Crustal S-velocity and Poisson's ratio structure beneath the INDEPTH IV Transect: NE Tibetan Plateau to Qaidam Basin

During the INDEPTH IV controlled-source seismic experiment in June 2007, 949 vertical-component seismographs (IRIS-PASSCAL Texans) at 100-650 m station spacing and 20 broadband and 29 short-period three-component seismographs (34 from GIPP, Germany and 15 from SEIS-UK) at 5-6 km station spacing recorded 5 large shots (1000-2000 kg) and 100 small shots (80 kg) along a 270 km long profile across the Kunlun mountains in northeast Tibet. A detailed tomographic inversion of the well-recorded Sg arrivals observed by these 998 instruments reveals the S-velocity structure of the upper crust down to about 10 km depth beneath the profile. The major lateral variation in upper crustal S-velocities along the profile is from lower velocities beneath the Qaidam basin in the north to higher velocities beneath NE Tibet in the south. Beneath NE Tibet, there are also lateral variations with lower S-velocities beneath the valleys (basins) along which the North Kunlun and South Kunlun Faults run and higher velocities in the mountainous regions between and to the north and south, where older rocks are generally exposed at the surface. From the S-velocity model, there is no evidence within the upper crust for major overthrusting of NE Tibet over the Qaidam basin. Following the controlled-source experiment, 50 broadband seismographs (35 from GIPP, Germany and 15 from SEIS-UK) were deployed for a period of one year along two profiles across the Kunlun mountains and the Jinsha river suture in northeast Tibet. The profile across the Kunlun mountains consisting of 24 stations at 5 km spacing was coincident with the central 120 km of the controlled-source profile. It recorded a number of local earthquakes. Some of these events were in-line and thus the P and S arrivals from these in-line events provided useful additional information regarding the velocity structure below the profile. Together with the S waves generated from the shots a crustal S-velocity model is being derived and will be presented. For this model the boundaries from the now published P-velocity model are incorporated. Initial S velocities for the deeper parts of the model are derived from the P velocities of the published model divided by 1.732. Together, the published P-velocity model and the S-velocity model will provide a Poisson’s ratio model along the INDEPTH IV profile across the Kunlun mountains. From this model, crustal lithologies may be derived which in turn may constrain modes of formation of the Tibetan plateau.