亚像素相位相关法在获取汶川地震近场形变中的应用

APPLICATION OF SUB-PIXEL IMAGE CORRELATION TO NEAR FIELD GROUND DEFORMATION MEASUREMENTS OF WENCHUAN EARTHQUAKE

震后地表实际破裂带的分布及其近场的形变特征,是理解块体运动学特性、断层破裂特征、地震发生机制等科学问题的十分重要的约束条件。基于InSAR获取的汶川地震同震形变场,由于发震断层附近同震形变梯度巨大,沿断层带出现了非相干条带,以致于无法获得断层附近的形变量。而基于亚像素级的光学影像偏移量法为获取断层附近大形变分布提供了可能。文中以SPOT卫星影像为数据源,采用光学影像偏移量法获得了什邡及茂县地区的水平位移形变场。结果显示龙门山断裂带上至少2条断裂同时发生破裂,形成了主要地表破裂带(龙门山镇-高川破裂带)和次级地表破裂带(汉旺破裂带),沿龙门山镇-高川破裂带平均位移量为4～6m,在高川附近伴随的平均右旋水平位移为1～3m;汉旺破裂带因逆冲导致水平缩短,平均位移量一般为1～2m。汶川-茂县断裂带没有明显的地表破裂带。研究表明,利用光学影像相位相关法能够获得近断层位错量,可以成为InSAR手段的重要补充。

The distribution and characteristic of ground deformation is a key issue in geodesy,which brings insight into the geometry of the ruptured fault and seismic hazard assessment in the future in the sur- rounding areas. It also provides better constraint conditions for geophysical inversion. Compared with field research, satellite imagery regularly provides detailed and spatially comprehensive images and is a most valuable alternative especially for the study in remote areas. So, observing seismic rupture is ur- gent after earthquake. InSAR is useful for measuring ground displacement,but the technique has se- vere limitations that are mainly due to data decorrelation and signal saturation,and it does not general- ly provide measurements in the near-fauh area where large displacements occur. In this paper, the sub-pixel correlation method and SPOT image are used to map the Wenchuan earthquake rupture and to identify the faults activated by the earthquake. A computation is introduced of the inverse projec- tion matrices for which a rigorous resampling is proposed. Image registration and correlation is a- chieved with an iterative unbiased processor that estimates the phase plane in the Fourier domain for subpixel shift detection,then the earthquake deformation field is derived. The results indicate that the Wenchuan earthquake produced at least surface ruptures on two faults along the Longmenshan Fault, the main rupture named Beichuan-Yinxiu rupture zone（ Longmenshan town-Gaochuan in this map） and the secondary rupture named Hanwang rupture zone. The former is characterized by dextral-slip thrusting with a horizontal displacement of 4 -6m in average and a dextral-slip displacement of 1 ~ 3m near Gaochuan town. The latter is characterized by pure thrusting,with horizontal displacement 1 -2m in average. There is no obvious ground-rupture along Wenchuan-Maoxian Fault. The research indi- cates that sub-pixel correlation using optical image can be a powerful complement to differential radar interferometry,which can measure ground displacement near the fault zone. The study also shows that earthquake displacement fields can be calculated by remote sensing technology. The surface rupture can be traced and the meizoseismal area can be located by this method. Compared with field re- search,satellite imagery regularly provides detailed and spatially comprehensive images and is a most valuable alternative especially for the study in remote areas.