Evidence for a shear velocity discontinuity in the lower mantle beneath India and the Indian Ocean

Abstract

SH and sSH seismograms are modeled to determine the shear velocity structure in the D" region beneath India and the Indian Ocean. The signals show waveform complexities similar to those observed in data sampling the D" region beneath Alaska, the Caribbean, and Eurasia (Lay and Helmberger), which have been attributed to a 2.7% shear velocity discontinuity ~ 280 km above the core-mantle boundary. The new data set consists of long-period tangential component recordings at WWSSN stations in Africa, the Middle East, and Europe for 11 intermediate and deep focus Indonesian earthquakes. In the distance range 70-82[deg] the waveforms show an arrival between SH and ScSH with systematic moveout. From 89 to 94[deg] there is a strong distortion of the SH waveforms, indicating the arrival of several phases closely spaced in time. The relative time shifts of similar complexity in the corresponding sSH phases requires a deep mantle origin. The depth dependence and moveout of the interference effects are well-predicted for both SH and sSH phases by a model with a lower mantle discontinuity. Alternative explanations of the interference as resulting from receiver reverberations, SKS contamination, multiple source complexity, or near source multipathing are ruled out by systematic tests. While it is apparent that lateral variations in the lower mantle velocity structure prevent any single model from fitting all of the data, synthetic waveform modeling (using generalized ray theory and reflectivity) shows that the data can be well-fit by a model with a discontinuity similar in size and depth to that proposed for the previously investigated regions (Lay and Helmberger), but with a negative velocity gradient within the D" layer.