Abstract: On August 25, 2008, an M_W6.7 earthquake struck Zhongba County, central Lhasa block. Subsequently, an aftershock of M_W6.0 occurred on September 25. The rupture caused by this earthquake is rather complicated. There are some differences in focal positions and fault parameters given by different institutions. In addition, a deeper understanding of the tectonic significance of this earthquake is also needed.
Firstly, we use interferometric synthetic aperture radar data collected by the environmental satellite(ENVISAT)of European Space Agency and the advanced land observing satellite(ALOS)of Japan Aerospace Exploration Agency to obtain eight coseismic deformation fields covering the whole epicenter region based on InSAR technology. Because the terrain in the earthquake area fluctuates greatly and there are many objects with low coherence(eg. lake), we choose 30-resolution SRTM DEM data as reference DEM, the more robust Goldstein as filtering method, and Delaunay Minimum Cost Flow as phase unwrapping method. The interferograms show that the surface deformation caused by this earthquake is about 50km long and is divided into two lobes, north and south. The shape of the deformation in the north is similar to that of Palung Co Lake, and the maximum signal is hidden by the lake. The deformation in the south has two centers, located at two ridges respectively. The aftershock also caused two minor deformations at the east and north of Palung Co Lake.
Secondly, we use uniform sampling method to downsample 8 interferograms, and set the sampling interval of near-field data to be much smaller than that of far-field region, to ensure the observation data characteristic and sampling density of the main deformation region. In order to better invert the rupture slip distribution of the main shock, we subtract the influence of aftershock deformation. Finally, 6 data sets for the main shock deformation are obtained. Smoothness of sliding distribution is applied to restrict the sliding amount of adjacent fault slices. The best-fit solution shows that at least two ruptures in the south and north are caused by the earthquake, mainly of normal dip-slip and partial sinistral strike-slip by Okada uniform elastic half-space dislocation model and SDM method. The northern rupture is related to the Palung Co Fault with NE strike, with the maximum deformation of -13.0cm and the maximum slip of 0.52m in the depth of~12km, and the southern rupture deformation is obviously strongly related to topography, with the maximum deformation of -38.7cm and the maximum slip of 1.15m in the depth of~14km. The maximum slip is located at(30.81°N, 83.45°E), between the positions determined by GCMT and NEIC. The results also show that normal fault earthquakes may play an important role in the uplift of Tibet Plateau.
Thirdly, we use 15 images obtained between 2008 and 2010 from ENVISAT to obtain the post-earthquake time series deformation to further understand the tectonic background of the earthquake using SBAS-InSAR technology. 54 pairs of good interferences are screened out for processing, of which 30 pairs were unwrapped by Delaunay MCF method. The velocity accuracy threshold is set to 2mm/a to ensure reliable estimation of deformation velocity value. After two step SBAS inversions, the time series of deformation after the earthquake is obtained, thereby revealing that the post-earthquake deformation is not obvious on both sides of the fault but in the denudation and deposition area. This shows that no obvious common phenomena such as afterslip or creep are found after the earthquake. From the three cumulative deformation profiles, it can be seen that the regional deformation is mainly denudation and subsidence related to topography and geomorphology, and the deformations of adjacent subsidence and uplift regions are basically the same. The result shows that the graben structure in Lhasa block is mainly vertical deformation caused by terrain difference. In order to explain this result, we processed GPS data from 1991 to 2015 and obtained the principal strain rate in the western region of Lhasa block. The result shows that the east-west extension in Lhasa block is obvious but uneven. The strain is mainly stretching or squeezing perpendicular to deep and large faults, and the strain decreases near the grabens. The tensile strain near the Palung Co fault graben is~2.4×10⁸/a. This also shows that estimates of the tectonic activity based on geomorphology may be underestimated on some normal faults that have not been mapped or have no clear large-scale surface expression in the Tibet Plateau.
This study combines multi-orbit InSAR data to constrain the focal mechanism solution of the Zhongba earthquake, proving that abundant interferometric results can complement each other, which is helpful to analyze the deformation distribution caused by the earthquake more clearly and completely, especially in the absence of surface rupture.

Key words: 2008 Zhongba earthquake, InSAR, earthquake deformation, focal mechanism, post-earthquake deformation

摘要： 2008年8月25日拉萨块体中部的仲巴县发生了M_W6.7地震，之后发生了M_W6.0余震。然而，针对此次地震，不同机构给出的震源位置和发震断层参数存在一定差异，同时对其所揭示的构造意义也需要更深刻的认识。文中利用不同波长、不同入射倾角的升降轨ENVISAT ASAR和ALOS PALSAR卫星数据，基于InSAR技术获取了该地震的8幅同震形变场图像，得到此次地震引起的地表形变长约50km，且分为南、北2瓣。为了更好地反演主震的破裂滑动分布，在数据降采样过程中，设置近场区域的采样间隔远小于远场区域，以保证主要形变区域的观测数据特征和采样密度，并且扣除了余震形变的影响。基于Okada均匀弹性半空间位错模型和SDM方法反演的结果表明，此次地震的发震断层为E倾的帕龙错断裂，破裂滑动主要为正断倾滑兼部分左旋走滑性质，南段最大滑动量为1.15m，北段最大滑动量为0.52m。为了进一步理解此次地震的构造背景，文中利用SBAS-InSAR技术获得了震后仲巴地区2008-2010年的形变时间序列，发现形变主要是由剥蚀导致的沉降区堆积所引起的，而在断裂两侧并不明显。结合1991-2015年的GPS数据分析，认为拉萨块体内部EW向的伸展作用是明显且不均匀的，地堑附近伸展量逐渐减小。正断型地震在高原隆升过程中可能起着重要的调节作用。

关键词: 2008年仲巴地震, InSAR, 地震形变, 震源机制, 震后形变