GEOINFORMATICS FOR TERRAIN CHANGE STUDIES IN TECTONICALLY ACTIVE KUTCH REGION, INDIA

Abstract

Naturally occurring events such as earthquake causes impulsive deformations inside the earth as well as on the surface. Measurement of deformation rate on ground is a tedious job as it would require meticulous detailed observations within short time involving lot of sophisticated instrumentation and manpower. However, microwave (RADAR) remote sensing technique endowed with special capability of all time and weather condition imaging and measuring the distance of object (on earth surface) from the satellite. This microwave technique, where Synthetic Aperture Radar (SAR) images are used to quantify the extent of ground deformations, is known as Interferometric Synthetic Aperture Radar (InSAR or IfSAR) and Persistent Scatterer Interferometry (PSI). PSI is a radar-based technique that belongs to the group of differential SAR image interpretation. Land deformation studies based on InSAR has been in practice, however, in only 2000 the Permanent Scatterer Interferometry (PSI) technique was first developed by a research team at the Politecnico di Milano (POLIMI), Italy. The principal aim of this development was to overcome the limitation of temporal and geometrical decorrelation of synthetic aperture radar (SAR) interferometry. Though geodetic observations, such as aseismic and co-seismic InSAR studies, have provided important insights into the earthquake deformation cycle for inter plate regions for different parts of the world, however prior to this work little post seismic intra plate tectonic deformation observations have been made by researchers for Kutch region, India. The present study intends to provide an improved understanding of the earthquake deformation cycle for the unstable continental plate region of Kutch rift basin. This study is the systematic effort, using satellite based InSAR, PSI, multispectral image, digital elevation models supported by topographical, geological, tectonic and earthquake information to observe and interpret surface deformation associated with tectonic processes for the Kutch rift basin India. The work is motivated by the relatively incomplete understanding of the post-seismic ground changes due to the occurrence of large intensity, damaging, earthquake in Bhuj on 26th January 2001 as well as subsequent earthquakes and earlier tectonically affected geological set up. The landscape of Kutch is one of the most unique examples of tectonically active intraplate region enduring rift reversal. Since geological time, the basin has never attained ii stability and is continuously subjected to many phases of tectonic deformations. In an active tectonic belts the potentially seismogenic zones are indicated by major faults. These faults bear certain geomorphic signatures which can explain the origin, stress condition and geometry of the fault and therefore survey across active faults can reveal idea about paleo-seismic events. The devastating earthquake of magnitude 7.7Ms (IMD) / 7.7Mw (USGS) that struck Bhuj on January 26, 2001 is an example of such seismic activity. As per report, the event had caused maximum uplift of approximately 1.575 meters near the epicenter. Widespread liquefaction and corresponding deformations were reported in the epicentral area around Bhuj and Bhachau city after the earthquake. The Rann of Kutch earthquake of 1819 was the severest in the region which caused visible faulting on the earth surface and created a high mound. This earthquake was followed by a large number of aftershocks which continued for nearly eight years. By means of morphometric analysis, realistic synthetic two-pass InSAR and PSI technique the region has been carefully examined for neotectonic activity aided by generation of suitable time series interferograms from SAR image pairs for Kutch area. As reported by Institute of Seismological Research (ISR), Gujarat, the small scale deformation distributed in the Kutch rift basin are some of the least investigated using InSAR and the present study demonstrates the capability of the technique to provide important observations on small-magnitude post-seismic events. Ground deformation (change) is expressed in terms of negative i.e. ground has moved towards satellite and positive i.e. ground has moved away from the satellite in the line-of-sight direction (LOS). The multi-year time-series of descending InSAR data from the Envisat, ALOS-PALSAR and Sentinel 1-A satellite are used and inter-seismic ground deformation which is estimated in the Kutch region is up to ±15 to 30 mm/yr in the satellite-to-target line-of-sight direction. Initially a series of interferogram produced from Envisat and ALOS-PALSAR satellites images for a time period of 2003 to 2010 are used to investigate long-term surface deformation processes for the Kutch, India. A deformation pattern could be detected across the Kutch mainland, Island area and the KBF and KMF associated areas to the South of Banni plains in the Central Kutch basin. Further the geomorphic study of the different basin suggests expected neotectonic activity along with presence of numerous faults as observed from optical and iii microwave images. The estimated deformation may be occurring consistently with the marked faults. In this study InSAR and PSI datasets (one pair ALOS PALSAR, four pair Envisat ASAR, three pair Sentinel 1-A and eleven scenes of Sentinel 1-A) have been utilized to analyse the nature of ground deformation in the Kutch region. The ALOS-PALSAR data pair of years 2007 and 2010 have been analysed for the area bounded by Kutch Mainland Fault (KMF) and Katrol Bhuj Fault (KBF) supposed to have going through relaxation phase of a stressed crustal layer. Three different regions in the study area illustrated countable significant fringes, indicating different amount of positive and negative ground deformations (-35cm, -35cm and 24cm) to the north and east of Bhuj city. The two interferogram images generated from Envisat dataset between 2003-05 indicate an episodic ground change followed by a negative and positive ground deformations. The interferogram derived from 2003-05 pair indicates resultant subsidence in the mentioned area along with a positive prominent change (subsidence) on ground to the NE, NW and south of Bhuj. Further deformation map generated followed by interferogram images from 2003 and 2005 Envisat data pair deciphers a positive ground deformation up to 17 cm in and around the Bhuj region covering mostly the Mainland region of Kutch basin. Elevation profile along the locations of deformation are drawn for SRTM (2000) and ALOS PALSAR (2007) DEM. Further an elevation difference is calculated employing change detection technique for the Bhuj area between the two DEMs, indicating a relative elevation variation between 2000 and 2007 for Bhuj. The interferograms generated from Sentinel 1-A data pairs showed a small scale ground deformation. In order to study more precisely the PSI technique has been deployed to observe the terrain change over one year period. The interferogram generated from Sentinel 1-A data pair reveals a positive deformation of about 18 mm, followed by a small scale ground deformation up to mm scale around Bhuj, in the upper mainland, along KMF, KBF fault zones and along the IBF in Khadir mound and along the major lineament cross cutting the KMF, east to the Khadir uplift. In order to compute rate of terrain change over the last few years, persistent scattering technique has been employed for central Kutch region covering Bhuj area. The PSI study reveals a negative ground displacement of 20 to 30 mm per year in the W and NW of Bhuj city to the east of Mainland along the LOS. A subsidence rate of 25-30 mm is observed in the south and east of the Anjar city from the PSI study. The Interferogram iv generated based on the Envisat and Sentinel 1-A data exhibits ground deformations in the area in and around KMF and KHF. Further, as per ISR report these two faults were mapped as active. Morphometric analysis is the quantitative assessment of form features of the earth surface and any landform unit. Tectonic features such as dislocation and displacement of mountain ridges, valleys, clearly recognisable lineaments, and drainage offsets have been identified in the Landsat true colour composite image. Also, the morphometric analysis and shaded relief models (SRMs) created from DEM, further substantiates the presence of faults. In this study the geomorphic evidences of active faults were also inferred. A significant offset is observed to the West along KBF; a major offset in drainages is observed and marked to the extreme WWS along KBF. Another series of 3 offsets have been identified to the south of KBF roughly parallel to each other. A displacement of intertrappean beds and some other sedimentary rocks is observed to the south of KBF. A lineament is marked along the KBF having step like fashion (en-echelon). Two prominent ridge displacements are witnessed at the extreme north of KBF, which concludes the presence of fault. Further two other sets of ridge displacements are observed at the extreme NW of Kutch basin. The faults have been identified and marked on both the Landsat true colour image and Sentinel 1-A power image from which the DEM is also derived to infer the existence of faults on hillshade and aspect map. Further the faults are marked for the eight locations on a three dimensional prospective view obtained by draping high resolution Digital Globe image over the ALOS PALSAR 12.5m resolution DEM. Hypsometric curves have been obtained for the associated drainage basins from SRTM-DEM for the eight locations where the faults have been identified. The stream length index found to be highest for two locations. The basin asymmetry factor (AF) shows a proportioned tilting towards east and west controlled by the marked fault at three locations. The Basin shape index (Bs) for three locations indicates nearly circular shape basins due to lower tectonic activity. Finally, the present study concludes that InSAR, PSI, morphotectonic and morphometric study can make a useful contribution towards better understanding of earthquake induced effects and seismic hazard in the Kutch region, India.