Radial Anisotropy and it's Relation to Fast and Slow Moving Tectonic Plates
Authors: Ho Tak , Priestley Keith , Debayle Eric 
Published: March 2021

&lt;p&gt;We present a new radially anisotropic (&lt;strong&gt;&amp;#958;)&lt;/strong&gt;&amp;#160;tomographic model for the upper mantle to transition zone depths derived from a large Rayleigh (~4.5 x 10&lt;sup&gt;6&amp;#160;&lt;/sup&gt;paths) and Love (~0.7 x 10&lt;sup&gt;6&lt;/sup&gt;&amp;#160;paths) wave path average dispersion curves with periods of 50-250 s and up to the fifth overtone. We first extract the path average dispersion characteristics from the waveforms. Dispersion characteristics for common paths (~0.3 x 10&lt;sup&gt;6&lt;/sup&gt;&amp;#160;paths) are taken from the Love and Rayleigh datasets and jointly inverted for isotropic V&lt;sub&gt;s&amp;#160;&lt;/sub&gt;and&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;. CRUST1.0 is used for crustal corrections and a model similar to PREM is used as a starting model. V&lt;sub&gt;s&lt;/sub&gt;&amp;#160;and&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;&amp;#160;are regionalised for a 3D model. The effects of azimuthal anisotropy are accounted for during the regionalisation. Our model confirms large-scale upper mantle features seen in previously published models, but a number of these features are better resolved because of the increased data density of the fundamental and higher modes coverage from which our&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;(z) was derived. Synthetic tests show structures with radii of 400 km can be distinguished easily. Crustal perturbations of +/-10% to V&lt;sub&gt;p&lt;/sub&gt;, V&lt;sub&gt;s&lt;/sub&gt;&amp;#160;and density, or perturbations to Moho depth of +/-10 km over regions of 400 km do not significantly change the model. The global average decreases from&amp;#160;&lt;strong&gt;&amp;#958;~&lt;/strong&gt;1.06 below the Moho to&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;~1 at ~275 km depth. At shallow depths beneath the oceans&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;&gt;1 as is seen in previously published global mantle radially anisotropic models. The thickness of this layer increases slightly with the increasing age of the oceanic lithosphere. At ~200 km and deeper depths below the fast-spreading East Pacific Rise and starting at somewhat greater depths beneath the slower spreading ridges,&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;&lt;1. At depths &amp;#8805;200 km and deeper depths below most of the backarc basins of the western Pacific&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;&lt;1. The signature of mid-ocean ridges vanishes at about 150 km depth in V&lt;sub&gt;s&lt;/sub&gt;&amp;#160;while it extends much deeper in the&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;&amp;#160;model suggesting that upwelling beneath mid-ocean ridges could be more deeply rooted than previously believed. The pattern of radially anisotropy we observe, when compared with the pattern of azimuthal anisotropy determined from Rayleigh waves, suggests that the shearing at the bottom of the plates is only sufficiently strong to cause large-scale preferential alignment of the crystals when the plate motion exceeds some critical value which Debayle and Ricard (2013) suggest is about 4 cm/yr.&lt;/p&gt;