Abstract
Observations of shear-wave anisotropy are key for understanding the mineralogical structure and flow in the mantle. Several researchers have reported the presence of seismic anisotropy in the lowermost 150–250 km of the mantle (i.e., D” layer), based on differences in the arrival times of vertically (SV) and horizontally (SH) polarized shear waves. By computing waveforms at period > 6 s for a wide range of 1-D and 3-D Earth structures we illustrate that a time shift (i.e., apparent splitting) between SV and SH may appear in purely isotropic simulations. This may be misinterpreted as shear wave anisotropy. For near-surface earthquakes, apparent shear wave splitting can result from the interference of S with the surface reflection sS. For deep earthquakes, apparent splitting can be due to the S-wave triplication in D”, reflections off discontinuities in the upper mantle and 3-D heterogeneity. The wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 3909-3922 |
| Number of pages | 14 |
| Journal | Journal of Geophysical Research: Solid Earth |
| Volume | 123 |
| Issue number | 5 |
| DOIs | |
| State | Published - May 11 2018 |
Bibliographical note
KAUST Repository Item: Exported on 2020-10-01Acknowledgements: Waveforms simulations are available to the reviewers in this repository https://drive.google.com/drive/folders/0B1zZpyGk7o6GUnlZZEhNR1V3RVU?usp=sharing. If the manuscript is accepted for publication, the waveforms will be available in the eQuake-RC platform (http://equake-rc.info/). We thank the two anonymous reviewersand the associated-editor for helpful comments. This research was carried out on the High Performance Computing Cluster supported by the Research and Specialist Computing Support services at the University of East Anglia and on Archer, the UK's National Supercomputing Service. We thank Andy Nowacki, James Wookey and the CoMITAC project for providing us with the version of SPECFEM3D GLOBE modified for short{ period simulations (funded by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC Grant agreement 240473) and for fruitful discussions. Some figures were built using Generic Mapping Tools (GMT; Wessel & Smith 1998). This work also benefited from funding by the European Commission's Initial Training Network project QUEST (contract FP7-PEOPLE-ITN-2008- 238007, http://www.quest-itn.org) and discussions offered by the COST Action TIDES (http://www.tides-cost.eu/). AMGF and LP also thank funding from the Leverhulme Trust (project F/00 204/AS), followed by support from NERC project NE/K005669/1. In addition, AMGF also thanks support from NERC project NE/N011791/1. JR acknowledges support via NSF grant EAR-1565511.