Crustal velocity structure from surface wave dispersion tomography in the Indian Himalaya

Abstract

Background A network of 15 broadband seismographs in a ~500 km long, N-S array recorded 12 months of data in 2002-2003 (Rai et al. 2006). The array traverses the NW Himalaya, from the Indian plain in the south, across the Indus-Tsangpo Suture and the Tso Morari Dome, to the southern flank of the Karakoram in the north. Magnetotelluric (MT) studies in this region reveal low-resistivity zones which may be indicative of fluids, graphite, or partial melts in the mid-crust. We have tested these hypotheses by creating 1-D models of crustal shear wave velocity. The models contain lowvelocity zones at 25–40 km depth; these may be indicative of fluids or partial melts. Methods Our models are obtained by inverting group velocity dispersion curves of Rayleigh waves in the period range of roughly 4–60 s. Numerous magnitude 4 events, several magnitude 5 events, and one magnitude 6 event occurred 900 km or less from the array. We find dispersion curves by analyzing the z-component of fundamental mode Rayleigh waves using Robert Herrmann’s Computer Programs in Seismology (Herrmann and Ammon 2002). We invert the dispersion curves using these programs to create 1-D models of crustal shear wave velocity structure. The inversion is done to 150 km depth, but we consider only the upper 60 km of the models. Results