汶川地震破裂带白沙河段同震水平缩短量的估算

同震位移向量由垂直、走滑和水平缩短(或拉张)三分量构成，合理估计各分量是全面认识地震地表变形特征的基础。汶川地震后的现场调查，获得了大量的同震位移数据，但是绝大部分是垂直和走滑两个分量，水平缩短(或拉张)分量十分缺乏，这必然导致我们对汶川地震破裂特征认识的偏差。本文通过对变形的天然地貌面和人工建筑的测量和复原，获得了白沙河破裂段8个观察点水平缩短量数据。同震水平缩短量在白沙河段的分布显示出了其由南西向北东逐渐减小的趋势，它与其他研究获得的垂直和走滑同震位移的分布基本一致。此外，同震水平缩短与同震垂直位移量在该破裂段上的分布，表现为峰、谷互补，可能暗示了地震破裂面倾角沿该破裂段的变化过程： 南段为高角度，中段逐渐转变为低角度，最后在北段再次转为高角度。

CO­SEISMIC HORIZONTAL SHORTENING ON BAISHAHE SEGMENT OF WENCHUAN EARTHQUAKE RUPTURE ZONE

Co­seismic slip vector includes three components: strike slip，vertical and horizontal shortening(or extension). A precise estimate of all three components is essential to understanding the co­seismic surface deformation. After Wenchuan earthquake，through field investigation the surface rupture data of co­seismic displacement have been obtained，most of which however are strike slip and vertical offset，with a little data about horizontal shortening. The Wenchuan earthquake engendered two surface ruptures along the central fault and the front range fault of the Longmenshan thrust fault zone, respectively. The Baishahe rupture segment is one of the most havoc places on the central rupture and close to the epicenter of the earthquake. It is located at the south of the central rupture，with horizontal shortening observed. By measuring and recovering deformed natural and man­made reference surfaces we estimated eight horizontal shortening amounts，four natural(3.37m,1.44m,0.36m，and 0.15m)and four man­made(2.40m,2.49m,1.20m，and 1.49m). Man­made reference surface show more obvious deformation than natural reference surface. The northeastward downtrend of horizontal shortening along the rupture segment is consistent with the strike slip and vertical offset measured by other researchers. High co­seismic horizontal shortening amount is also found to be related with low vertical offset，and vice versa，which might be explained by the change of dip angle of the rupture plane­higher angle in two ends of the Baishahe segment，and lower angle in the middle.