Midcrustal low‐velocity layer beneath the central Himalaya and southern Tibet revealed by ambient noise array tomography

Ambient noise tomography has been becoming an important tool to image the shallow lithospheric structure of the Earth. Using 2 months of ambient noise data from 20 stations of the Himalayan Nepal Tibet Seismic Experiment, we investigate the upper and middle crustal structure in the central Himalaya and southern Tibet. About 120 interstation Rayleigh wave empirical Green's functions with sufficient signal‐to‐noise ratio are obtained and used for group velocity dispersion analysis in the period range 6–25 s using frequency‐time analysis technique. The obtained dispersion data are then used to construct 2‐D group velocity maps. At the short periods from 9 to 15 s, the distribution of Rayleigh wave velocities delineates several distinct low‐ and high‐velocity zones separated mainly by geological boundaries. The high group velocity zone is located mainly around regions with plutonic rocks, and the low group velocity zone is located around regions with sedimentary or metasedimentary rocks. Finally, we invert for the shear velocity structure in the upper and middle crust along a N‐S trending cross section at the longitude 86.5°E. We observe a clear low‐velocity layer in the middle crust (about 10–25 km depth) distributed on both sides of the Indus Yarlung Suture zone. The existence of this midcrustal low‐velocity zone suggests a mechanically weaker middle crust beneath the central Himalaya and southern Tibet, which might decouple the upper crustal deformation from that of the lower crust in the Tibetan‐Himalayan orogenic processes.