scholarly journals Lowermost Mantle Anisotropy Beneath Africa From Differential SKS ‐ SKKS Shear‐Wave Splitting

2019 ◽  
Vol 124 (8) ◽  
pp. 8540-8564 ◽  
Author(s):  
M. C. Reiss ◽  
M. D. Long ◽  
N. Creasy
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Wave Splitting ◽  
Mantle Anisotropy ◽  
Lowermost Mantle

scholarly journals A potential post-perovskite province in D″ beneath the Eastern Pacific: evidence from new analysis of discrepant SKS–SKKS shear-wave splitting

2020 ◽  
Vol 221 (3) ◽  
pp. 2075-2090 ◽  
Author(s):  
Joseph Asplet ◽  
James Wookey ◽  
Michael Kendall
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Azimuthal Anisotropy ◽  
Eastern Pacific ◽  
Core Mantle Boundary ◽  
The Core ◽  
Wave Splitting ◽  
Mantle Anisotropy ◽  
Lowermost Mantle

SUMMARY Observations of seismic anisotropy in the lowermost mantle—D″—are abundant. As seismic anisotropy is known to develop as a response to plastic flow in the mantle, constraining lowermost mantle anisotropy allows us to better understand mantle dynamics. Measuring shear-wave splitting in body wave phases which traverse the lowermost mantle is a powerful tool to constrain this anisotropy. Isolating a signal from lowermost mantle anisotropy requires the use of multiple shear-wave phases, such as SKS and SKKS. These phases can also be used to constrain azimuthal anisotropy in D″: the ray paths of SKS and SKKS are nearly coincident in the upper mantle but diverge significantly at the core–mantle boundary. Any significant discrepancy in the shear-wave splitting measured for each phase can be ascribed to anisotropy in D″. We search for statistically significant discrepancies in shear-wave splitting measured for a data set of 420 SKS–SKKS event–station pairs that sample D″ beneath the Eastern Pacific. To ensure robust results, we develop a new multiparameter approach which combines a measure derived from the eigenvalue minimization approach for measuring shear-wave splitting with an existing splitting intensity method. This combined approach allows for easier automation of discrepant shear-wave splitting analysis. Using this approach we identify 30 SKS–SKKS event–station pairs as discrepant. These predominantly sit along a backazimuth range of 260°–290°. From our results we interpret a region of azimuthal anisotropy in D″ beneath the Eastern Pacific, characterized by null SKS splitting, and mean delay time of $1.15 \, \mathrm{ s}$ in SKKS. These measurements corroborate and expand upon previous observations made using SKS–SKKS and S–ScS phases in this region. Our preferred explanation for this anisotropy is the lattice-preferred orientation of post-perovskite. A plausible mechanism for the deformation causing this anisotropy is the impingement of subducted material from the Farallon slab at the core–mantle boundary.

Sensitivity of SK(K)S and ScS phases to heterogeneous anisotropy in the lowermost mantle from global wavefield simulations

2021 ◽  
Vol 228 (1) ◽  
pp. 366-386
Author(s):  
Jonathan Wolf ◽  
Maureen D Long ◽  
Kuangdai Leng ◽  
Tarje Nissen-Meyer
Keyword(s):  
Wave Propagation ◽  
Shear Wave ◽  
Shear Wave Splitting ◽  
Forward Modelling ◽  
Finite Frequency ◽  
Specific Flow ◽  
Frequency Wave ◽  
Wave Splitting ◽  
Mantle Anisotropy ◽  
Lowermost Mantle

SUMMARY Observations of seismic anisotropy at the base of the mantle are abundant. Given recent progress in understanding how deformation relates to anisotropy in lowermost mantle minerals at the relevant pressure and temperature conditions, these observations can be used to test specific geodynamic scenarios, and have the potential to reveal patterns of flow at the base of the mantle. For example, several recent studies have sought to reproduce measurements of shear wave splitting due to D″ anisotropy using models that invoke specific flow and texture development geometries. A major limitation in such studies, however, is that the forward modelling is nearly always carried out using a ray theoretical framework, and finite-frequency wave propagation effects are not considered. Here we present a series of numerical wave propagation simulation experiments that explore the finite-frequency sensitivity of SKS, SKKS and ScS phases to laterally varying anisotropy at the base of the mantle. We build on previous work that developed forward modelling capabilities for anisotropic lowermost mantle models using the AxiSEM3D spectral element solver, which can handle arbitrary anisotropic geometries. This approach enables us to compute seismograms for relatively short periods (∼4 s) for models that include fully 3-D anisotropy at moderate computational cost. We generate synthetic waveforms for a suite of anisotropic models with increasing complexity. We first test a variety of candidate elastic tensors in laterally homogeneous models to understand how different lowermost mantle elasticity scenarios express themselves in shear wave splitting measurements. We then consider a series of laterally heterogeneous models of increasing complexity, exploring how splitting behaviour varies across the edges of anisotropic blocks and investigating the minimum sizes of anisotropic heterogeneities that can be reliably detected using SKS, SKKS and ScS splitting. Finally, we apply our modelling strategy to a previously published observational study of anisotropy at the base of the mantle beneath Iceland. Our results show that while ray theory is often a suitable approximation for predicting splitting, particularly for SK(K)S phases, full-wave effects on splitting due to lowermost mantle anisotropy can be considerable in some circumstances. Our simulations illuminate some of the challenges inherent in reliably detecting deep mantle anisotropy using body wave phases, and point to new strategies for interpreting SKS, SKKS and ScS waveforms that take full advantage of newly available computational techniques in seismology.

scholarly journals Insight into NE Tibetan Plateau expansion from crustal and upper mantle anisotropy revealed by shear-wave splitting

2017 ◽  
Vol 478 ◽  
pp. 66-75 ◽  
Author(s):  
Zhouchuan Huang ◽  
Frederik Tilmann ◽  
Mingjie Xu ◽  
Liangshu Wang ◽  
Zhifeng Ding ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Shear Wave ◽  
Upper Mantle ◽  
Shear Wave Splitting ◽  
Upper Mantle Anisotropy ◽  
Wave Splitting ◽  
Mantle Anisotropy ◽  
Insight Into ◽  
Ne Tibetan Plateau

scholarly journals Crustal seismic anisotropy of the Northeastern Tibetan Plateau and the adjacent areas from shear-wave splitting measurements

2019 ◽  
Vol 220 (3) ◽  
pp. 1491-1503 ◽  
Author(s):  
Nan Hu ◽  
Yonghua Li ◽  
Liangxin Xu
Keyword(s):  
Tibetan Plateau ◽  
Shear Wave ◽  
Seismic Anisotropy ◽  
Active Faults ◽  
Shear Wave Splitting ◽  
Crustal Anisotropy ◽  
Western Qinling ◽  
Northeastern Tibetan Plateau ◽  
Wave Splitting ◽  
Mantle Anisotropy

SUMMARY The Northeastern Tibetan Plateau has thickened crust and is still undergoing strong active crustal shortening and deformation. Crustal anisotropy can provide clues to how the crust is currently deforming and evolving. We use an automatic method to analyse the upper-crustal anisotropy of the NE Tibetan Plateau and the adjacent region using local earthquakes recorded at 39 permanent seismic stations during the period 2009–2018. The majority of the dominant fast directions are consistent with the maximum horizontal stress orientation, suggesting that the upper-crustal anisotropy is mainly controlled by the regional or local stress field. Several fault-parallel measurements are observed for stations on or near to the main faults. These fault-parallel fast directions indicate that the main mechanism of upper-crustal anisotropy is associated with shear fabric caused by deformation. Fast directions neither fault-parallel nor stress-parallel are observed at stations lying several kilometres away from fault zones, likely reflecting the combined influence of stress-aligned microcracks and active faults. A comparison between our upper-crustal anisotropy parameters and those inferred from previous anisotropy studies that used receiver function and teleseismic shear wave splitting measurements suggests that the crust has the same deformation mechanisms as mantle anisotropy in the southern part of the Western Qinling Fault, whereas the upper-crustal anisotropic mechanism is different from those of lower crust and mantle anisotropy in the northern part of the Western Qinling Fault. These observations imply that the Western Qinling Fault may be an important boundary fault.

Upper mantle anisotropy beneath North China from shear wave splitting measurements

2012 ◽  
Vol 522-523 ◽  
pp. 235-242 ◽  
Author(s):  
Lijun Chang ◽  
Chun-Yong Wang ◽  
Zhifeng Ding
Keyword(s):  
Shear Wave ◽  
Upper Mantle ◽  
North China ◽  
Shear Wave Splitting ◽  
Upper Mantle Anisotropy ◽  
Wave Splitting ◽  
Mantle Anisotropy
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