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Lithosphere of Indus Block in the Northwest Indian Subcontinent through Genetic Algorithm Inversion of Surface-Wave Dispersion

Seismology Division, India Meteorological Department, Lodi Road, New Delhi 110003, India gsureshimd{at}yahoo.com

Saurabh Jain and S. N. Bhattacharya^*

Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India sinku.jain{at}gmail.com

^* Present address: 108 Mausam Apartments, West Enclave, Pitampura, Delhi 110034, India; sn_bhattacharya{at}hotmail.com.

The Indus block in the Northwest Indian subcontinent is a continental area lying between the Chaman fault in the west, the Aravalli range in the east, and the main boundary fault of Himalaya in the north. We evaluate the lithospheric structure of this crustal block through inversion of Love- and Rayleigh-wave group velocities obtained using broadband records at Bhuj from the Kashmir earthquake of 8 October 2005 and its aftershocks; the recording station lies in the southern edge and epicenters lie in the northern edge of this block. The wave paths are mostly in the study area in a direction parallel to the Aravalli trend. The period of the group velocity data ranges from 5.4 to 74.1 sec, and the inversion of these data resolves the structure down to the lithosphere–asthenosphere boundary (LAB). A nonlinear inversion has been carried out through a genetic algorithm with a wide solution space; some new concepts of solution space of a layered structure and of a misfit function are used. The mean and standard deviation of the 50 accepted solutions with low misfit give the structure IB11 for the Indus block; the standard deviation gives the estimation of the uncertainty and the resolution of the corresponding parameter. In IB11, the sedimentary thickness is 5.6 km within two layers. The total thickness of the crust is 44.2 km. The S-wave velocity below the crust is 4.393 km/sec, while this velocity is 4.603 km/sec in the Indian region to the east of the Aravalli range. In IB11, the LAB is at a depth of 79 km, which is much shallower than the corresponding depth of 120 km in the Indian region. On the other hand, the S-wave velocities below the crust as well as the depth of the LAB are similar to those of the Arabian shield. These similarities support the hypothesis that the Indus block is a detachment of the Arabian–Nubian shield.