逆冲断裂的视走滑位移分析及其在5·12汶川大地震同震破裂调查中的应用

Oblique Slip Displacement of Thrust Faulting and Its Application to the 5·12 Wenchuan Co-seismic Rupture Earthquake

逆冲走滑断裂中走滑分量大小的确定是判断断裂性质和应力场方向的重要参数,在野外主要依靠被断裂错开的标志线来判断。标志线的恰当选择,直接影响断裂走滑性质的判断及大小的测定。本文在对5.12汶川大地震同震破裂野外地质调查的基础上,详细分析了在同震逆冲断裂上所观察到的位移特征,指出当地表标志线与断裂斜交时,逆冲运动伴随的地表水平缩短将造成标志线产生沿断裂走向上的视走滑位移现象,并给出了基于不同地震陡坎模式的计算视走滑位移量的公式。根据视走滑位移计算公式,对实测走滑结果进行了计算、校正。校正结果显示彭灌(安县-灌县)断裂同震破裂带基本无走滑运动,为近纯逆冲性质;小鱼洞断裂和彭灌断裂上各自相隔数米的两个标志线分别出现左旋和右旋视走滑的例子进一步说明影响视位移误差的主要因素是水平缩短量,而非断裂走向的局部变化。本文的分析表明,野外测定逆冲走滑断裂的走滑位移时需尽量选择与断裂直交(垂直)的标志线,或注明标志线与断裂的夹角等几何关系;只有经视走滑位移校正后的测量才能代表断裂本身的真实走滑特征。

The horizontal and vertical components of large oblique thrusts are important parameters in determining the kinematics of earthquake ruptures and regional stress orientation, but their values are mainly based on field measurement of offsets using marker lines. Choosing proper marker lines will directly affect judgment and measurement of the nature of slip fault. Based on our field observation during mapping the 5 -12 Wenehuan earthquake surface rupture, we analyzed the features of displacement in the co-seismic thrust fault and found that when the marker lines are cross the fault, thrust motion accompanying horizontal shortening resuhs in the oblique displacement. Based on the modes for different scrap this study also gives an equation to calculate oblique slip displacement, and calculated and corrected the displacements in site. Obliquity correction results show no slip motion in the Anxian--Guanxian fault （Pengguan fault）, suggesting a pure thrust. Two marker lines （Xiaoyudong fault and Pengguan fault） about a few meters apart are characterized by leftqateral and right-lateral slip respectively, further implying that the main factor affecting displacement error is horizontal shorting but not local change of fault direction. This study indicates that when measuring displacement of thrust faults on site, those marker lines which are perpendicular to fault or mark the angle between marker line and fault should be selected. Only after correction can this measurement stand for the real slip displacement of faults.