Shear wave splitting as a diagnostic of variable anisotropic structure of the upper mantle beneath central Fennoscandia

2007 ◽  
Vol 438 (1-4) ◽  
pp. 57-77 ◽  
Author(s):  
L. Vecsey ◽  
J. Plomerová ◽  
E. Kozlovskaya ◽  
V. Babuška
Keyword(s):  
Shear Wave ◽  
Upper Mantle ◽  
Shear Wave Splitting ◽  
Anisotropic Structure ◽  
Wave Splitting

Seismic anisotropy of the crust and upper mantle beneath western Tibet revealed by shear wave splitting measurements

2018 ◽  
Vol 216 (1) ◽  
pp. 535-544 ◽  
Author(s):  
Changhui Ju ◽  
Junmeng Zhao ◽  
Ning Huang ◽  
Qiang Xu ◽  
Hongbing Liu
Keyword(s):  
Shear Wave ◽  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Crust And Upper Mantle ◽  
Wave Splitting

scholarly journals Measuring Crustal Seismic Anisotropy through Shear Wave Splitting

2021 ◽  
Author(s):  
◽  
Kenny Graham
Keyword(s):  
Numerical Simulation ◽  
New Zealand ◽  
Shear Wave ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Anisotropic Structure ◽  
Crustal Anisotropy ◽  
Physical Mechanisms ◽  
Alpine Fault ◽  
Wave Splitting

<p>This thesis involves the study of crustal seismic anisotropy through shear wave splitting. For the past three decades, shear wave splitting (SWS) measurements from crustal earthquakes have been utilized as a technique to characterize seismic anisotropic structures and to infer in situ crustal properties such as the state of the stress and fracture geometry and density. However, the potential of this technique is yet to be realized in part because measurements on local earthquakes are often controlled and/or affected by physical mechanisms and processes which lead to variations in measurements and make interpretation difficult. Many studies have suggested a variety of physical mechanisms that control and/or affect SWS measurements, but few studies have quantitatively tested these suggestions. This thesis seeks to fill this gap by investigating what controls crustal shear-wave splitting (SWS) measurements using empirical and numerical simulation approaches, with the ultimate aim of improving SWS interpretation. For our empirical approach, we used two case studies to investigate what physical processes control seismic anisotropy in the crust at different scales and tectonic settings. In the numerical simulation test, we simulate the propagation of seismic waves in a variety of scenarios.  We begin by measuring crustal anisotropy via SWS analysis around central New Zealand, where clusters of closely-spaced earthquakes have occurred. We used over 40,000 crustal earthquakes across 36 stations spanning close to 5.5 years between 2013 and 2018 to generate the largest catalog of high-quality SWS measurements (~102,000) around the Marlborough and Wellington region. The size of our SWS catalog allowed us to perform a detailed systematic analysis to investigate the processes that control crustal anisotropy and we also investigated the spatial and temporal variation of the anisotropic structure around the region. We observed a significant spatial variation of SWS measurements in Central New Zealand. We found that the crustal anisotropy around Central New Zealand is confined to the upper few kilometers of the crust, and is controlled by either one mechanism or a combination of more than one (such as structural, tectonic stresses, and gravitational stresses). The high correspondence between the orientation of the maximum horizontal compressive stress calculated from gravitational potential energy from topography and average fast polarization orientation around the Kaikōura region suggests that gravitationally induced stresses control the crustal anisotropy in the Kaikōura region. We suggest that examining the effect of gravitational stresses on crustal seismic anisotropy should not be neglected in future studies. We also observed no significant temporal changes in the state of anisotropy over the 5.5 year period despite the occurrence of significant seismicity.   For the second empirical study, we characterized the anisotropic structure of a fault approaching failure (the Alpine Fault of New Zealand). We performed detailed SWS analysis on local earthquakes that were recorded on a dense array of 159 three-component seismometers with inter-station spacing about 30 m around the Whataroa Valley, New Zealand. The SWS analysis of data from this dense deployment enabled us to map the spatial characteristics of the anisotropic structure and also to investigate the mechanisms that control anisotropy in the Whataroa valley in the vicinity of the Alpine Fault. We observed that the orientation of the fast direction is parallel to the strike of the Alpine Fault trace and the orientations of the regional and borehole foliation planes. We also observed that there was no significant spatial variation of the anisotropic structure as we move across the Alpine Fault trace from the hanging wall to the footwall. We inferred that the geological structures, such as the Alpine Fault fabric and foliations within the valley, are the main mechanisms that control the anisotropic structure in the Whataroa valley.    For our numerical simulation approach, we simulate waveforms propagating through an anisotropic media (using both 1-D and 3-D techniques). We simulate a variety of scenarios, to investigate how some of the suggested physical mechanisms affect SWS measurements. We considered (1) the effect on seismic waves caused by scatterers along the waves' propagation path, (2) the effect of the earthquake source mechanism, (3) the effect of incidence angle of the incoming shear wave. We observed that some of these mechanisms (such as the incidence angle of the incoming shear wave and scatterers) significantly affect SWS measurement while others such as earthquake source mechanisms have less effect on SWS measurements. We also observed that the effect of most of these physical mechanisms depends on the wavelength of the propagating shear wave relative to the size of the features. There is a significant effect on SWS measurements if the size of the physical mechanism (such as scatterers) is comparable to the wavelength of the incoming shear wave. With a larger wavelength, the wave treats the feature as a homogeneous medium.</p>

scholarly journals Relation of crustal and upper mantle deformation beneath Sunda-Banda Island Arc inferred from shear-wave splitting analysis

2019 ◽  
Vol 1204 ◽  
pp. 012069
Author(s):  
Syuhada Syuhada ◽  
Nanang Tyasbudi Puspito ◽  
Titi Anggono ◽  
Nugroho Dwi Hananto ◽  
Hendra Grandis ◽  
...  
Keyword(s):  
Shear Wave ◽  
Upper Mantle ◽  
Shear Wave Splitting ◽  
Island Arc ◽  
Wave Splitting

Shear wave splitting as diagnostics of variable tectonic fabrics across the Eastern Alps – Bohemian Massif contact zone

2020 ◽  
Author(s):  
Luděk Vecsey ◽  
Jaroslava Plomerová ◽  
Vladislav Babuška ◽  
the AlpArray-EASI Working Group ◽  
the AlpArray Working Group
Keyword(s):  
Shear Wave ◽  
Bohemian Massif ◽  
Upper Mantle ◽  
Large Scale ◽  
Broad Band ◽  
Eastern Alps ◽  
Shear Wave Splitting ◽  
Splitting Methods ◽  
Mantle Lithosphere ◽  
Wave Splitting

&lt;p&gt;We examine lateral variations of shear-wave splitting evaluated from data recorded during the passive seismic experiments AlpArray-EASI (2014-2015) and AlpArray Seismic Network (2016-2019). The swath about 200 km broad and 540 km long along 13.3&amp;#176; E longitude was selected to study the large-scale anisotropy in the mantle lithosphere beneath the Bohemian Massif (BM) and the Eastern Alps. The region is covered by about 200 broad-band temporary and permanent stations.&lt;/p&gt;&lt;p&gt;The shear-wave splitting evaluation consists of several steps: it starts by automated identification and pre-processing of SKS waveforms, filtering and quality check. Then we analyse and, if needed, also correct seismic waveforms for seismometer mis-orientations of all stations used. To improve results of splitting analysis of signals distorted by noise, we carefully apply two splitting methods (eigenvalue, transverse energy). We stack splitting measurements for waves closely propagating within the upper mantle and include particle motion analysis. The modified version of the splitting methods (Vecsey et al., 2008) enables us to retrieve 3-D orientation of large-scale anisotropic structures in the mantle lithosphere and deformations within the sub-lithospheric part of the upper mantle.&lt;/p&gt;&lt;p&gt;Both the evaluated shear-wave splitting parameters and the particle motions are consistent within sub-regions of the Alpine and BM upper mantle and exhibit significant and often sudden lateral changes across the whole region. We relate such changes to sharply bounded anisotropic domains with uniform fossil fabrics in the mantle lithosphere.&lt;/p&gt;

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