Local earthquake tomography of the Cachar fold and thrust belt in lower Assam, Northeast India

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 P- and S-wave velocity structure in three dimensions. The entire crust down to a depth of ≈ 40 km is seismically active with about 70% of the microearthquakes ranging in magnitude from 0.5 to 3.5 occurring in the upper crust. The velocity tomograms reveal alternating patterns of high and low velocity anomalies down to a depth of ≈8 km that correlate with the NE-SW trending en-echelon pattern of the anticlines and synclines of the Cachar fold belt (CFB). These structures are distinctly demarcated up to depths of ≈ 4 km revealing the deformation of the low velocity sediments (Vp ≈ 1.8 to 4.3 km/s) beyond which they are correlatable with the relief in the high velocity medium (Vp ≥ 5 km/s). The sediment (Vp < 5 km/s) thickness varies from ≈ 4 km above the anticlinal structural highs to ≈ 8 km in the broad synclines. The shallow (≤ 10 km) microseismicity is aligned in a NE-SW direction correlating predominantly with the major anticlines. The study reveals the geometry and depth extent of the structural features of CFB and their association with seismicity indicating faulting resulting from deformation due to the compressional tectonics in northeast India.