Lithospheric and upper mantle structure of southern Tibet from a seismological passive source experiment

Fifteen seismic stations were operated with about 20-km spacing in southern Tibet across the Zangbo suture (the collision zone between India and Asia) between May and October 1994 as part of the International Deep Profiling of Tibet and the Himalaya project (INDEPTH II) for wide-angle recording of the controlled source experiment and for passive earthquake recording. In addition, a dense deployment (4-km spacing) of stations within the German Depth Profiling of Tibet and the Himalayas (GEDEPTH) project also recorded a number of teleseismic earthquakes. The third data source used in this study is the records of the permanent broadband station at Lhasa. Teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from P-to-S converted phases. Important results are as follows. The Moho at 70-80 km and a second discontinuity at 50-60 km depth are observed over the entire profile south and north of the Zangbo suture. The data from the GEDEPTH dense array enable the detection of inclined structures penetrating the crust at the Zangbo suture. A pronounced low-velocity zone exists north of the Zangbo suture at about 10-20 km depth. The locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model IASP91 [Kennett, 1991] over a large region of the Himalaya and southern Tibet.