Relocation of earthquakes in the Northeast Indian region using joint hypocentre determination method

A set of 1941 earthquake events recorded in the northeast region of India during January 1993 - December 1999 was used for relocation by the Joint Hypocentre Determination (JHD) technique. We have utilized both P- and S-wave arrivals recorded by closely spaced 77 temporary and permanent seismic stations in the region. Results of this analysis show that (a) the average root mean square travel time residual becomes smaller than the corresponding single event locations by the HYPO71; (b) station correction varies from -2.43 to 2.32 s for P-wave, and from -2.84 to 2.84 s for S-wave, indicating large crustal velocity variations in the region; (c) positive station corrections are obtained in the Shillong Plateau, Assam valley and in the Manipur fold belt, and negative station corrections are obtained in the Mikir hills, and (d) lateral variation of the velocity structure inferred from station corrections is comparable with that obtained by 3D velocity inversion using the local earthquake tomography method.