Study of Earth’s Layered Structure on a Global Scale Using Broadband Seismic Datasets - fig. 1

Study of Earth’s Layered Structure on a Global Scale Using Broadband Seismic Datasets - fig. 1 Figure 1. Distribution of ray-piercing points of P-to-S converted phase at 1000 depth.
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Whether the Earth’s lower mantle is layered or has steady velocity gradients has important implications on various models related to the dynamics of the Earth’s deep interior. We are being funded by the National Science Foundation to systematically search for discontinuities in the Earth’s lower mantle.
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Networks/experiments that contributed to the project include GDSN, GEOSCOPE, US Advanced National Seismic Systems, Southern African Seismic Experiment, and Northern and Southern California Seismic Networks and a few smaller-scale portable experiments. About 80% of the seismograms were obtained from the IRIS Data Management Center. We have manually checked all the seismograms and have converted them into P-to-S receiver functions (RFs). About 60,000 high-quality radial receiver functions are moveout-corrected and stacked to image possible sharp velocity discontinuities in the lower mantle. Stacking of ray-piercing points beneath caps of different sizes is being finalized.
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Figures 1, 2, and 3 show preliminary results beneath the largest cap (i.e., the whole earth). Possible discontinuities at depths around 850, 900, 1100, 1220, and 1600 km are observable. For each observed seismogram, we are generating a synthetic seismogram using the real focal mechanism and station-event geometry. We will then convert the synthetics into receiver functions and stack them for the purpose of identifying multiples and other non P-to-S phases originated from known discontinuities.
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Since early this year, the same data set has also been used to map crustal thickness and Vp/Vs beneath the stations by stacking hundreds of high-quality P-to-S converted phases at the Moho recorded by each of the stations. The main scientific objective of the NSF-funded project is to answer the critical question of whether continental formation processes have been the same over geologic times, or have been changing temporally. This project will also produce a brief description of tectonic history and geologic setting in the vicinity of each of the stations.

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