Featured Research

Surface Displacements of the 12 November 2017 Iran – Iraq Earthquake

The 2017 Mw 7.3 Iran–Iraq earthquake occurred in the seismically less active northern Zagros Mountains. In this study, DInSAR technique is used to derive the surface displacements of the earthquake using Sentinel-1 and ALOS-2 images.

Figure: 3D displacements showing (a) up–down (U–D), (b) east–west (E–W) and (c) north–south (N–S) components derived from the LOS measurements of three different geometries. The arrows in (b) represent the magnitude and direction of the displacement.

One preseismic, three coseismic and a postseismic interferogram are generated. The coseismic interferograms did not show any signs of a surface rupture and indicate a maximum of 80 cm uplift and 50 cm subsidence. The zenith total delay maps derived from the Generic Atmospheric Correction Online Service for InSAR are used to correct tropospheric delays in the interferograms. After correction, an error of 0–2 cm is observed in the line of sight measurements. The preseismic displacement reveals an indication of ground motion before the earthquake occurrence. The postseismic phase indicates afterslip after the earthquake. Localized displacements corresponding to landslides are observed on the mountainous slopes. The 3D displacements reveal ∼40 cm of surface motion suggesting westward movement and reverse faulting.

Divya Sekhar Vaka, Y. S. Rao, Avik Bhattacharya, ‘Surface Displacements of the 12 November 2017 Iran – Iraq Earthquake derived using SAR Interferometry’, Geocarto International, 2019. DOI: 10.1080/10106049.2019.1618927.

Comparison of HH and VV Polarizations for Deformation Estimation using Persistent Scatterer Interferometry

Persistent Scatterer Interferometry technique (PSI) exploits time series of interferometric SAR data to estimate deformation over an area. The process involves analysing stable phase pixels from differential interferograms formed with respect to a single master. In this study, eight RADARSAT2 C-band fine quad polarization images from 2014 to 2016 are used to identify Persistent Scatterer (PS) points and estimate deformation over Vijayawada city in India. With a full graph approach, 29 interferograms are generated. All the interferograms are unwrapped to aid stable deformation estimation. The prime objective of our research is to study the effect of HH and VV polarizations in PS point selection and deformation estimation. Initially, pixels with Amplitude Stability Index > 0.75 are selected as PS points. Later, PS pixels exhibiting temporal coherence > 0.75 and reflectivity > 1 are selected for deformation estimation.

Figure: Deformation near the river coast and around three canals in HH and VV polarizations. Moving far from the coast, VV shows uplifted areas, which are not observed in HH polarization. RADARSAT-2 Data and Products © MacDonald, Dettwiler and Associates Ltd. 2014 – All Rights Reserved. RADARSAT is an official trademark of the Canadian Space Agency

Before refinement, PS points for HH and VV polarization are identified as 49409 and 48395 respectively, whereas after refinement, they are about 24063 and 34389. Atmospheric Phase Screen (APS) is estimated and removed using a filtering approach. After APS removal, mean line of sight (LOS) displacement and velocity maps are generated. A linear model is used to invert displacements into velocities. Velocity obtained using HH polarized data ranges from +16 to -14 mm/year, and for VV it ranges from +12 to -19 mm/year. The deformation trend is linear throughout the observation period except in October 2014. Deformation around this month shows up and down pattern. We attribute this observation to the devastating Hudhud cyclone that hit the state of Andhra Pradesh in October 2014. Atmospheric variations due to the presence of strong troposphere component varied radar signal delay before and after the cyclone. Excluding this phenomenon, time series plots of selected points showed linear deformation pattern in both polarizations.

Vaka, D. S., S. Sharma, and Rao, Y.S., “Comparison of HH and VV Polarizations for Deformation Estimation using Persistent Scatterer Interferometry.”, ACRS 2017 – 38th Asian Conference on Remote Sensing, New Delhi, 2017