Xiaogang Song

Ship detection from polarimetric sar images

SAR image from sea can constantly contain ships and their ambiguities in azimuth and range directions. For maritime applications, the ambiguities are visible due to their strong intensities in a low backscattering background of sea environment. Thus, the ambiguities can be often mistaken as ships and cause false alarms. Many approaches have been proposed for reducing the azimuth ambiguities in single channel SAR image. This paper analyzed scattering mechanisms of the azimuth ambiguities for PolSAR images and proposed a method for detecting ships from PolSAR images. By using eigenvector-eigenvalue decomposition, three eigenvalues can be used to differentiate ship targets, azimuth ambiguities and sea clutter. One C-band JPL AIRSAR polarimetric data have been chosen to evaluate the method. The experimental results show that the proposed method can effectively reduce false alarms caused by the azimuth ambiguities.

Coseismic displacement field of the Wenchuan Ms 8.0 earthquake in 2008 derived using differential radar interferometry

We used the radar data from satellite ALOS/PALSAR of Japan and D-InSAR technology to derive the coseismic displacement produced by the Wenchuan, China Ms 8.0 earthquake on 12 May 2008. The result shows that the coseismic displacement primarily concentrated in a near-field range about 100km width on both sides of the Yingxiu-Beichuan fault. The incoherent zone about 250km long and 15~35km wide nearby the fault suffered the largest deformation with surface ruptures. The secondary deformed areas are 70km wide on each side of the incoherent zone, where the displacements exhibit a sunk northern wall with maximum -110~120cm and an uplifted southern wall with maximum 120~130cm, respectively. In the far-field range of the fault, displacements are less than 10cm. Using the offset tracking, we found clear rupture traces and coseismic displacement of 3m along the faults. With a model of four fault sections, we retrieved slip distribution on the faults. The inversion result reveals two slips of 10m at depths 5~20km beneath the Yingxiu-Beichuan fault and one slip of 2.3m at depth 5~20km below the Guanxian-Jiangyou fault, respectively. Thrust faulting dominates the southwestern Yingxiu-Beichuan fault and the entire Guanxian-Jiangyou fault, while right-slip is the primary component along the northeastern Yingxiu-Beichuan fault.

Deriving 3D coseismic deformation field by combining GPS and InSAR data based on the elastic dislocation model

Abstract The density of GPS measurements is usually one of the key issues in resolving 3-D coseismic deformation field from integrating GPS and Interferometric Synthetic Aperture Radar (InSAR) measurements with pure mathematic interpolation methods An approach that combines the elastic dislocation model with the Best Quadratic Unbiased Estimator (BQUE) or a robust estimation method named IGG (Institute of Geodesy and Geophysics) is proposed to reconstruct 3-D coseismic deformation field, in which only a small amount of GPS data is needed to produce a reasonable initial 3-D coseismic deformation. Then the BQUE and IGG are used to weight the InSAR and GPS measurements to avoid computational issues caused by the negative variance problem and to decrease the impact from gross errors. The Wenchuan earthquake is used to test the proposed method. We find that the developed method makes it possible to use only a few GPSs and InSAR data to recover the 3-D coseismic deformation field, which offers extensive future usage for measuring earthquake deformation, particularly in some tectonically active regions with sparse GPS measurements.

Three-dimensional deformation field caused by the Gaize earthquake by Multi-LOS DInSAR measurement technology

This paper firstly presents the Multi-LOS DInSAR measurement result of the coseismic deformation field caused by the Gaize Ms6.9 mainshock and Ms6.0 aftershock in Tibet, China, and then obtain the 3D deformation field based on the 3D resolving mode. The characteristic analysis of coseismic deformation field shows the rupture of mainshock is majorly normal, left-lateral striking with a little rotation; and the aftershock is typical normal rupture nature. The mainshock and aftershock had induced the east and west rupture(maybe buried) successively, and produced the east and west two subsiding centers

GPS-Derived Fault Coupling of the Longmenshan Fault Associated with the 2008 Mw Wenchuan 7.9 Earthquake and Its Tectonic Implications

Investigating relationships between temporally- and spatially-related continental earthquakes is important for a better understanding of the crustal deformation, the mechanism of earthquake nucleation and occurrence, and the triggering effect between earthquakes. Here we utilize Global Positioning System (GPS) velocities before and after the 2008 Mw 7.9 Wenchuan earthquake to invert the fault coupling of the Longmenshan Fault (LMSF) and investigate the impact of the 2008 Mw 7.9 Wenchuan earthquake on the 2013 Mw 6.6 Lushan earthquake. The results indicate that, before the 2008 Mw 7.9 Wenchuan earthquake, fault segments were strongly coupled and locked at a depth of ~18 km along the central and northern LMSF. The seismic gap between the two earthquake rupture zones was only locked at a depth < 5 km. The southern LMSF was coupled at a depth of ~10 km. However, regions around the hypocenter of the 2013 Mw 6.6 Lushan earthquake were not coupled, with an average coupling coefficient ~0.3. After the 2008 Mw 7.9 Wenchuan earthquake, the central and northern LMSF, including part of the seismic gap, were decoupled, with an average coupling coefficient smaller than 0.2. The southern LMSF, however, was coupled to ~20 km depth. Regions around the hypocenter of the 2013 Mw 6.6 Lushan earthquake were also coupled. Moreover, by interpreting changes of the GPS velocities before and after the 2008 Mw 7.9 Wenchuan earthquake, we find that the upper crust of the eastern Tibet (i.e., the Bayan Har block), which was driven by the postseismic relaxation of the 2008 Mw 7.9 Wenchuan earthquake, thrust at an accelerating pace to the Sichuan block and result in enhanced compression and shear stress on the LMSF. Consequently, downdip coupling of the fault, together with the rapid accumulation of the elastic strain, lead to the occurrence of the 2013 Mw 6.6 Lushan earthquake. Finally, the quantity analysis on the seismic moment accumulated and released along the southern LMSF show that the 2013 Mw 6.6 Lushan earthquake should be defined as a “delayed” aftershock of the 2008 Mw 7.9 Wenchuan earthquake. The seismic risk is low along the seismic gap, but high on the unruptured southwesternmost area of the 2013 Mw 6.6 Lushan earthquake.

Near-field motion of the Haiyuan fault zone in the northeastern margin of the Tibetan plateau derived from InSAR permanent scatterers analysis

Abstract The Haiyuan fault zone is a major discontinuity in the northeastern margin of the Tibetan plateau. A magnitude 8.5 earthquake occurred there in 1920. Both geological investigations and GPS measurements show that this fault zone is still highly active, with a slip rate of 3 to 10     mm / year , exhibiting a large range of variance in both space and time. We attempt to use the permanent scatterers interferometric (PSI) synthetic aperture radar technique to better detect the near-field motion of this fault zone. We process and analyze 38 scenes of ENVISAT/ASAR images from two neighboring descending orbits using the PSI method. The results show a remarkable velocity gradient of about 5     mm / year across the central segment of the fault zone and a rate of about 5 to 6     mm / year on its eastern segment. The motion senses are consistent with a left-lateral strike slip. The motion histories of most PS points show a stable linear variation trend in time series. In addition to these motion features that agree with those from geological, GPS and other observations, the dense PS analysis also reveals spatially continuous variations of crustal motion around the fault zone.

Monitoring deformation evolution in Longtan Reservoir area by multitemporal interferometric synthetic aperture radar using time sequences of spaceborne synthetic aperture radar images

Abstract In order to discover the relationship between the earth surface deformation evolution and the water impoundment/discharge and penetration process in the Longtan Reservoir area, we process a time series of images from Envisat ASAR and ALOS PALSAR instruments using an MT-InSAR technique enhanced by corner reflectors. This allows us to successfully obtain a deformation map from this densely vegetated reservoir area with karst topography. We obtain time series of precise deformation images, which show clearly, for the first time, the complete spatio-temporal earth surface deformation evolution in the whole reservoir area during the impoundment/discharge process. The results from the two kinds of data show a similar underlying pattern: (1) The water load causes subsidence in almost all of its immediate surroundings; (2) the subsidence magnitude shows cyclic variation matching seasonal water level variation. Every year, the water level starts to increase in June and quickly reaches its peak in July to August, and the subsidence immediately follows and also reaches its peak magnitude in July to August; then the water level decreases slowly, and the area rebounds slowly. (3) Statistical analysis of time-series deformation shows that about 60% of the deformation occurs during the rain season (June to August) can be restored, and the remaining 40% become permanent.

Coseismic deformation field derived from ENVISAT/ASAR data and fault slip inversion of Ms7.1 Yushu earthquake, China in 2010

This work estimated the coseismic deformation field of the Yushu earthquake in 2010 using the C band ASAR data, and inverted the fault slip distributions and simulated the interferograms. The results demonstrated that the slip distributions were mainly presented in shallow depth above 15km.There were two slip-concentration areas, maximum slip was nearby the Jiegu town, and fault motion sense was left-lateral strike-slip, which were all reconciled with field observations.

Co-seismic displacement observation in decorelate belt along seismic fault of 2008 Wenchuan earthquake

In this work we employ the pixel offset tracking technique to capture large displacements in incoherent zone nearby the Yingxiu-Beichuan fault caused by 2008 Wenchuan Ms8.0 earthquake. The used data of 6 tracks is from ALOS/PASAR dataset of Japan. The result shows that the entire surface rupture belt is 238km long, extending almost linearly in a direction NE42°. It is offset left laterally by a NW-striking fault at Xiaoyudong and turns at Gaochuan, where the rupture belt shifts toward south by 5km largely keeping the original trend. In terms of features of rupture traces, the rupture belt can be divided into 5 sections and 3 types. North to rupture belt, surface displacements are 2.95m on average, mostly in 2.0∼3.5m with the maximum 7.0∼9.0m at individual places nearby Beichuan. South to the rupture belt, the average displacement is 1.75m, dominated by 1.0∼2.0m, with 3.0∼4.0m at a few sites. Along the Guanxian-Jiangyou fault, there is a uplift zone in the radar line of sight, which is 66km long, 1.5∼6.0km wide, with vertical displacements about 2m but no observable rupture traces.

Study of coseimic displacement field of the YUSHU MS 7.1 earthquake derived from Envisat/Asar data

We used the radar data from the satellite ENVISAT/ASAR of ESA and the differential interferometric synthetic aperture radar (D-InSAR) technology to derive the coseismic displacement field produced by the Yushu, Qinghai Province, China Ms 7.1 earthquake on 14 April 2010. Based on processing SAR data acquired before and after the event by the two-pass method, we obtained the interferometric fringe map which covered the causative fault (Ganzi-Yushu fault) and determined the deformation scope caused by the shock. Three arc second SRTM DEM was used for removing the topographic phase. Atmospheric delayed phase related to terrain was eliminated. MCF phase unwrapping method was performed to the interferometric fringe map for yielding digital image of the interferometric displacement field, which was analyzed by displacement contours and the profile across the fault. The result shows that the Yushu Ms 7.1 earthquake has produced a surface deformation area of about 89km×59km, along the Ganzi-Yushu fault. But in the vicinity of the fault, about 70km×27km, the deformation fringes are dense and thin, meaning the deformation gradient is big, where should be main devastated region. The whole deformation field is captured by a set of ellipse-like fringes, which converge towards the fault indicative of increasing gradient and amplitude of displacements, exhibiting sunk north wall and uplifted south wall in sight line. This movement direction in sight line just agrees with the left motion faulting of Ganzi-Yushu fault. We can count at least 6 fringes, equivalent to a displacement of 17cm in southern side and 10 fringes equivalent to a displacement of 28cm in northern side. The maximum relative displacement between the north and south walls at least reach 45cm that appears nearby the Jiegu Town of Yushu county in Qinghai Province, where is the most destroyed area in the event. Along the fault strike, there are three locally distorted areas, respectively present at the epicenter and around the Jiegu Town. The profile across the fault indicates that deformation is highly variable gradients with profound heterogeneity near the fault. All these results are coincide with the field investigation. Interferometric synthetic aperture radar (InSAR) has become an important geodetic imaging technique to map deformation of Earths surface. Particularly, this method has been successfully employed to measure coseismic surface displacements produced by several great earthquakes [4, 5].