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Application of GPS in crustal deformation studies: Some case studies
Some case studies
Deccan trap region in Western Maharashtra The Deccan trap region of western India generally considered as a seismically stable continental region (SCR), has been experiencing significant level of seismicity in the past two decades including the devastating Latur earthquake ( M 6.3) of September 29, 1993. This occurrence in the Central Indian shield has led to serious introspection among geo-scientists and motivated the Department of Science and Technology, Government of India, to sponsor the programme ‘Seismological upgradation and related studies in peninsular Shield’. Under this programme, to estimate the crustal deformation, a 73 site GPS geodetic network, distributed in an area of 200 x 350 km2 (shaded region in the inset of Fig. 1) has been designed. The first campaign started during January, 1995. The results presented here are based on GPS data collected for 3 campaigns during 1997-99 at 21 sites. Velocity vectors ( in ITRF96) obtained from 3 GPS campaigns during 1997-99, indicate that the magnitude of the horizontal velocity of Deccan trap region is in the range of 40-60 mm/yr with an average of 51 mm/yr in N47o E (Fig.1). The estimated dilatational strain (Fig.2 ) is about 0.4 micro strain/yr in average. An extensional strain regime is observed along the west coast and south of Koyna and Warna reservoirs, transcending into a region of compressive strain towards the interior of the shield area. The extensional strain regime coincides with the West Coast Geothermal Province and intersecting fault system south of Koyna-Warna reservoirs. And the compressional strain regime could possibly be correlated with the India- Eurasia collision forces in the NS to NNE-SSW direction. Reddy et al.(2000) gives more details of this study. Bhuj region in Gujarat The Bhuj earthquake on 26th January 2001 with magnitude Mw 7.7 (Ms 8) is considered most devastating in last 50 years. About 25,000 people died and 400, 000 houses were destroyed. This region had experienced many high magnitude and damaging earthquakes in the past too. The earthquake during 1819 in the northwestern part of the Great Rann of Kutch and the Anjar earthquake of 1956 are two major earthquakes. Apart from these major earthquakes, the region has experienced several earthquakes in the magnitude range of 4-5. Considering the geo-tectonics and seismic history of the, occurrence of this Bhuj earthquake, though not predicted, it is not surprising. For studying the seismo-tectonics of the Bhuj earthquake affected region, many geophysical investigations are being carried out as a part of DST sponsored project. GPS measurements for monitoring post seismic deformation in this region considered one of the important studies. This zone demarcated as active zone V and extends approximately 250 km in east-west direction and 150 Km in the north-south direction. Fig.3 shows GPS network of 14 sites. Amongst these sites, 11 sites are located in the Kutch region. Both Trimble and Leica GPS receivers were used in GPS data collection. Fig.4 shows changes in East-West, North-south and vertical components of the baseline Lodai-Ratanpar ( which passes through the epicentral area) during 21-28 February,2001. During this time, though there were some after shock occurred, no displacement is seen in these components. However, a few days of GPS data analysis will not lead to any conclusion that there is no significant deformation taking place. Once enough data is collected at all the 14 sites, the data analysis will yield velocity distribution from which strain distribution can be estimated. This information can be supplemented by Interferometric Synthetic Aperture Radar (InSAR) (Reddy et al., 2000) results for better reliability on crustal deformation taking place the region  Fig. 4 East-West, North-South and vertical components of baseline Lodai-Ratanpar passing through the epicentral. The estimates are based on analysis of 6 hours data segments during 21-28 February 2001. The lower right corner graph shows the variations in baseline vector. Large errors are associated with the vertical component. |