Abstract: Most great(M≥8)earthquakes during modern times have occurred in interplate regions or major continental collision zones, such as Sumatra, the Japanese island arc or the San Andreas fault zone. Continental faults slip at a much lower rate than boundary faults, but they also have the potential of generating large earthquakes. For example, the 2008 Wenchuan earthquake with a magnitude of 7.9, the slip rate of seismic fault is less than 3mm/a. They also have the potential to be significantly deadlier than those on plate boundaries because of the long repeat times and lack of preparedness. The January 23rd 1556 Huaxian earthquake in Shaanxi Province, central China, is the deadliest in history with an estimated death toll of ~830 000 from building collapse, land-sliding, famine, and disease. The earthquake occurred in the graben of the Weihe River.
    The Weihe Graben in Shaanxi Province has recorded multiple earthquakes in history, whereas most active faults within the graben have a low slip rate over geological times (~1mm/a). The slip rate of faults is an important parameter for assessing the risk of earthquakes and the interval between major earthquake recurrences. In order to obtain the quantitative information of faults slip rate, traditional geological methods or geodetic observation techniques can be used. Interferometric synthetic aperture radar(InSAR), as a modern geodetic observation technology, has the characteristics of all-weather and day-and-night imaging capability, wide spatial coverage, fine resolution, and high measurement accuracy. InSAR offers the potential to measure interseismic slip rates on faults at a resolution of millimetres per year. In this study, we use InSAR data to analyze the present deformation of the Kouzhen-Guanshan, Weihe and North Qinling faults in the central part of the graben.
    We collected 32 European Space Agency(ESA's)Envisat ASAR images from descending track 161 between 2003 and 2010, and processed them using ROI_PAC. The precise orbit determination from the Delft Institute for Earth Oriented Space Research(DEOS)was applied to correct for orbital effects. The topographic contribution was simulated and removed using the 90m resolution Shuttle Radar Topography Mission(SRTM)Digital Elevation Model(DEM)from CGIAR-SCI. Each interferogram was downsampled to 64 looks in the range direction (1 280m). Before phase unwrapping, a weighted power spectrum filter was applied to improve the signal-to-noise ratio. The branch-cut method was used for phase unwrapping. Phase unwrapping errors were checked by summing around a closed loop. All the major unwrapping errors were identified and corrected manually. We obtained a total of 98 interferograms with a spatial baseline of smaller than 300m, and selected 33 interferograms whose coherence is well preserved for time-series analysis. The time-series analysis was implemented using the π-RATE software package. It uses the geocoded interferograms from ROI_PAC to create a minimum spanning tree(MST)network, from which the orbital and topographically-correlated atmospheric errors are estimated. The MST network connects all epochs with the most coherent interferograms,including no closed loops of interferograms. The network approach is able to improve the estimation of orbital error by ~9% compared to the independent interferograms approach. The orbital errors are empirically modelled as planar or quadratic ramps. The topographically-correlated atmospheric correction was applied to each interferogram after having corrected for the orbital errors. Following creating a minimum spanning tree network, correcting for orbital and topographically-correlated atmospheric errors, and calculating the covariance matrix, we obtained the 7-year average slip rate of the faults that we are focused on.
    Our results show that the faults across the Weihe graben all have a small slip rate of less than 2mm/a. The Kouzhen-Guanshan Fault does not show any evident deformation signal. The Weihe Fault seems to show 1mm/a normal faulting in the satellite line-of-sight direction. In addition, we find ~10mm/a surface subsidence of the Xi'an City between 2003 and 2010. We use the stable Ordos block as a reference to assess the accuracy of our InSAR time-series analysis. Assuming the Ordos block has no internal deformation, we calculated the error of the InSAR rate map to be (-0.1±1)mm/a, indicating that our result is reliable. This paper presents a preliminary result of the present deformation of the Weihe Graben. InSAR is a powerful technique for monitoring active faults on a timescale of tens of years, and can be used for seismic hazard assessment in the future.

Key words: Weihe Graben, interferometric synthetic aperture radar, fault slip rate

摘要： 与板块边界的断层相比, 块体内部的断层滑动速率较小, 但仍具备发生大地震的潜力。 渭河盆地历史上曾多次发生大地震, 而盆地内一系列正断层长期的滑动速率却相对较低。 文中以渭河盆地中部的口镇-关山断裂、 渭河断裂和北秦岭断裂为例, 利用合成孔径雷达干涉测量技术(InSAR)对盆地内断层现今的滑动速率进行研究和分析。 搜集了欧洲空间局Envisat卫星161号降轨2003—2010年的32幅影像数据, 通过ROI_PAC软件对渭河盆地的Envisat ASAR数据进行处理, 得到98幅空间基线长度≤300m的干涉图, 并从中挑选出质量较好的33幅干涉图用于后续的时序分析。 使用π-RATE对33幅干涉图创建最小生成树网络(MST)并进行轨道误差校正、 与地形相关的大气校正、 去除参考相位和计算协方差矩阵等, 获得渭河盆地中部3条主要断裂在7a内的平均滑动速率。 结果显示, 2003—2010年期间渭河盆地断裂的滑动速率较小, 不超过2mm/a, 其中口镇-关山断裂没有明显的形变信号, 渭河断裂有约1mm/a的卫星视线向形变; 西安市整体地表沉降速率在垂向上最大可达10mm/a。 文中以稳定的鄂尔多斯地块为参考评估InSAR时序分析的精度, 得到InSAR速率图的误差约为(-0.1±1)mm/a, 证明了结果的可靠性。 文中工作可为获取断层10a尺度的现今滑动速率提供重要的技术思路, 继而为评估地震危险性以及危害性预测提供参考依据。

关键词: 渭河盆地, 合成孔径雷达干涉测量, 断层滑动速率