Crustal & dam deformation studies using GPS


Case Study II: The GPS Network for Gujarat Earthquake of January 2001
The tragic earthquake that struck the Gujarat region of western India on 26 Jan. 2001, destroying thousands of lives and valuable property, has once again highlighted the need and importance of monitoring the post-earthquake deformations for understanding the complex earthquake mechanism. Keeping in view the urgent nature of the work, immediate Global Positioning System (GPS) observations were carried out in the area by the GPS team of Indian Institute of Technology Bombay (IITB). The aim is not only to understand the post-earthquake crustal deformation pattern, but also to establish precise GPS control for monitoring crustal dynamics in this earthquake-affected region in the future. 

Field Data Collection 
The existing geodetic control network in the earthquake-affected Bhuj region consists of several geodetic stations at approximately 20-40 km spacing, of the series of the Great Trignometrical (GT) Triangulation Network of India (Bendick et al, 2001). These series have been established during the mid-nineteenth century. GPS observations at these stations would yield valuable data about the cruatal deformations in the region due to various causes, including the earthquakes of 1919 and 2001. Hence, immediately after the January 2001 Bhuj earthquake, during the GPS field campaign of February, 2001, a total of 17 stations, including 5 old GT stations, which were found intact, and 12 new stations established close to the GT stations found destroyed/disturbed, have been observed by our team (Fig. 2). Four 4000SSI Trimble dual frequency geodetic GPS receivers were used for this. The observations were carried out in four campaigns, as shown in Fig. 1, with 48 hours of continuous observations at every station. The aim of establishing these stations is to monitor the deformations of the region from a reference point outside the region, and to monitor the deformations of the region near the epicentre. It is proposed to reoccupy some more geodetic stations, and carry out repeat observations at all these stations within a period of 6 months to 1 year, to study the deformation pattern. These observations will be useful in estimating the crustal deformation in the region, in order to understand the seismic activities there.

Data Analysis & Preliminary Results
GPS data collected by our team has been pre-processed using the GPSurvey software, and processed using Bernese 4.2 software developed by the University of Bern. The permanent IGS station at Indian Institute of Science (IISc), Bangalore, has been taken as a reference station. Precise satellite orbit files have been used for the data processing. The results obtained indicate that the maximum RMS error in the estimation of coordinates is less then 1 centimeter. The maximum RMS error in estimating the base line length is found to be 1.03 cm, which is somewhat higher than expected. The probable cause is that the distance to the reference station is quite large.

Even though a detailed analysis of the data to estimate the deformations is only possible after repeat observations, the following problems related to the data collection, processing and analysis may be mentioned here:
  • Due to the difficult conditions in the earthquake-affected region immediately after the earthquake, extensive field work was not possible.
  • The existing GT stations in the area were established over 150 years ago, and many were found destroyed/disturbed.
  • Due non-availability of data from any reference station in the vicinity, a reference station at about 1500 km from the area had to be taken for data processing.
  • The co-ordinates of the existing GT stations are in Everest Datum, which must be converted to WGS84 Datum for comparison with the GPS-derived co-ordinates at the observed stations. Precise transformation parameters required for such conversion are presently not available, hence comparison with old co-ordinates will not yield precise estimates of the deformations.
Conclusions and Future Work
The GPS network in Koyna will yield precise estimates of the deformations in the dam structure, as well as the crustal deformations in the vicinity of the dam and reservoirs, after sufficient number of repeat observations, over a long period. It is planned to carry out such repeat observations at least twice a year over next 4-5 years. An integrated analysis of these results, with those obtained from the dam instrumentation, will help us in understanding the behaviour of the dam structure and its surroundings under different loading conditions and seismic activities. 

The first phase of GPS measurements in the Bhuj region has given precise estimates of the coordinates and baseline lengths of the GPS stations now established, to few mm accuracy. Due to the various problems related to the data processing and analysis mentioned above, a detailed analysis of the data and initial estimates of the deformation vectors is possible only after repeat observations. In the next phase, it is planned to re-observe the same network, and also add few more old GT stations to the network, for future monitoring of crustal deformations in the region. A comprehensive analysis for drawing inferences about the earthquake-related deformations would be possible only by comparison of data collected through GPS campaigns in different phases over a period of few years. This would yield reliable estimates of the displacements and velocities of the GPS stations in the region. An integrated analysis with data from other sources and techniques is necessary before definite conclusions are drawn from such results.

Acknowledgements
This work is supported by the Department of Science & Technology, Government of India, and Indian Institute of Technology Bombay, through a research grant. The field work was carried out by V. S. Tomar & Rahul Chandvaskar, Research Fellows, and C. Venkateswarlu, Ankur Manake, & Vikas, students of Civil Engineering Department of IITB, along with the author. The GPS instrumentation is made available by DST.

References
  • Bendick, R., R. Bilham, E. Fielding, V.K. Gaur, S.E. Hough, G. Kier, M.N. Kulkarni, S.Martin, K.Mueller, M.Mukul (2001). The 26 January 2001 “Republic Day” Eartrhquake, India, Seismological Research Letters, Vol.72,No.3, May/June.
  • DST (1998). Report on National Programme for GPS & Geodetic Studies, National GPS Expert Group, Dept. of Science & Technology, Govt. of India.
  • Kulkarni, M.N. (1986), Geodetic Surveys for Dam Deformation Studies, Indian Surveyor, January, Journal of the Institution of Surveyors (India): 23-25.
  • Kulkarni, M.N. (1998) Application of Geodesy to monitor Earthquake Hazard: An Overview, Proc. Eleventh Symposium on Earthquake engineering, Roorkee University, Roorkee, India, December.
  • Kulkarni, M. N. and A. Manake (2000). Dam Deformation Studies using GPS & Geodetic techniques, proc. Indian Geotechnical Conference-2000, Civil Engg. Dept., I. I. T. Bombay, India: 253-256.
  • Kulkarni, M. N. & B.C . Roy (1995). Geodetic VLBI to monitor Crustal Dynamics in India, proc. International Workshop on Latur Earthquake, NGRI, Hyderabad, India.
  • Manake, A. and Kulkarni, M. N. (2001). Study of Deformation of Koyna Dam using GPS, Submitted to Survey Review, International Journal of Surveying & Mapping, The Commonwealth Association of Surveying & Land Economy, UK.
  • Roy, B.C. and M.N. Kulkarni (1995) Geodetic Aspects of monitoring Seismotectonics in India, proc. International Workshop on Latur Earthquake, NGRI, Hyderabad, India, Oct.

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