The investigators propose to perform a comprehensive search for lower mantle discontinuities by using tens of thousands seismograms recorded by about 450 broadband seismic stations worldwide in two global and one regional seismic networks, and two large portable seismic arrays. About 70,000 three-component, source-normalized seismograms with strong P (or P_diff) arrivals will be used to image the discontinuities by stacking P-to-S converted phases, and about 25,000 source-normalized seismograms with strong SS arrivals will be used to study discontinuities by stacking bottom-side reflected S waves at the mid-point of the source-receiver path. The same amount of synthetic seismograms will be stacked to verify the results. Preliminary work from stacking about 20,000 P-to-S receiver functions recorded by the Global Digital Seismic Network revealed several possible discontinuities at the depths of 1100, 1600, 2000, 2450, and 2650 km. The stacking amplitudes of those discontinuities are about 1/6 to 1/2 of that of P410S, implying that the velocity contrasts across those apparent discontinuities are in the order of 1 to 2%.
The proposed work includes improving the preliminary observations by using three times more data, complementing and verifying the P-to-S results by stacking SS precursors and using synthetic seismograms, mapping spatial variation of lower mantle discontinuities by stacking P-to-S conversions and SS precursors in small caps, and finally, making the processed about 100,000 three-component, broadband seismograms public by establishing a web-based database.
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