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Local earthquake tomography was performed to image the subsurface structure beneath the seismically active Cachar fold and thrust belt in lower Assam, northeast India. A total of 3341 P phases and 1833 S phases corresponding to 180 local microearthquakes recorded over 11 months by a temporary, dense network of 76 stations were used to simultaneously locate the hypocenters and estimate the velocity structure in three dimensions. The entire crust till a depth of ≈ 40km is seismically active with about 70% of the microearthquakes occurring in the upper crust. The velocity tomograms were well resolved up to a depth of 10km and reveal strong lateral heterogeneity in the shallow upper crust in terms of alternating patterns of high and low velocity anomalies that correlate with the NE-SW trending en-echelon pattern of the Cachar fold belt. The velocity perturbations upto ± 10% are observed in the shallow subsurface (< 5km) beyond which they are within ± 5% and reduce to within ± 2% at a depth of 10 km. The vertical sections of P wave velocity reveal deformation of the sediments leading to shallow subsurface structural highs and lows that correlate with the surface topography. About 50% of the observed seismicity occurs within the sediments at shallow depths(≤ 5km) revealing faulting due to the compressional tectonics in northeast India. The present study provides seismological evidence of deformation of the shallow sediments accompanied by faulting leading to the observed seismicity consistent with thin skinned tectonics for the evolution of the Cachar fold belt.

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