High resolution maping of the crustal architecture by ambient noise tomography in the North Western Himalaya

Abstract:

A sizeable mid-crustal low velocity layer is mapped beneath the North-Western (NW) Himalaya and clear evidence for a decollement plane is reported. We performed ambient noise tomography for the NW Himalaya region using data of 31 broadband seismic stations. The ray paths sample the Himalayan region, the south Tibetan detachment zone and the Indo-Tsangpo suture zone, therefore covering the boundary regions of Indian and Eurasian plates to the South of the Karakoram fault. This part of the Himalayan region has witnessed devastating earthquakes such as Kashmir earthquake of 2005 and Kangra earthquake of 1905 along with many strong events. Rayleigh and Love waves data is utilized to extract dispersion curves for more than 500 paths for each wave. The spatial regional difference for group velocities are mapped for the periods in the range 4-40 sec, however most of the data are for period lower than 30 sec. The 2D tomography maps of fundamental mode highlight high lateral variations that may account for sub-surface tectonic deformation and variable crustal thicknesses. Larger variations are depicted for high period Rayleigh waves rather than Love waves and this mainly for the paths passing close to India-Tibet tectonic boundary. This study based on latest data gives new detail of sub-surface structural setup of the western part of Himalaya highlighting a low velocity mid-crustal layer characterized by an absence of lower crustal seismicity below decollment plane . A clear discontinuity within the physical properties mimics a possible decollement plane that could transfer sizeable earthquakes. Specifically a minimum value of Rayleigh wave velocity close to the decollement zone is reported and may likely correspond to mineral anisotropy while its existence to the lower part can be due to partial melting as per previous finding. The influence of Moho on the dispersion data suggests that this discontinuity is dipping towards north. However in the northern part close to India-Tibet plate tectonic boundary the Moho should be deeper than the efficiency of available data.