断层破裂面倾角变化对断陷盆地强地面运动的影响

地震事件中,断层破裂面的倾角大小直接影响到地表强地震动的分布状态,尤其在断陷盆地中,强地面运动特征还可能受到盆地结构及盆地内多条围限断层的影响.本文模拟了银川断陷盆地内的活动断层——银川隐伏断层南段发生Mw6.5特征地震时,断层破裂面倾角在30°~85°范围内变化时引起的强地面运动,探讨了断层破裂面倾角变化对盆地内强地面运动分布特征和强度的影响.结果表明:破裂面倾角较缓时,银川盆地内的强地面运动分布区域不仅仅决定于发震断层的产状,同时还受到断层上盘距离最近的芦花台断层的影响,致使强地面运动集中于两条断层所围限的区域;随着发震断层破裂面的倾角逐渐增大,强地面运动以发震断层产状的影响为主,强震集中区向发震断层靠近并分布于发震断层上盘,且沿断层走向方向出现了强度不同的地震动反射区;尤其是发震断层破裂面倾角接近垂直时,受银川盆地"西陡东缓"结构和盆地西边界贺兰山东麓断裂反射作用的影响,竖向地震动反射区强度在远离发震断层的西北方向明显增大,致使芦花台断层附近区域与银川断层南段上盘区域成为地震发生时可能遭受震害最严重的地区.本文探讨结果提醒我们在类似区域的活动断层附近进行建(构)筑规划和地震工程设计时,有必要考虑发震断层破裂面倾角大小和盆地内其它断层构造的共同影响,综合评价潜在地震对盆地内近断层地表及各类建(构)筑物的危害性.

Impact on strong ground motion from change of fault rupture dip angle in faulted basin

For a large-size faulted basin with many active faults developed, the strong ground motion in basin will be influenced by the following several factors, the fault dips, the activity modes, the expand shape of faults inside basin or along basin edges. In order to research the relations between the fault dip angles and strong ground motion characters in faulted basin, Yinchuan basin and Yinchuan buried fault are suitable for us to carry out our plans. In Yinchuan faulted basin, there are two major faults inside and two enormous faults stretching along west and east sides. Yinchuan buried fault, a Late Pleistocene epoch-Holocene active fault, is a major one of those activity faults which perhaps affect the urban planning and development and earthquake resistance of Yinchuan City seriously. In our research, according to the earthquake risk evaluation results of southern segment of Yinchuan Fault, we defined the characteristic earthquake magnitude M_w6.5 and assume different dip angles (30°, 45°, 60°, 66°, 75°, and 85°) for simulations of strong ground motion. By an improved Finite-Element Method, we computed all above cases of ground motion and draw following conclusions. For a gently dipping fault, the intensive distribution of ground motion appeared in the area between the southern segment of Yinchuan buried fault and the Luhuatai fault which means that the Luhuatai fault blocks the spreading of seismic wave. With the increasing of the fault dip angle, the strong ground motion mainly distribute on the hanging-wall close to the fault which shows that the fault angle of inclination is the major factor controlling the ground motion distribution. Furthermore, along the direction of the trend of Yinchuan buried fault, several areas of weaker ground motion formed gradually. When the fault dip became larger and larger until 85°, influenced by edge faults of Yinchuan basin with the occurrence of "steep in west edge and gentle in east edge", special reflection region of strong ground motion in UD component become stronger and stronger in the southwest area far away from the Yinchuan buried fault which makes the regions near Yinchuan fault and Luhuatai fault become two major possible risk zones if potential earthquake occurred. All above results remind us that it is quite necessary to consider the combined action of active fault dip angle and reflected waves from edge faults in faulted basin when we undertake the seismic safety evaluation and resistance for buildings and constructions in the near-field region.