2008年汶川大地震小鱼洞地表破裂带精细填图

Detailed mapping of the Xiaoyudong coseismic surface rupture of Wenchuan earthquake

5·12汶川MW7．9级地震为罕见的、地壳尺度位移配分于多条平行断裂的板内逆冲走滑型地震。在2条北东走向、近平行的主要地表破裂问，发育北西走向的小鱼洞地表破裂。介绍了对小鱼洞北西向地表破裂的精细填图。小鱼洞地表破裂空间上位于灌县一江油与映秀一北川断裂间，全长约8km，总体走向310°，为南西盘抬升、逆冲兼具左旋走滑性质。地表破裂在南东端走向变化较大．从300-310°变为南北向。并与灌县一江油地表破裂带的磁峰段相连。小鱼洞地表破裂的垂向位错自北西往南东方向递减，北西端陡坎高度最大3．4m，南东端则小于0．2m，衰减梯度约为0．5m／km。左旋走滑位移测量点较少，集中在中段的小鱼洞镇附近．所测最大左旋走滑位移约为2．2m,一般走滑位错与同处垂直位错具有同步变化的特征。小鱼洞断裂近地袁的倾角较缓。为300±15°。结合已有地貌、地球物理和地质研究结果，提出小鱼洞断裂是向下与灌县一江油断裂交会的侧向断坡，位于映秀一北川断裂中南段间的断面倾角差异的撕裂部位，连接映秀一北川和灌县一江油断裂。在运动学上，认为小鱼洞断裂是以斜向断坡为几何形态的撕裂断裂，调节了北东走向的主断裂的运动学横向差异。小鱼洞断裂上的同震位移矢量与N70°、80°E的区域主压应力场方向匹配。这一方向与龙门山高原边界斜交。

The 12th May 2008 Wenchuan earthquake is one of the very few reported continental oblique reverse-faulting events with crustal-scale co-seismic slip partitioning on parallel reverse faults. Between the two principal NE-striking ruptures, the NW-striking Xiaoyudong also broke during the quake. This paper reports a detailed mapping and coseismic slip measurements along the Xi- aoyudong rupture. The generally N55°W-striking Xiaoyudong rupture extends for -8kin and accommodates a left-lateral and SW- side up oblique motion. In the map view, the rupture zone has a relatively stable strike over the most part of the fault, but changes rapidly at its southeast end, where the strike changes from N55~65~W to nearly north-south southward, and then connects with the Cifeng section of the Pengguan fault. Fault scarp is highest in the northwest part, reaching 3.4m, and decreases southeastward to lessthan 0.2m at the southeastern end, with the average slip gradient of 6°104 along the strike. Left-lateral offset measurements are sparser than the vertical, and are concentrated in the nfiddle reach near Xiaoyudong Town. The maximum observed left-lateral displacement is 2.2m. Left-lateral offsets co-vary with the vertical component. The near-surface fault has low to medium dip angle, being 30°＋ 15% Geological and geophysical evidence suggests that the Xiaoyudong fault is likely a -30°SW-dipping lateral ramp that soles into the Pengguan fault, and at its northwestern end intersects with the Beichuan fault, where the latter has a step in the fault plane. Kine- matically, the Xiaoyudong fault likely functions as a tear and conjugate fault, connecting and transferring coseismic slip transfer between the Beichuan and Pengguan faults. Xiaoyudong rupture is a tear fault with the geometry of oblique ramp, adjusting the lateral varia- tions of the northeast-trending main fault kinematics. Slip vector on the Xiaoyudong fault is compatible with the regional N70° and 80°E principal compressional stress direction, oblique to the current mountain front orientation.