Estimation of the drift and deformations of the Indian plate using GPS repeat measurements

M. N. Kulkarni, H. V. Save
Department of Civil Engineering, I.I.T., Bombay,
Tel: +91-22-5767308
kulkarni@civil.iitb.ernet.in, save6uc@yahoo.com

Abstract
The Global Positioning System is a satellite-based navigation and surveying system for determination of precise position and time using radio signals from satellites, in real-time or in post-processing mode. GPS is being used all over the world for determining the co-ordinates and baselines of important monuments specially established for crustal deformation studies. There are number of tectonically active plates on the earth's crust which are in constant motion, causing catastrophic earthquakes. Accurate measurements of these plate movements: their magnitude, velocity field and direction are crucial for understanding the crustal dynamics and the associated earthquake processes. GPS technique can do this accurately and economically. In this study, this important application of GPS has been investigated. The tectonic studies by various models have estimated the movement of the Indian Plate to be 55 mm/year in the North Easterly direction. In this case study, effort has been made to validate this estimated movement through GPS measurements, in addition to estimating the regional (intra-plate) and local deformations in the area of study. The results presented here are of intermediate nature, as the data analysis is still being done.

Introduction
The Global Positioning System (GPS) has found numerous applications in various fields, including navigation, surveying, mapping, remote sensing, developmental activities etc.

For everyday surveying, GPS has become highly competitive technique to the terrestrial surveying methods using theodolites and EDMs. Whereas, in the geodetic fields, GPS is likely to replace most techniques currently in use for determining precise horizontal positions of points more than a few tens of kms apart, thus revolutionizing the concept of geodetic surveying. In India, many organizations are actively involved in the use of GPS, and more are planning to join the ever-expanding GPS user community.

In this case study, an effort has been made to estimate the plate motion, and the intra plate and local crustal deformations. Detailed analysis of the data, and its interpretation are still continuing, hence the results presented here are not final.

GPS for Crustal Dynamics
With the high accuracy achieved by GPS in estimation of base line lengths, this relatively new geodetic positioning technique has assumed great importance in crustal dynamics studies. Precise GPS repeat measurements and data processing to achieve high accuracy, yield the estimates of deformations of the Earth's crust over the period of the repeat observations, both in the horizontal and vertical directions. Analyzing these results one can estimate the crustal dynamic parameters, which may lead to the prediction of an earthquake in future (Bilham et. al., 1995). Thus GPS data is valuable for understanding the complex process of Earthquakes (Kulkarni 1999).

Data Collection and Processing
Indian Institute of Geomagnetism (IIG), Mumbai, has been carrying out extensive GPS campaign in the Western Maharashtra region as a part of the 'All India GPS for Geodynamics Program' initiated by the Department of Science & Technology (DST) of the Government of India. Data from this campaign for 3 stations for the March 98 and March 99 epochs collected by IIG GPS teams have been obtained, and processed using Trimble GPSurvey 2.30 Software. The three stations occupied by IIG in the Western Maharashtra region selected for this study are Mandangad, Unhere and Pune. The reference stations used for the estimation of the deformations of the Indian Plate are Bishkek (pol2), an International GPS Service (IGS) permanent station in Kyrghyzstan, situated on the Eurasian plate and Indian Institute of Science (IISc), Bangalore, a station established by the Centre for Mathematical Modelling and Computer Simulation (C-MMACS), Bangalore, in the Peninsular shield of the Indian Plate. The data for these two IGS permanent stations have been downloaded from the IGS data centre website on the internet. For processing the data, the GPSurvey 2.30 software of Trimble Navigation Ltd. has been used.


Fig 1

Analysis of Results
The location of the 3 stations in the area under investigation, and the two reference stations, is shown in Fig. 1. The results obtained have been tabulated in Tables 1-3. The results for estimating the plate motion are given in Table 1, and plotted in Fig. 2. It can be seen from the results that the deformations estimated in the three baselines across the plate boundary are homogenous, and significant in comparison with the deformation standard deviations, indicating compression, as expected. Application of plate velocity model will yield actual estimates of plate motion. In the results for Intra-plate motion, presented in Table 2 deformations of the baselines between IISc station and the three field stations are also significant compared to the errors, and quite uniform, indicating expansion between the two regions. The local deformations in the area under investigation, tabulated in Table 3 and plotted in Fig. 4, appear to be significant, but they need further detailed analysis, before drawing inference. These results are of intermediate nature, and are being subjected to detailed analysis, as highlighted in the next paragraph.


Fig 2