近断层强地震动的特点

本文全面地总结和解释了近断层地震动的主要特点.近断层地震动与远场地震动不同,显著地受到下列因素的影响:断层破裂机制,相对于场地的破裂传播方向,以及由断层滑动产生的、可能的永久地面位移.这些因素导致破裂方向效应和“F ling step”效应.对于倾滑断层,还导致上盘效应.而且破裂方向效应和“F ling step”效应分别引起了双向和单向长周期速度脉冲.上盘效应导致上盘场地的地震动大于下盘相同断层距场地的地震动.对于垂直走滑断层,在给定的距断层最近的范围内,破裂方向效应使地震动产生强烈的空间变化.对于倾滑断层,有两个显著的效应,即破裂方向效应和上盘效应.上盘效应主要是由断层的大部分接近于上盘之上的场地引起的.破裂方向效应是由破裂传播和辐射图效应引起的.长周期速度脉冲产生于垂直断层面的方向上,使得垂直断层走向的地震动大于平行断层方向的地震动.而“F ling step”效应是由地震时断层两盘的相对运动造成的,且发生在断层错动的方向上.

Characteristics of near-fault strong ground motions

Main characteristics of near-fault strong ground motions are summarized and explained.Near-fault ground motions are significantly different from those further away from the seismic source.And they are significantly influenced by the rupture mechanism,the direction of rupture propagation relative to the site,and possible permanent ground displacements resulting from the fault slip.These factors result in effects referred to as "rupturedirectivity" and "fling step",and also result in effects referred to as "hanging wall".The effects of "rupture-directivity" and "fling step" result in different velocity pulses respectively: two sided and one sided.The hanging-wall effect causes larger ground motions on the hanging wall than on the foot wall at the same closest distance.For vertical strike-slip faults,the rupture directivity effect causes as strong spatial variation in ground motions for a given closest distance to the fault.For dip-slip faults,there are two prominent effects: the rupture directivity effect and hanging wall effect.The hanging wall effect is due mainly to the proximity of much of the fault to hanging wall sites. The rupture directivity effect is due to rupture propagation and radiation pattern effects.The radiation pattern of the shear dislocation on the fault causes length-period velocity pulse is oriented in the direction perpendicular to the fault plane,causing the strike-normal component of ground motions to be larger than the strike-parallel component at periods longer than0.5 seconds.The fling step effect in near-fault strong ground motions is caused by the relative movement of the two sides of the fault on which the earthquake occurs,and oriented in the direction parallel to the fault dislocation.