基于Landsat 8光学影像的巴基斯坦Awaran M_w7.7地震形变监测及参数反演研究
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摘要
2013年9月24日巴基斯坦俾路支省(Balochistan)境内的阿瓦兰县(Awaran)发生了Mw7.7级地震.本文利用覆盖该地区的Landsat 8数据,基于影像配准的方法获取了该次地震的同震形变场,并运用地统计的方法对形变结果进行精度评定.针对传统四叉树算法中近场和远场中采样密度的不均匀性,以及噪音区域对数据降采样和反演结果收敛性的影响,本文提出了改进的四叉树算法对点的密度和形变梯度进行合理兼顾.最后利用光学影像获取的形变结果和数据的精度水平,基于Okada弹性半空间形变模型反演了该地震的震源参数和断层滑动分布.结果表明,地震断层北倾47°,滑动以左旋走滑为主,断层的西南部兼具少量的倾滑运动分量,断层滑动主要集中分布在断层面0~15km深度范围,最大滑动量达10m.反演获得的地震标量矩为4.68×1020 N·m,震级约为Mw7.75级.本文的研究结果可以为该地区的地壳应力变化研究和地震灾害评估提供依据,同时为Landsat 8光学影像应用于地震的形变研究提供参考.
On September 24 th 2013,an earthquake of Mw7.7occurred North-Northeast(NNE)of Awaran,Balochistan Province,Pakistan,causing extensive damages to cities and counties along the Hoshab fault.At least 825 people were killed and hundreds more were injured.However,there were quite limited SAR and geodetic data available,because the ENVISAT/ASAR satellite was out of service and ALOS/PALSAR stopped working at that time.Fortunately,this event was completely mapped by the Apr.2013 launched Landsat8optical satellites,which is usuallyfor the change detection of the earth.So this earthquake provides a unique chance to study how to map the coseismic deformation of great earthquake with the Landsat 8optical images.In this study,we utilize the Landsat 8images to map the coseismic deformation based on the cross-spectrum correlation method,which is usually used in SAR pixel offset tracking measurement.A linear model is used to remove the long wavelength phase ramp caused by the orbital error of satellite.Our results indicate there is a curved oblique strike-slip faulting along the pervious Hoshab fault and the maximum horizontal slip is up to 12 min the center of fault trace.In order to weigh the different datasets in modeling,we propose a geostatistic approach to assess accuracy of the North-South(NS)and East-West(EW)offsets derived from the Landsat 8optical images.The results show their accuracy is about 0.6 mand 0.5 mrespectively.In order to reduce the effects from topography change,atmosphere and low coherence area,we mask the measurements on the water and cloud regions.We propose a modified quadtree resampling method that not only takes account of density of samples but also gradient of deformation to overcome the limitation of conventional quadtree resampling method near and far earthquake rupture area.We keep both the largest and smallest windows as 128×128and 8×8pixels,respectively,to ensure sufficient point density in the non-deforming area and proper samples near the surface rupture.We locate the curved fault traces and construct the fault geometry based on the discontinuity in surface offsets.We use a two-step approach,combining the simulated annealing(SA)optimization and subsequent derivative-based Levenberg-Marquardt inversion,to solve for the fault parameters in nonlinear modeling.It is a simplification to assume uniform slip on fault planes since heterogeneity in fault displacement and rake is observed on all scales of faults.Therefore,we construct a complex curved fault plane with dip angle estimated above to match the fault trace we mapped and divide it into 34×5subfaults(7 km ×7 km)to estimate variations in slip on the fault.Based on Okada half-space elastic model,the slip distribution of this earthquake is inferred from the Landsat 8 measurements.The inversion results show that the left-lateral strike-slip dominates the earthquake fault slip concentrating at the depth less than 15 km,with a slight dipslip component and the maximum slip is up to 10 m.The inverted seismic moment is 4.68×1020 Nm,corresponding to Mw7.75.We also find that some thrust faults in the Kirthar range and strikeslip faults within the Chaman fault system have been brought closer to failure by this earthquake.This research can help the earthquake disaster evaluation in this area,and also shed some lights on the earthquake study with the Landsat 8optical images in future.
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