The earthquake hazard associated with the Main Himalayan Thrust (MHT) is a critical issue for India and its neighbouring countries in the north. We used data from a dense seismic network in Uttarakhand, India, to model the lateral variations in the depths of MHT (2–6% drop in Vs at 12–21 km depths), Moho (a sharp increase in Vs (by ~ 0.5–0.7 km/s) at 39–50 km depths) and lithosphere (a marked decrease in Vs(~ 1–3%) at 136–178 km depths), across the Himalayan collisional front. Our joint inversion of radial PRFs and group velocity dispersion data of Rayleigh waves detects three NNE trending transverse lithospheric blocks segmenting the lithosphere in Uttarakhand Himalaya, which spatially correlate well with the northward extension of the Delhi -Haridwar Indian basement ridge, an inferred tectonic boundary and great boundary fault, respectively. Our radial receiver function imaging detects highly deformed and segmented crustal and lithospheric structures associated with three mapped transverse lithospheric blocks, suggesting a reduction in rupture lengths of future earthquakes, thereby, reducing earthquake hazards in Uttarakhand.
Cratons are the oldest regions of the Earth with generally undeformed crust since the Paleoproterozoic (Lee et al., 2011). They form the interior shield of continents with very little topography and are surrounded by mobile belts that are successively younger toward the continental margins. Most of the cratons are underlain by thick (150-250 km), cold and buoyant lithosphere composed of depleted mantle with 20%-30% melt removed. Partial melting and melt extraction in the Earth's upper mantle can significantly modify the composition and therefore the geophysical properties of the solid residues (Afonso & Schutt, 2012). The highly depleted rocks can be significantly less dense and generally seismically faster than their fertile counterparts. The cratonic lithosphere is generally characterized by high shear wave velocity (>4.6 km/s), low surface heat flow (<40 ± 11 mW/m 2 ) and lower density due to the presence of highly melt-depleted peridotites abundant in forsterite-rich olivine
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