PIs: Kelly Liu and Stephen Gao
06/2002 - 05/2005 by National Science Foundation
So far the most of the discontinuities were found in the top 1/3 of the Earth's mantle, and there have been only a limited number of direct observations or indirect indications for discontinuities in the lower mantle. In addition, most of them were found in localized areas. We search for discontinuities in the lower mantle by using about 40,000 three-component source-normalized broadband seismograms recorded by two global (GDSN and GEOSCOPE), two regional (SCSN and NCSN) permanent seismic networks, as well as those from several large-scale portable seismic experiments. The discontinuities are imaged by stacking P-to-S converted phases. The results of the stacking are checked against those using synthetic seismograms to avoid mis-identification of the phases. Results so far revealed several possible discontinuities at the depth of 1100, 1600, 2000, 2450, and 2650 km. The velocity contrasts across those discontinuities are in the order of 1 to 2 percent. Comparison of our results with previous ones, and the implications of those findings on the dynamics and composition of the mantle will be discussed.