| The continent-continent convergence of India with Asia 40-60 Myr ago produced some of the most spectacular topographical features on Earth. The Himalayan Nepal Tibet Seismic Experiment (HIMNT), a temporary broadband seismic network in eastern Nepal and the southern Tibetan Plateau from 2001-2003, was an attempt to better define the upper lithospheric structure, deformation processes, and to examine how 40 mm/yr of plate convergence is absorbed in the region. This study focused on (1) background seismic noise analysis, (2) earthquake processes, and (3) body wave attenuation and its implications. Spectral analysis of the ambient seismic signals shows levels within low and high global models and noise variations independent of vault type.; Synthesis of body waves and moment tensor inversion from local earthquakes within and near the HIMNT network show that convergence processes are accommodated not only in the shallow crust, but in the upper mantle of India beneath the Himalaya. From the pressure and tension axes orientations, body forces appear to contribute to earthquake deformation in the crust, whereas convergent forces dominate in the upper mantle. Focal depths determined near and below the crust-mantle boundary supports the bimodal strength model for the continental lithosphere.; Spectral modeling of P and S phases reveals nearly equal seismic attenuation in the upper lithosphere and large attenuation sources in the Tibetan crust. These attenuation measurements suggest that sources of seismic energy loss other than partial melt in the Tibetan Plateau crust, a key component for many Himalayan formation scenarios, may be the dominant attenuation mechanisms. These attenuation mechanisms include partially saturated crust from metamorphic dehydration processes or crustal heating from crustal thickening processes. |
