芦山7.0级地震近断层地震动的方向性

基于芦山7.0级地震中断层距小于100 km自由场台站的强震动记录观测数据，研究此次地震近断层地震动的方向性特性，并探讨方向性特性与震源破裂机制、断层距离和空间方位的关系.研究结果表明：（1）与距断层较远记录不同，近断层地震动在不同的观测方向上表现出显著的强度差异，存在明显的极大和极小作用方向.在不同的方向上，最大加速度反应可达最小值的4倍以上；（2）这种方向性差异在T=1.0 s以上的长周期段更为明显，在T=0.1 s以下的短周期段，地震动随方向变化的差异较小.地震动强度随方向变化的差异随周期增大而增大，不同方向上的加速度反应谱值的最大值与最小值之比从周期T=0.01 s时的约1.7增大到周期T=10 s时的约2.4；（3）在距离断层约35 km以内，地震动具有明显方向性，地震动卓越方向具有垂直断层走向的特征，随断层距的增大，这种方向性不明显.从不同方向上地震动强度的差异来看，随断层距增大，地震动强度在不同方向上的差异在减小，表现为各个周期的最大值/中值和最大值/最小值比值均随断层距离增大缓慢减小；（4）近断层地震动的方向性特性主要受断层上、下盘的相对运动所控制，其在长周期的卓越方向与水平同震位移方向一致，且该卓越方向上的地震动强度绝对大小与地震破裂造成的静态位移明显相关，表现为地震动强度随水平同震位移的增大而增大.

Variations of near-fault strong ground motion with directions during the 2013 Lushan MS7.0 earthquake

This work is based on the strong ground motion recordings of 22 free-field stations within 100 km to the causative fault of the 2013 Lushan M_S7.0 earthquake. We investigate the variations of near-fault ground motion with orientations, and its relationship to focal mechanism, fault distances and spatial locations. The results show that (1) the near-fault strong motion exhibits distinct variations with orientations, with clear maximum and minimum response directions. In various directions, the largest acceleration response can exceed 4.0 times the smallest. (2) The variation with orientation is obvious at long periods, while relatively small at short periods of T<0.1 s.Variation in Intensity Measures (IMs) with orientation increases with increasing periods. The average maximum-to-minimum ratio increases from about 1.7 at 0.01 s to about 2.4 at 10.0 s. (3) Ground motion exhibits strong directionality within about 35 km from the fault, with maximum-value direction perpendicular to the fault strike, which becomes unclear with the increase of the fault distance. Variations in IMs decrease with the increase of the fault distance, both the maximum-to median and maximum to minimum ratios decrease skowly with the increasing distance. (4) The directionality is determined by the relative motion between the hanging wall and footwall of the fault. The prominent orientation of long-period motion coincides with the direction of co-seismic displacement, and the maximum IMs are correlated with the static displacement, shown as increase of IMs with the increasing static displacement.