| Earthquakes are one of the most destructive natural disasters on earth, and the earthquake activities in China are very common. Accurate measurement of coseismic ground deformation is the most direct way to understand the mechanisms of earthquakes. Meanwhile, the combination of observed coseismic deformations and simulations with geophysical model is important to characterize regional active faults and assess the mid- and long-term seismic risks.Differential Interferometric Synthetic Aperture Radar (DInSAR), a novel space-based geodetic technique that emerged in the 1990s, has been developed into an important tool in mapping global earthquake-induced ground deformation. Benefiting from the day and night operational capabilities, large-scale coverage, and high-resolution imaging, DInSAR has been applied to map and model more than 100 global earthquakes.To measure the complex deformation field caused by an earthquake,however,DInSAR still faces several limitations, such as the phase de-correlation, atmospheric long-wavelength artifacts, and the ambiguity in characterizing ground displacements with data from only one orbit. Phase de-correlation often happens at places where the ground deforms with large gradients (e.g.,near the epicenter),making the DInSAR measurements have many null values.The usage of long-wave length (e.g., L-band) SAR data has advantages in the reduction of phase de-correlation and the retrieval of large-gradient deformation. However, long-wavelength data are vulnerable to ionosphere perturbation, and may cause measurement errors in range directions from several centimeters to several tens of centimeters. In addition, the surface deformation measured from DInSAR is actually the projection of true ground displacement vectors along the satellite line of sight (LOS) direction. Therefore, it is often difficult to fully reveal the complex coseismic deformation features with only one interferometric pair.The limitations of DInSAR hinder the correct understanding of physics of earthquakes,and accurate estimation of source parameters. To overcome the limitations of DInSAR, this paper systemically studied the Multi-Aperture InSAR (MAI), a technique that measures ground displacements along the SAR azimuth direction. With the azimuth offsets measured from MAI, this paper then proposed an azimuth-offset-based method to correct the ionospheric phase error in the SAR interferograms. Additionally, this paper proposed a novel method for retrieving three-dimensional (3D) coseismic deformation field by using the DInSAR and MAI measurements. The proposed methods were validated on several seismic events by mapping and modeling the associated ground deformation.MAI is a technique that exploits the interferometric phase of sub-aperture SAR images to measure the azimuth displacements based on one interferometric pair. MAI complements the missing information of DInSAR,thus extending the capability of DInSAR for measuring two-dimensional surface displacements. This paper systematically studied the principles of sub-aperture splitting of SAR images, interferometric model, and data processing of MAI. The Kumamoto earthquake (Mw7.1) occurred in Japan in 2016 was explored to demonstrate the data processing of MAI. The MAI measurements show that the Kumamoto earthquake caused a ground surface rupture of about 45 km. Moreover, it can easily conclude that the earthquake is a dextral strike-slip event from the displacement patterns on the north and south fault walls,while which cannot be inferred from the DInSAR measurements. The measurement precision of MAI was also assessed through theoretical calculations and real data-based experiments.The analyses indicate that the precision of MAI strongly relies on the de-correlation condition of interferometric phase, and is closely related to the radiomatic parameters of SAR satellite.The theoretical calculation shows that the measurement precision of MAI based on ALOS PALSAR is smaller than 0.08 m when the coherence is 0.8.This paper proposed a method by the integration of SAR azimuth offsets and fault model to correct ionospheric errors in coseismic interferomgrams. The retrieval of azimuth offsets due to ionophseric disturbances is vital in this method. To achieve this aim, a strategy that combines the MAI and the pixel offset tracking (POT) techniques was presented to gain a high resolution SAR azimuth offset field first, by exploiting both the SAR amplitude and phase information. Then, a fault model was used to remove the azimuth offsets that caused by the coseismic deformation. The proposed method was applied to the 20080229-20080531 ALOS PALSAR interferometric pair that covers the 2008 Wenchuan (China) earthquake for a test.The test results show that the maximum ionosphere phase delay in the coseismic SAR pair is about 17 rad, corresponding a deformation error of about 32 cm in the line of sight. The corrected results recovered three continous deformation fringes on the southern wall of the fault. Calculations show that the coseismic displacements of three small zones in far-filed regions have a reduction of their standard deviations with 50%, 65%, and 84%, respectively.For the issues that the one-dimensional measurements of DInSAR cannot fully characterize the coseismic deformation, this paper proposed an integrated method to map 3D coseismic deformation field by combining DInSAR, MAI, and displacement gradient tensor(DGT) model, here refers to InSAR-DGT. The method first generates initial 3D coseismic deformation maps through a linear inversion method by combining the ascending/descending DInSAR and MAI measurements. A functional relationship is then constructed between the valid points (VPs) with high quality in the initial 3D displacements and the low quality points based on the DGT model. Finally, the 3D displacements are re-estimated through the weighted Least Square adjustment. The proposed method was then applied to map the 3D coseismic deformation field related to the 2011 Mw6.8 Tarlay earthquake (Myanmar). The results show that the InSAR-DGT method can remarkably reduce the numbers of null points near the epicenter. Comparing with the conventional linear inversion method, the InSAR-DGT improved the precisions of the calculated displacements by 22%, 36%, and 24% in east-west(E-W), north-south (N-S), and vertical (U-D) directions, respectively. Analyses show that if the number of VPs is more than 80, the precisions of displacements in the E-W and U-D directions are less than 1 cm. If the number of VPs is more than 166, the precisons of the N-S displacements are less than 2 cm. It is revealed from the 3D deformation field that the causative fault for the Tarlay earthquake generated a left-lateral slip that was accompanied by a minor normal dip-slip component.In order to further explore the improvements of 3D coseismic deformation field for the earthquake inversion, this paper inferred the 3D coseismic deformation field and source parameters of the 2010 Yushu earthquake (Mw6.9) in Qinghai by combing the DInSAR and MAI measurements. The results show that the maximum coseismic displacements in E-W ,N-S, and U-D directions are -40.4, 113.8, and -65.7 cm, respectively, and the peak-to-peak slip offset between the southern and northern fault wall is 1.82 m. Especially, this paper found that the localized area 6-30 km northwest of the Yushu County was affected by a significant thrust component with apparent vertical deformation, which was underestimated or ignored by the previous studies. The 3D deformation field reveals that the Yushu earthquake ruptured the surface about 74 km along a strike of about 300°. The non-linear inversion performed on five fault segments based on the Okada dislocation model reveals that the faults are located in the shallow crust with the upper depth smaller than 2 km. Four fault segments are nearly vertical (84°-88°) and the one fault segment dips 66.2° to the southwest. The estimated seismic moment is 2.43×1019 Nm (Mw 6.92), which is close to the solution provided by the Global Centroid Moment Tensor (GCMT) project. The slip distribution derived from linear inversion for each fault patch shows that the slip occurrence extends down to 12 km beneath the ground surface, and a peak slip of 2.23 m appears at the depth of about 5 km right near the epicenter.Comparison with the results derived from the seismic waves, field investigation, GPS (Global Positioning System) and conventional DInSAR observations indicates that the source parameters of the Yushu earthquake can be improved by the combined use of DInSAR and MAI measurements. |
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