Quantifying Intrinsic and Extrinsic Contributions to Elastic Anisotropy Observed in Seismic Tomography Models

2021 ◽  
Author(s):  
John Keith Magali ◽  
Thomas Bodin ◽  
Navid Hedjazian ◽  
Yanick Ricard ◽  
Yann Capdeville
Keyword(s):  
Seismic Tomography ◽  
Large Scale ◽  
Seismic Anisotropy ◽  
Texture Evolution ◽  
Scaling Law ◽  
Preferred Orientation ◽  
Elastic Model ◽  
Effective Anisotropy ◽  
Crystallographic Preferred Orientation ◽  
Radial Anisotropy

<p>Large-scale seismic anisotropy inferred from seismic observations has been loosely interpreted either in terms of intrinsic anisotropy due to Crystallographic Preferred Orientation (CPO) development of mantle minerals or extrinsic anisotropy due to rock-scale Shape Preferred Orientation (SPO). The coexistence of both contributions misconstrues the origins of seismic anisotropy observed in seismic tomography models. It is thus essential to discriminate CPO from SPO in the effective anisotropy of an upscaled/homogenized medium, that is, the best possible elastic model recovered using finite-frequency seismic data assuming perfect data coverage. In this work, we investigate the effects of upscaling an intrinsically-anisotropic and highly-heterogeneous Earth's mantle. The problem is applied to a 2-D marble cake model of the mantle with a binary composition in the presence of CPO obtained from a micro-mechanical model. We compute the long-wavelength effective equivalent of this mantle model using the 3D non-periodic elastic homogenization technique. Our numerical findings predict that overall, upscaling purely intrinsically anisotropic medium amounts to the convection-scale averaging of CPO. As a result, it always underestimates the anisotropy, and may only be overestimated due to the additive extrinsic anisotropy from SPO. Finally, we show analytically (in 1D) and numerically (in 2D) that the full effective radial anisotropy ξ<sup>*</sup> is approximately just the product of the effective intrinsic radial anisotropy ξ<sup>*</sup><sub>CPO</sub> and the extrinsic radial anisotropy ξ<sub>SPO</sub>:</p><p>ξ<sup>* </sup>= ξ<sup>*</sup><sub>CPO </sub>× ξ<sub>SPO</sub></p><p>Based on the above relation, it is imperative to homogenize a texture evolution model first before drawing interpretations from existing anisotropic tomography models. Such a scaling law can therefore be used as a constraint to better estimate the separate contributions of CPO and SPO from the effective anisotropy observed in tomographic models.</p>

Crystallographic preferred orientation of akimotoite and seismic anisotropy of Tonga slab

Nature ◽  
2008 ◽  
Vol 455 (7213) ◽  
pp. 657-660 ◽  
Author(s):  
Rei Shiraishi ◽  
Eiji Ohtani ◽  
Kyuichi Kanagawa ◽  
Akira Shimojuku ◽  
Dapeng Zhao
Keyword(s):  
Seismic Anisotropy ◽  
Preferred Orientation ◽  
Crystallographic Preferred Orientation

Lattice preferred orientation of talc and implications for seismic anisotropy

2020 ◽  
Author(s):  
Jungjin Lee ◽  
Haemyeong Jung ◽  
Reiner Klemd ◽  
Matthew Tarling ◽  
Dmitry Konopelko
Keyword(s):  
High Pressure ◽  
Subduction Zones ◽  
Seismic Anisotropy ◽  
Preferred Orientation ◽  
Azimuthal Anisotropy ◽  
Hydrous Minerals ◽  
Ultra High Pressure ◽  
Lattice Preferred Orientation ◽  
Radial Anisotropy ◽  
First Time

<p>Strong seismic anisotropy is generally observed in subduction zones. Lattice preferred orientation (LPO) of olivine and elastically anisotropic hydrous minerals has been considered to be an important factor causing anomalous seismic anisotropy. For the first time, we report on measured LPOs of polycrystalline talc. The study comprises subduction-related ultra-high-pressure metamorphic schists from the Makbal Complex in Kyrgyzstan-Kazakhstan and amphibolite-facies metasomatic schists from the Valla Field Block in Unst, Scotland. The here studied talc revealed a strong alignment of [001] axes (sub)normal to the foliation and a girdle distribution of [100] axes and (010) poles (sub)parallel to the foliation. The LPOs of polycrystalline talc produced a significant P–wave anisotropy (AVp = 72%) and a high S–wave anisotropy (AVs = 24%). The results imply that the LPO of talc influence both the strong trench-parallel azimuthal anisotropy and positive/negative radial anisotropy of P–waves, and the trench-parallel seismic anisotropy of S–waves in subduction zones.</p>

Preferred orientation and elastic anisotropy in shales

Geophysics ◽  
2007 ◽  
Vol 72 (2) ◽  
pp. D33-D40 ◽  
Author(s):  
Ivan Lonardelli ◽  
Hans-Rudolf Wenk ◽  
Y. Ren
Keyword(s):  
Seismic Anisotropy ◽  
Elastic Anisotropy ◽  
Rietveld Method ◽  
Orientation Distribution ◽  
Preferred Orientation ◽  
Crystallographic Preferred Orientation ◽  
Clay Particles ◽  
Crystal Properties ◽  
Overlapping Peaks ◽  
Structure And Microstructure

Anisotropy in shales is becoming an important issue in exploration and reservoir geophysics. In this study, the crystallographic preferred orientation of clay platelets that contributes to elastic anisotropy was determined quantitatively by hard monochromatic X-ray synchrotron diffraction in two different shales from drillholes off the coast of Nigeria. To analyze complicated diffraction images with five different phases (illite/smectite, kaolinite, quartz, siderite, feldspar) and many overlapping peaks, we applied a methodology based on the crystallographic Rietveld method. The goal was to describe the intrinsic physical properties of the sample (phase composition, crystallographic preferred orientation, crystal structure, and microstructure) and compute macroscopic elastic properties by averaging single crystal properties over the orientation distribution for each phase. Our results show that elastic anisotropy resulting from crystallographic preferred orientation of the clay particles can be determined quantitatively. This provides a possible way to compare measured seismic anisotropy and texture-derived anisotropy and to estimate the contribution of the low-aspect ratio pores aligned with bedding.

scholarly journals Large-scale variation in seismic anisotropy in the crust and upper mantle beneath Anatolia, Turkey

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Cédric P. Legendre ◽  
Li Zhao ◽  
Tai-Lin Tseng
Keyword(s):  
Upper Mantle ◽  
Large Scale ◽  
Seismic Anisotropy ◽  
Crust And Upper Mantle ◽  
Wave Splitting ◽  
Mantle Processes ◽  
Open Question ◽  
Subduction System ◽  
Anisotropic Phase ◽  
Regional Tectonics

AbstractThe average anisotropy beneath Anatolia is very strong and is well constrained by shear-wave splitting measurements. However, the vertical layering of anisotropy and the contribution of each layer to the overall pattern is still an open question. Here, we construct anisotropic phase-velocity maps of fundamental-mode Rayleigh waves for the Anatolia region using ambient noise seismology and records from several regional seismic stations. We find that the anisotropy patterns in the crust, lithosphere and asthenosphere beneath Anatolia have limited amplitudes and are generally consistent with regional tectonics and mantle processes dominated by the collision between Eurasia and Arabia and the Aegean/Anatolian subduction system. The anisotropy of these layers in the crust and upper mantle are, however, not consistent with the strong average anisotropy measured in this area. We therefore suggest that the main contribution to overall anisotropy likely originates from a deep and highly anisotropic region round the mantle transition zone.

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