Waveform Modeling of D" Discontinuity Structure

Waveform Modeling of D" Discontinuity Structure The SH-velocity wave field is shown at propagation time of 600 sec for a 500 km deep event with dominant source period of 6 sec. Selected wave fronts are labeled with black double-sided arrows. Ray paths are drawn in black for an epicentral distance of 75o. The calculation is done for the D" discontinuity (indicated with a dashed green line) model of Fig. 1. Non-linear scaling was applied to the wave field amplitudes to magnify lower amplitude phases.
The 3-D velocity structure of the deep mantle has been inferred from imaging procedures such as migration, tomography, stacking, and waveform modeling, all which utilize localized 1-D reference structures. As these methods often have limiting assumptions it is essential to assess to what extent 3-D solution models are self-consistent with the imaging procedures from which they were produced; this is possible through synthesizing waveforms in laterally varying media. We use a 3-D axi-symmetric finite difference algorithm (SHaxi) to model SH-wave propagation through cross-sections of recent 2- and 3-D lower mantle models along a north-south corridor roughly 700 km in length beneath the Cocos Plate. Synthetic seismograms with dominant periods up to 3 sec are computed to assess fit of 3-D model predictions to data. Model predictions show strong wave- form variability not observed in 1-D model predictions. It is challenging to predict 3-D structure based on localized 1-D mod- els when lateral structural variations are on the order of a few wavelengths of the energy used. Iterative approaches of computing synthetic seismograms and adjusting model characteristics by considering path integral effects are necessary to accurately model fine-scale D" structure.
</p><p>Thorne, M.S., Lay, T., Garnero, E.J., Jahnke, G., Igel, H., Seismic imaging of the laterally varying D" region beneath the Cocos Plate, Geophys. J. Int. (170), pp. 635-648, doi: 10.1111/j.1365-246X.2006.03279.x, 2007.</p>


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