Post‐seismic crustal deformation and strain rate in Bhuj region, western India, after the 2001 January 26 earthquake

SUMMARY The M w 7.6 Bhuj earthquake in Gujarat, western India, which occurred on 2001 January 26 was a major intraplate event in the Indian subcontinent. To study the characteristics of transient post‐seismic deformation at the earth's surface, to elucidate the distribution of strain accumulation rate and so understand the earthquake recurrence process, five GPS campaigns were conducted at 14 sites, in Bhuj region during 2001–2002. The daily variations in the site position coordinates and the baselines during the early after‐shock period (i.e. for 21 d starting from 2001 February 10–March 2) showed no short‐term post‐seismic crustal deformation. Neither logarithmic nor exponential function could be well fitted to any of the components of the time‐series of the position co‐coordinates, indicating no discernable contribution from viscoelastic relaxation, poroelastic rebound or afterslip mechanisms. The estimated velocity field in ITRF2000 spanning five epochs during 2001–2002 has an average of 50 mm yr –1 in north–northeast direction. To obtain a better perspective of post‐seismic deformation in the epicentral track, the residual velocity field has been expressed in an India fixed reference frame. The residual velocity of 11.62 mm yr −1 in a north–northwest direction and 5.24 mm yr −1 towards the south seen at sites south and north of the epicentre, respectively, indicates localized ongoing convergence in the epicentral region. At sites away from the epicentre no significant deformation took place. The estimated principal strain from the velocity field gives average compression and extension rates of 0.07 and 0.04 micro‐strain yr −1 , respectively. A zone of maximum compressive strain rate of 0.30 micro‐strain yr −1 with azimuth of 11° delineated north of the epicentral region shows good agreement with seismic deformation along the blind fault derived from earthquake focal mechanisms. These results suggest ongoing transpressional deformation across the area with a blocked structure embedded between the North Wagad Fault and South Wagad Fault. This deformation may be related to the present neo‐tectonic compressive stress regime of the Indian Plate due to its NNE movement against the collision front in the north and its proximity to the triple junction in the western continental margin of the study area.