THE mega thrust Dec 26, 2004 Sumatra earthquake occurred due to thrust faulting on the interface of the subducting Indian and the overriding Burman plates. The rupture propagated 1300 km northwards within a duration of ~10 minutes at an average propagation rate of 2.1 km/s (Jade et al. 2005, Bilham et al. 2005). A large number of aftershocks followed the earthquake along the entire rupture plane for several days. GPS derived displacements indicate eastward motion of 12±3 mm and 6±3 mm at Bangalore (IISC) and Hyderabad (HYDE) IGS stations on the Indian plate due to this mega event. GPS sites in Northeast India suffered 3 to 5mm of southward deformation (Jade et al. 2007).

Northeast India is a highly complex tectonic zone bounded by Indo-Eurasian Himalayan thrust belt on the north and Indo-Burman range on the east. It lies far north of the Sumatra earthquake’s rupture line. The objective of this work is to estimate the daily change in the triangular area between the sites and to investigate the reason for the large sudden changes in the daily triangular area .

GPS Geodesy has been recognized in providing the most precise numbers for crustal deformation worldwide. Department of Science and Technology has launched a National GPS programme for monitoring crustal deformation due to earthquake occurrence and other geodynamic phenomena. As a part of this National programme CSIR Centre for Mathematical Modelling and Computer Simulation (C-MMACS), Bangalore established 12 permanent GPS stations and over 100 campaign stations across India in collaboration with various regional institutes to quantify intra-plate and inter-plate deformation in Indian subcontinent. The northeast network consists of 8 permanent stations whose details are given in Table 1. Other C-MMACS permanent stations used in the analysis are Bhopal (BHOP), Kodaikanal (KODI), Leh (RSCL), and Hanle (IAOH) (Table1, Figure 1). The IGS stations used in the analysis are also listed in same table and figure

Daily precise geocentric coordinates in XYZ Cartesian coordinate system for all the GPS sites obtained in ITRF 2000 reference Frame using GAMIT/GLOBK software were taken as the input for the estimation of areal strain for various triangles. The IGS stations Bangalore (IISC), Kunming (KUNM), Lhasa (LHAS), and Hyderabad (HYDE) along with the Northeast permanent network stations were used for triangular area calculations. The triangular area between a set of three stations each at one of its vertices were computed for the projection planes XY, YZ and ZX (Shinji et al. 2005).

A MATLAB program was written for this purpose which can read an input file consisting of X, Y, Z geocentric coordinates of 3 stations with their corresponding errors. It then estimates triangular area projected in all three planes for all the available days and its daily change ratio (Strain) with errors. The estimated Strain values and respective errors are then plotted in PPM scale.