THE DEFORMATION OF THE 2008 ZHONGBA EARTHQUAKES AND THE TECTONIC MOVEMENT REVEALED |
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| Authors: | QIU Jiang-tao LIU Lei LIU Chuan-jin WANG Jin-shuo |
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| Institution: | The Second Crust Monitoring and Application Center, China Earthquake Administration, Xi'an 710054, China |
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| Abstract: | On August 25, 2008, an MW6.7 earthquake struck Zhongba County, central Lhasa block. Subsequently, an aftershock of MW6.0 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. |
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| Keywords: | 2008 Zhongba earthquake InSAR earthquake deformation focal mechanism post-earthquake deformation |
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