源场地地形放大效应显著频段与汶川M_S8.0地震的能量集中频段基本吻合,汉源老县城建筑物的自振频率恰恰位于该频段,产生共振效应,从而造成更显著的放大效应,这也是汉源震害异常的主要原因之一.由此可见,河谷地形对地表地震动有重要的影响,在工程选址和抗震设计时应考虑其影响.

Effects of river valley topography on anomalously high intensity in the Hanyuan town during the Wenchuan MS8.0 earthquake

The Hanyuan town is located approximately 200 km to the macro-epicenter of the great Wenchuan M_S8.0 earthquake. However, it is the only one within the intensity VIII zone, surrounded by a region of intensity VI. In landscape, this town lies on the second and third terraces of the Liusha River. The river valley topography has important influence on earthquake damage. Authors have obtained several geological profiles during the scientific survey and carried out the measurements of shear velocity in the typical boreholes in Hanyuan. To quantitatively analyze the effects of river valley topography on the intensity anomalies in this area, a profile, perpendicular to the Liusha River and passing through the town, is selected as the computation model reflecting the differences of damage. Taking the pulse waves as the base inputs, the site amplification effects of the selected profile are studied by employing the finite difference method of wave propagation to analyze the effects of river topography on the high intensity anomalies. The results show that the frequencies of 1.0 to 10 Hz are the most amplified by the site of Hanyuan town and the magnifying effects are more significant in the old town than in the new town. The sites in the old town have significant amplification effects on the ground motion of main shock during the Wenchuan M_S8.0 earthquake and the peak accelerations of the old town greatly exceeded the specified value in the code of seismic design of buildings. The most significant frequencies amplified by the sites of the Hanyuan town are consistent with the concentrated energy frequency bands of the Wenchuan M_S8.0 earthquake, and the natural frequencies of buildings in the old town are just within these frequencies, thus leading to resonance effects to cause more significant amplification effects. This is one of the main causes inducing the intensity anomalies in the town during the Wenchuan M_S8.0 earthquake. Therefore, the valley terrain has important influences on the ground motion, which should be considered in selecting project sites and seismic design.