2008年汶川地震近断层竖向与水平向地震动特征

Characteristics of near-fault vertical and horizontal ground motion from the 2008 Wenchuan earthquake

选取分布在北川映秀中央断裂两侧断层距120 km以内的40个强震动台站的记录,对汶川地震近断层地震动竖向和水平向加速度峰值、速度峰值、竖向和水平向加速度反应谱及谱比值进行了统计分析.研究表明:(1)地震动加速度峰值有显著的上盘效应,经验衰减模型的结果表明,在距地表破裂3～60 km的范围内,龙门山发震断层上盘一侧竖向与水平向的加速度峰值要比衰减模型得到的平均值大30%～40%.上盘的加速度峰值残差大部分是正值,而断层下盘残差大部分为负;水平地震动的东西分量幅值总体要大于南北分量,东西分量衰减相对较慢.(2)地震动长周期成分较弱,加速度反应谱值随周期增大而迅速减小,在周期1.0 s时,即使在靠近中央断裂的最大加速度反应谱值也只有0.5 g;地震动加速度反应谱谱比值(竖向/水平向)沿龙门山断层周围的分布,在较长周期(T=0.2 s,0.5 s,1.0 s)与短周期(T=0.05 s,0.1 s)有明显的不同.(3)近断层竖向地震动显著,地震动加速度峰值比在(竖向/水平向)可达1.4.在龙门山发震断层的上盘,地震动加速度峰值比整体上比下盘要大,竖向地震动尤为剧烈.部分近断层记录的地震动谱比值(竖向/水平向)在短周期(<0.1 s)甚至超过1.5,统计分析还表明谱比值在短周期段(<0.1 s)随断层距的增大而减小.

Characteristics of near-fault vertical and horizontal ground motion from 2008 Wenchuan earthquake are investigated in this paper. Records from 40 strong motion stations along the Beichuan-Yingxiu fault with fault distance smaller than 120 km are used as database. Based on the analysis of peak ground acceleration, peak velocity, spectra acceleration and spectra ratio of vertical to horizontal ground motion, the following conclusions can be made： （1） Hanging wall effects are shown in PGA for near fault ground motions within 60kin to the fault rupture, the empirical model results in up to a 30%-40% increase in peak horizontal and vertical accelerations on the hanging wall over the distance range of 3 to 60km relative to the median attenuation for the Wenchuan earthquake. The peak acceleration residuals on the hanging wall appear to be biased to positive values whereas the residuals for footwall sites are negative. The EW component of horizontal accelerations is larger than the NS component and attenuates more slowly. （2） There is a lack of long period component in near-fault ground motion of Wenchuan earthquake, the value of spectrum acceleration decrease rapidly in the long period range. The maximum spectrum acceleration is only 0.5g even at locations very close to the main causative fault at period 1.0 s. Large distinctions are shown in the distribution of acceleration spectrum ratio （Vertical/ Horizontal） for near-source ground motion at longer periods （T=0. 2 s, 0. 5 s, 1. 0 s） with respect to short periods （T=0.05 s, 0. 1 s）. （3） High intensity of UD component is shown for near-fault ground motion, the PGA ratio （Vertical/Horizontal） can be as large as 1. 4. High intensity of UD component is more obvious on the hanging wall, and the PGA ratios on the hanging wall are much larger compared with the footwall. For some near-fault records, the acceleration spectrum ratio （Vertical/Horizontal） can even be larger than 1. 5. It can also be concluded that the acceleration spectrum ratio decrease with the increasing of fault distance at short periods （〈0.1 s）.