【摘要】 同震地表破裂带是地震破裂最直观的表现。地表破裂带的精细填图对理解地震破裂过程及发震机理有着重要意义。我们对5.12汶川地震映秀-北川同震破裂带南部的虹口乡区段开展了较为精细的填图工作,该区段地表破裂带的平面几何和同震位移特征沿走向均显示出公里尺度的横向变化和复杂性。根据同震破裂不连续性特征可把地表破裂分为4段,自南向北依次为深溪沟段、庙坝段、高原新村段和八角庙段。其中,深溪沟段和八角庙段同震垂直位错较大,达5～6m;庙坝段同震垂直位移量普遍较低,1～2m或更低;在高原新村附近,地表破裂带分为近平行的南北两支,北支以右旋走滑为主,南支以南东盘逆冲抬升为主,这与与映秀-北川同震破裂总体北西盘抬升的性质相反。本文对高原新村附近的反向陡坎进行了深入的探讨,并综合前人的研究结果,认为是破裂在从深部往上扩展的过程中,在浅表部位顺着发育在虹口东南部的大鱼洞-龙溪飞来峰后缘的具南东倾向的断面继承而成。与北川沙坝附近的反向陡坎一样,说明老断裂带构造几何特征对单次地震破裂的扩展有牵引作用。最后,讨论了八角庙有切割关系的两组断层擦痕的地震学和地质学意义,指出可能指示了映秀-北川断裂面上局部区段震前的初始应力水平较低。更多还原

【Abstract】 Surface rupture is most straightforward demonstration of earthquake rupture features.Detailed and exhaustive mapping of surface rupture zones is important for understanding better earthquake mechanism and dynamic rupture processes.In this paper,we document our effort in fine-scale mapping of the 5.12 Wenchuan earthquake surface rupture on the Yingxiu-Beichuan fault near Hongkou.Surface rupture in this short stretch shows clearly large alongstrike variations in both map-view geometry and coseismic slips.Based on the discontinuities in slip and geometry, the rupture can be divided into four sections,from southwest to northeast: the Shenxi Gou,the Miaoba,the Gaoyuan and the Bajiao Miao sections,respectively.The vertical offsets are larger in the Shenxi Gou and the Bajiao Miao sections,reaching locally 5～6m in maxima,and generally lower in the Miaoba section,1～2m or less.Near the Gaoyuan village,the surface rupture consists of two sub-parallel branches.The northern branch shows mostly rightlateral slip with minor SE side-up thrusting,and the southern one mostly SE side-up thrusting.The sense of vertical throw is thus opposite to the general NW side-up thrusting of the Wenchuan rupture.We propose that it is due to the inheritance at shallow depth of the SE-dipping fault of the klippen system in the south.Together with the locally reversed fault plane near the Shaba village,Beichuan,it demonstrates that the structures of old fault zones in guiding earthquake ruptures in the up-dip propagation of earthquake rupture.We also discuss the cutting relationship of slickenside striations of two different plunges and their geological and seismological implications,by pointing out that the curved striations may indicate the low initial shear stress at some localities on the Yinxiu-Beichuan fault before the Wenchuan earthquake.更多还原