基于高分辨率卫星影像估算汶川地震同震水平缩短量:以白沙河段为例

2008年汶川地震(Mw7.9)造成了严重的人员伤亡和重大经济损失。震后现场调查获得大量的垂直和走滑同震位移数据,但缺乏同震水平缩短量数据,直接影响了对汶川地震的同震形变特征的研究。文中尝试从一个新的角度,通过地震前、后QuickBird卫星影像对比,同时结合野外实地测量来分析汶川地震地表破裂带同震水平缩短量。白沙河破裂段位于中央主破裂带的西南端,距震中区映秀镇最近仅有15 km。在该破裂段的两侧各有一条乡间公路,相距300～900 m,我们通过对比地震前、后QuickBird卫星影像的公路间距变化,对比震后实测公路位置与震前的差异,获得了一系列的同震水平缩短量,其平均值为6.8 m(RMS),最大值和最小值分别为13.2 m和1.1 m,综合误差为2.33 m。该估计值大于沿地表破裂带通过复原变形人工构造物以及变形地层所获得的同震水平缩短量,说明汶川地震在地表造成的水平缩短分布在一个较大的范围内,而破裂带附近所观测到的仅仅是其中的一部分。

Co-seismic horizontal shortening associated with Wenchuan earthquake from high resolution satellite images: An example by Baishahe section

The May 12th,2008 Wenchuan earthquake(Mw 7.9) in Sichuan Province led to severe casualties and a huge economic loss.Field investigations after the earthquake have obtained a lot of co-seismic deformation data.However,most of them are strike and vertical offset;the horizontal shortening which is as important as the strike and vertical offset was rarely reported.In order to estimate the horizontal shortening,we suggested a new method: drawing reference geometric features in QuickBird images first;then calculating the space between adjacent features before and after earthquake,respectively;finally obtaining the co-seismic horizontal shortening based on the contrast of the spaces before and after earthquake.The Baishahe rupture section was selected as the sample place to test this new method for estimating co-seismic horizontal shortening because there are two roads,one on the northern side of the rupture and another on the southern with 300 m to 900 m spacing.By contrasting the space between two roads before and after earthquake,we obtained a series of horizontal shortening amounts;the average,maximum and minimum of those are 6.8 m,13.2 m and 1.1 m,respectively,and the error is 2.33 m.Checking the field measurement of the two roads after earthquake,we inferred that about 1.18 m may have been overrated using this new method.Moreover,our estimated results using this new method are larger than the calculated results based on restoration of deformed constructions and strata near the surface rupture,suggesting that the co-seismic horizontal shortening should have been distributed in a wider area,including the immeasurable permeated deformation far from the rupture and the measurable near the rupture.