POSSIBLE SEISMIC DISCONTINUITIES IN THE LOWER MANTLE

Seismic velocity discontinuities reflect abrupt changes in composition, mineralogy, temperature, or mantle fabric as a function of depth. Seismic studies of the existence, velocity contrast, depth, and sharpness of those discontinuities have played an important role in the understanding of the structure, dynamics, and evolution of various parts of the Earth. In particular, positive detection of a ubiquitous discontinuity in the depth near 1000 km may provide the most critical supporting evidence for a layered convective system in the mantle. The detection of another discontinuity in the depth range of 1500-1900 km may support recent models on mantle dynamics and composition made based on geodynamic modeling and geochemical observations.

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 depths 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.