Current computational resources and physical knowledge of the seismic wave generation and propagation processes allow for reliable numerical and analytical models of waveform generation and propagation. From the simulation of ground motion, it is easy to extract the desired earthquake hazard parameters. Accordingly, a scenario-based approach to seismic hazard assessment has been developed, namely the neodeterministic seismic hazard assessment (NDSHA), which allows for a wide range of possible seismic sources to be used in the definition of reliable scenarios by means of realistic waveforms modelling. Such reliable and comprehensive characterization of expected earthquake ground motion is essential to improve building codes, particularly for the protection of critical infrastructures and for land use planning. Parvez et al. (Geophys J Int 155:489-508, 2003) published the first ever neo-deterministic seismic hazard map of India by computing synthetic seismograms with input data set consisting of structural models, seismogenic zones, focal mechanisms and earthquake catalogues. As described in Panza et al. (Adv Geophys 53:93-165, 2012), the NDSHA methodology evolved with respect to the original formulation used by Parvez et al. (Geophys J Int 155:489-508, 2003): the computer codes were improved to better fit the need of producing realistic ground shaking maps and ground shaking scenarios, at different scale levels, exploiting the most significant pertinent progresses in data acquisition and modelling. Accordingly, the present study supplies a revised NDSHA map for India. The seismic hazard, expressed in terms of maximum displacement (Dmax), maximum velocity (Vmax) and design ground acceleration (DGA), has been extracted from the synthetic signals and mapped on a regular grid over the studied territory.
S U M M A R YWe investigate the seismic attenuation along a 160 km profile from the Lower to the High Himalaya in the Garhwal region, India, through the analysis of Lg waveforms from regional earthquakes recorded on 18 broadband seismographs with interstation spacing of 7-10 km. Lateral variability in attenuation is derived through the inversion of 36 two-station Q 0 (Lg Q at ∼1 Hz) measurements using a global optimization scheme. We observe a contrasting attenuation property in the two Himalayan belts: the Lower Himalaya has Q 0 of 742 ± 235 similar to those observed in the Indian shield, whereas the High Himalaya is characterized by an unusually low Q 0 value of 30-60. The seismic attenuation is also well correlated with the P-wave teleseismic traveltime residual pattern: faster arrival (approximately 0.2 s) at stations in the Lower Himalaya as compared to azimuthally independent time delay of ∼0.75 s for the High Himalayan stations. The high attenuation and low velocity in the High Himalaya suggests that low viscosity and partial melt in the crust can be correlated to the presence of Miocene leucogranite plutons in this Himalayan belt, a magmatic product of the Indo-Asian collision and presumably evidence of a partial melting event. This shallow low viscosity channel possibly connects the mid-crustal channel beneath Tibet.
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