Determination of the crustal structure in southern Tibet by dispersion andamplitude analysis of Rayleigh waves

Surface waves recorded by the nine broad-band stations of the INDEPTH II ex periment are analysed to study the crustal structure of southern Tibet. The ir frequency range is between approximately 0.015 and 0.050 Hz (i.e. betwee n 20 and 60 s period). Phase velocity dispersion curves are calculated for the regions north and south of the Tsangpo suture, using Wiener filtering o f fundamental mode Rayleigh waves incident upon the array with different ba ckazimuths, The two dispersion curves are inverted to obtain the S-wave cru stal models north and south of the suture, They show that a low-velocity la yer is present in the lower crust north but not south of the Tsangpo suture , To confirm these findings, variations in the amplitudes of the Rayleigh w aves across the suture are interpreted by numerical simulation. The strong amplitude variations at a 30 s period are reproduced using a model with a l ow-velocity layer in the lower crust north of the suture, a normal high-vel ocity layer to the south, and a sharp transition between these layers.