Transition Zone Structure Beneath the Tibetan Plateau From P-Wave Triplications

We use upper-mantle triplication waveforms to determine upper mantle velocity structure beneath the Tibetan plateau. The data were recorded from 1500 earthquakes in the distance range between 12 and 30 degrees by more than 300 temporary and permanent seismic stations around Tibet since 1990. To eliminate source signatures in the waveforms and isolate structural responses, we developed a new method to estimate the source time function (STF) using records at teleseismic distances. A time-domain iterative deconvolution algorithm was used to remove the estimated STF from the data. The source-free triplicated P waveforms were then stacked and modeled using generalized ray theory. Preliminary results show that the 660-km discontinuity beneath the plateau is deeper than the global average, and the velocity contrast across the discontinuity is smaller in central Tibet.