基于带状震源破裂机制的斜坡动力响应

Dynamic response of slope based on fracture mechanisms of strip-shape hypocenter

对紧邻发震断裂带的崩滑体进行动力响应分析时,考虑断裂带宏观破裂机制,即带状震源形成的地震动力作用对斜坡的影响更符合实际。依据汶川地震震源破裂机制与其空间位置差异,将该带状震源从发震断裂起破点至终破点依次分段为逆冲震源、逆冲兼少量走滑震源、逆冲兼走滑震源与走滑兼少量逆冲震源,基于此对汶川地震触发的四川安县大光包崩滑体在龙门山发震断裂带即带状震源作用下的动力响应特征进行了离散元数值模拟,揭示了其动力形成机制、触发主控因素和损伤、崩滑及堆积动态特征。研究表明:(1)依据该崩滑体离散元数值模型临界崩滑状态形成时间与此时发震断裂带破裂前锋所处位置关系,可判断其临界破坏是受到逆冲兼少量走滑震源引起的地震动力作用所致,而临界崩滑之前的坡体损伤主要由纯逆冲震源所致,其后的抛射碰撞破碎与堆积则主要受临界破坏时的地震惯性力和自身重力耦合作用所致,但由逆冲兼走滑与走滑兼少量逆冲震源形成的地震力仅对上述两个破坏过程起到了一定影响;(2)该崩滑体在带状震源作用下的动力响应过程为:在损伤至崩滑临界破坏阶段,坡体整体向其临空面发生了较大程度的水平位移后,潜在滑床又向坡体临空面反方向发生了一定程度的水平位移,致使潜在滑体完全破碎并处于与滑床彻底分离的临界状态;在坡体崩滑抛射阶段,坡体滑床发生了相当规模的反方向水平位移,其后滑床又开始做向坡体临空面方面的水平位移并直至其总体位移为0,而在此过程中竖向位移相对较小。对滑体而言,其在损伤、临界崩滑和抛射阶段则主要做向其临空面的水平位移,直至堆积自稳阶段其位移趋于稳定;(3)该坡体的损伤和临界崩滑破坏主要受纯逆冲震源及其少量走滑震源形产生的水平地震力作用所致,而在坡体抛射碰撞破碎与堆积阶段,滑体的动力响应主要是基于地形因素控制上的地震惯性力与自身重力作用所致,而后两种类型震源机制形成的水平和竖向地震力仅起到一定影响。

According to the realistic damage characteristics of slopes on the seismogenic faults as well as the fracture mechanismss of the faults in the Wenchuan earthquake, a so-called strip-shape hypocenter is employed to assess the dynamic responses of the typical slopes. Furthermore, the strip-shape hypocenter can be divided into four stages in accordance with their spatial locations from the initial break to the end break of the seismogenic fault. These stages consist of the thrust fault hypocenter, the thrust and a bit strike-slip fault hypocenter, the thrust and strike-slip fault hypocenter and the strike-slip and a bit thrust fault hypocenter. Thus, the distinct element method is applied to simulate the dynamic response of the Daguangbao landslide on the Longmenshan seismogenic fault in Sichuan province in China. Then, the dynamic formation mechanism, the triggered main controlling factors, and dynamic characteristics of damage, collapse and accumulation are revealed. The results show that the critical damage of the slope is mainly caused by the thrust and a bit strike-slip fault hypocenter, by considering the location of the front end of the fault break and the formation time of the critical damage. While before the critical damage of the slope, its damage is induced by the thrust fault hypocenter between the initial break of the faults and Wenchuan county. The following processes of ejecting, colliding, crushing and accumulating are mainly triggered by the coupled inertia force and the gravitational force, which are also influenced at a certain degree by the above two kinds of hypocenters with respect to their fracture mechanisms. Before the critical damage, the whole slope underwent a positive horizontal distance towards its free face. Then, the slide bed begins a negative horizontal distance away from its free face with a continuous positive horizontal distance of the slide mass, which results in a critical separation between them. At the ejecting, colliding and crushing stage, the slide bed continues its negative horizontal displacement to an extreme limit, then a positive horizontal one and followed by its zero horizontal displacement. The vertical displacement at this stage is relatively small. The motion characteristics of the slide bed in this process is in accordance with that of the monitoring seismic data, for example, the acceleration, the velocity and the displacement. Meanwhile, the slide mass continues its positive displacement to a constant limit with a colliding, crushing, accumulating to self-stabilization process. As far as the mechanical factors are concerned, the horizontal seismic force formed by the single thrust fault hypocenter and the thrust and a bit strike-slip fault hypocenter provides a key contribution to the whole slope including the slide bed and the slide mass before the critical damage state. At the ejecting, colliding, crushing and accumulating stage, the dynamic responses of the slide mass is mainly influenced by the inertia force and the gravitational force. However, on the basis of a special topography, the horizontal and the vertical seismic forces from the following two hypocenters give their finite influences to the slide mass.