Lithospheric imaging beneath North India using surface wave tomography

We provide a shear wave velocity model of the Indo-Gangetic plain (IGP) that extends down to a depth of 100 km.  Using vertical component seismograms of 108 broadband (BB) stations (50 and 58) of IRIS-DMC and National Centre for Seismological (NCS) respectively, located in and around the Northern Indian plate. The group velocity dispersion of the Rayleigh wave has been picked along  ~3000 paths across the study region over a period range of 8 to 80s. To construct the 3D shear wave velocity structure, we employ a two-step surface wave tomography procedure. In the first step, regionalized dispersion maps are prepared for each period of correlation length of 60km, and subsequently, we employ the Markov chain Monte Carlo (McMC) trans-dimensional Bayesian inversion algorithm to obtain the shear wave velocity structure. In regionalized dispersion maps, at short periods (~8s) we see slow velocity in northern IGP and region reported thick basement (~6km) from previous studies. For moving towards increasing periods map indicate slow velocity anomalies in the Himalayan and Tibetan plateau region are associated with a thick crust (>50) in contrast to the typical crust (~40km) of IGP. The fast and slow velocities areas are identified which are associated with the Indian shield and thick crust in the Himalayas. Further, we inverted regionalized geographical locations to get shear wave velocity at each point to make a 3D lithospheric model. We have used 20 chains with 600k burn-in phase and 300k in the main phase for sampling the posterior distribution and from the final posterior distribution best 500k models with of 5% deviation has been selected after removing those model that has outlier chains with unrealistic models.

Keywords:  Surface waves, Bayesian inversion, Seismic tomography, Northwestern Himalayas.