Earthquake data collected by the INDEPTH-II Passive-Source Experiment show that there is a substantial south to north variation in the velocity structure of the crustal beneath southern Tibet. North of the Zangbo suture, beneath the southern Lhasa block, a mid-crustal low-velocity zone is revealed by inversion of receiver functions, Rayleigh-wave phase velocities, and modeling of the radial component of teleseismic P-waveforms, Conversely, to the south beneath the Tethyan Himalaya, no low-velocity zone was; observed, The presence of the midcrustal low-velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern Yadong-Gulu rift and possibly much of southern Tibet.