汶川8.0级地震地表破裂白鹿镇段的变形特征

Deformation Features of Surface Rupture of the Wenchuan Earthquake (M8.0) at the Bailu Town

汶川8.0级地震在彭县—灌县断裂上形成的地表破裂带主要位于彭州磁峰与安县睢水之间,全长约90km,白鹿镇位于该断裂地表破裂带的南段。应用全站仪和GPS对地表破裂带的垂直位错量、水平位错量、构造缩短率进行精确测量,结果显示,沿彭县—灌县断裂,白鹿镇地表破裂的垂直位错量和水平位错量都是最大的,其中最大垂直位错量位于白鹿老街,为(2.7±0.2)m,最大水平位错量位于白鹿河堤岸,为(0.7±0.2)m。究其原因,可能是沿彭县—灌县断裂,白鹿地区煤层含量最为丰富,煤层厚度最大,因而该地区软弱岩层的屈服强度相对最低,从而导致该地区断层两侧岩体的滑动量最大。另外,通过白鹿中学附近垂直于断层走向的探槽,甄别出两条呈叠瓦状排列的分支断层,该探槽所揭示的地表破裂面倾角度数及倾角随深度的变化趋势与通过震源机制解推断的结论一致。

Surface rupture zone of Wenchuan earthquake（M8.0） on the Pengxian—Guanxian fault located between Cifeng and Suishui,the whole length was about 90 km where Bailu town located at southern part of surface rupture zone.The vertical offsets,horizontal offsets and tectonic shortening rate were measured precisely with total station instrument and GPS.The results showed that vertical offsets and horizontal offsets in Bailu town were most obvious along Pengxian—Guanxian fault,the largest slip amount was（2.7±0.2） m vertical offset in Bailu old street and（0.7±0.2） m horizontal offset in Bailu river embankment,Pengzhou.The were reasons were presented as follows： the coal seam in Bailu town was most abundant and most thick along Pengxian—Guanxian fault,the yield strength of soft rock was relatively low,and slippage of rock located both sides of fault was largest.Two branch faults which were imbricate arrangement were discriminated by the trial trench near Bailu middle school.Dip angle and change trend of dip angle with depth were consistent with the conclusion inferred by focal mechanism solution.