Crustal seismic structure beneath the Garhwal Himalaya using regional and teleseismic waveform modelling

We investigate the crustal seismic structure of the Garhwal Himalayan region using regional and teleseismic earthquake waveforms, recorded over 19 closely spaced broad-band seismic stations along a linear profile that traverses from the Sub Himalayas to Higher Himalayas. The regional earthquake traveltime analysis provides uppermost mantle P - and S-wave velocities as 8.2 and 4.5 km s(-1), respectively. The calculated receiver functions from the teleseismic P waveforms show apparent P-to-S conversions from the Moho as well as from intracrustal depths, at most of the seismic stations. These conversions also show significant azimuthal variations across the Himalayas, indicating complex crustal structure across the Garhwal Himalaya. We constrain the receiver function modelling using the calculated uppermost mantle (Pn and Sn) velocities. Common conversion point stacking image of P-to-S conversions as well as receiver function modelling results show a prominent intracrustal low shear velocity layer with a flat-ramp-flat geometry beneath the Main Central Thrust zone. This low velocity indicates the possible presence of partial melts/fluids in the intracrustal depths beneath the Garhwal Himalaya. We correlate the inferred intracrustal partial melts/fluids with the local seismicity and suggest that the intracrustal fluids are one of the possible reasons for the occurrence of upper-to-mid-crustal earthquakes in this area. The results further show that the Moho depth varies from similar to 45 km beneath the Sub Himalayas to similar to 58 km to the south of the Tethys Himalayas. The calculated lower crustal shear wave velocities of similar to 3.9 and 4.3 km s(-1) beneath the Lesser and Higher Himalayas suggest the presence of granulite and partially eclogite rocks in the lower crust below the Lesser and higher Himalayas, respectively. We also suggest that the inferred lower crustal rocks are the possible reasons for the presence and absence of the lower crustal seismicity beneath the Lesser and Higher Himalayas, respectively.