Similarities between the Madeira and Canary Hotspots Revealed by Seismic Anisotropy from Teleseismic and Local Shear-Wave Splitting with the SIGHT Project

2021 ◽  
Author(s):  
David Schlaphorst ◽  
Graça Silveira ◽  
João Mata ◽  
Frank Krüger ◽  
Torsten Dahm ◽  
...  
Keyword(s):  
Shear Wave ◽  
Canary Islands ◽  
Delay Time ◽  
Seismic Anisotropy ◽  
Vertical Component ◽  
Mantle Flow ◽  
Crust And Upper Mantle ◽  
Length Scales ◽  
Flow Models ◽  
Using Data

<p>The Madeira and Canary archipelagos, located in the eastern North Atlantic, are two of many examples of hotspot surface expressions, but a better understanding of the crust and upper mantle structure beneath these regions is needed to investigate their structure in more detail. With the study of seismic anisotropy, it is possible to assess the rheology and structure of asthenosphere and lithosphere that can reflect a combination of mantle and crustal contributions.</p><p>Here, as part of the SIGHT project (SeIsmic and Geochemical constraints on the Madeira HoTspot), we present the first detailed study of seismic anisotropy beneath both archipelagos, using data collected from over 60 local three-component seismic land stations. Basing our observations on both teleseismic SKS and local S splitting, we are able to distinguish between multiple layers of anisotropy. We observe significant changes in delay time and fast shear-wave orientation patterns on short length-scales on the order of tens of kilometres beneath the western Canary Islands and Madeira Island. In contrast, the eastern Canary Islands and Porto Santo the pattern is much more uniform. The detected delay time increase and more complex orientation patterns beneath the western Canary Islands and Madeira can be attributed to mantle flow disturbed and diverted on small-length scales by a strong vertical component. This is a clear indication of the existence of a plume at each of those archipelagos, nowadays exerting a strong influence on the western and younger islands. We therefore conclude that a plume-like feature beneath Madeira exists in a similar way to the Canary Island hotspot and that regional mantle flow models for the region should be reassessed.</p><p>This is a contribution to project SIGHT (Ref. PTDC/CTA-GEF/30264/2017). The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.</p>

Seismic anisotropy and mantle flow constrained by shear wave splitting in central Myanmar

2021 ◽  
Author(s):  
Enbo Fan ◽  
Yumei He ◽  
Yinshuang Ai ◽  
Stephen S. Gao ◽  
Kelly H. Liu ◽  
...  
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Mantle Flow ◽  
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

Mantle flow under the Central Alps: Constraints from non-vertical-ray SKS shear-wave splittting

2020 ◽  
Author(s):  
Eric Löberich ◽  
Götz Bokelmann
Keyword(s):  
Shear Wave ◽  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Single Layer ◽  
Depth Resolution ◽  
Shear Wave Splitting ◽  
Mantle Flow ◽  
Central Alps ◽  
Upper Mantle Anisotropy ◽  
Wave Splitting

<p>The association of seismic anisotropy and deformation, as e.g. exploited by shear-wave splitting measurements, provides a unique opportunity to map the orientation of geodynamic processes in the upper mantle and to constraint their nature. However, due to the limited depth-resolution of steeply arriving core-phases, used for shear-wave splitting investigations, it appears difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To change that, we take advantage of the different backazimuthal variations of fast orientation <em>φ</em> and delay time <em>Δt</em>, when considering the non-vertical incidence of phases passing through an olivine block with vertical b-axis as opposed to one with vertical c-axis. Both these alignments can occur depending on the type of deformation, e.g. a sub-horizontal foliation orientation in the case of a simple asthenospheric flow and a sub-vertical foliation when considering vertically-coherent deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Combining high-quality manual shear-wave splitting measurements of three datasets leads to a dense station coverage. Fast orientations <em>φ</em> show a spatially coherent and relatively simple mountain-chain-parallel pattern, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the whole study area together, our non-vertical-ray shear-wave splitting procedure points towards a b-up olivine situation and thus favors an asthenospheric anisotropy source, with a horizontal flow plane of deformation. We also test the influence of position relative to the European slab, distinguishing a northern and southern subarea based on vertically-integrated travel times through a tomographic model. Differences in the statistical distribution of splitting parameters <em>φ</em> and <em>Δt</em>, and in the backazimuthal variation of <em>δφ</em> and <em>δΔt</em>, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of asthenospheric flow, likely a depth-dependent plane Couette-Poiseuille flow around the Alps, the origin in the southern subarea remains more difficult to determine and may also contain effects from the slab itself.</p>

scholarly journals Seismic Anisotropy at the Rotokawa and Ngatamariki Geothermal Fields in the Taupo Volcanic Zone

2021 ◽  
Author(s):  
◽  
Stefan Mroczek
Keyword(s):  
Shear Wave ◽  
Delay Time ◽  
Seismic Anisotropy ◽  
Horizontal Stress ◽  
Shear Wave Splitting ◽  
Taupo Volcanic Zone ◽  
Volcanic Zone ◽  
Geothermal Fields ◽  
Wave Splitting ◽  
Maximum Horizontal Stress

<p>In order to investigate the cracks/fractures in the geothermal fields of Rotokawa and Ngatamariki, we measure seismic anisotropy across both fields and interpret the results in the context of stress aligned microcracks. Cracks aligned perpendicular to the direction of maximum horizontal stress close and their fluid is forced into cracks aligned with maximum horizontal stress (SHmax). Seismic anisotropy is the directional dependence of a seismic wave's velocity and provides a measure of crack orientation and density.  To measure seismic anisotropy we conduct shear wave splitting measurements on 52,000 station-earthquake pairs across both Rotokawa and Ngatamariki from earthquakes recorded during 2015. Both fields are the subject of other geophysical and geological studies. Thus they are excellent subjects for studying seismic anisotropy. We cluster our measurements by their station-event path and fit the parameters from these clusters to those from theoretical crack planes. We also apply 2-D tomography to shear wave splitting time delays (𝛿t) and spatial averaging to shear wave splitting fast polarisations (∅). In addition, we compare time delays with P-wave to S-wave velocity ratios (νP / vS).  Local measurements of stress within Rotokawa and regional measures of stress within the Taupo Volcanic Zone provide a comparison for the shear wave splitting measurements. We measure ∅ which agrees with the NE-SW regional direction of SHmax across Ngatamariki and parts of Rotokawa. Within Rotokawa, we observe a rotation of ∅ away from NE-SW toward N-S that agrees with borehole measurements of direction of SHmax of 023° and 030°. Spatial averaging of ∅ reveals mean orientations close to the strike of nearby active faults.  The theoretical crack planes, that fit best to the shear wave splitting measurements, correspond to aligned cracks striking 045° outside of both fields, 035° within Ngatamariki, and 035° through to 0° within Rotokawa.  The average percent anisotropy for the full dataset, approximately 4%, is close to the upper bound for an intact rock. Delay time tomography shows regions of higher delay time per kilometre of path length (s=km) within both fields and possibly associated with the production field fault in Rotokawa.  vP =vS shows a wide range of normally distributed values, from 1.1 through to 2.4 with a mean of 1.6, indicating a mixture of gas filled and saturated cracks. A positive correlation between delay time per kilometre (𝛿tpkm) and νP /νS indicates that the majority of the cracks are saturated.</p>

scholarly journals Shear-Wave Splitting in the Alpine Region

2021 ◽  
Author(s):  
Götz Bokelmann ◽  
Gerrit Hein ◽  
Petr Kolinsky ◽  
Irene Bianchi ◽  
AlpArray Working Group
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Eastern Alps ◽  
Shear Wave Splitting ◽  
Western Alps ◽  
Alpine Region ◽  
Mantle Flow ◽  
Ocean Bottom ◽  
Wave Splitting ◽  
Ocean Bottom Seismometers

&lt;p&gt;To constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear-wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. 4 years of teleseismic earthquake data were processed automatically (without human intervention), from 724 temporary and permanent broadband stations of the AlpArray deployment including ocean-bottom seismometers. We have obtained an objective image of anisotropic structure in and around the Alpine region, at a spatial resolution that is unprecedented. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the central Alps. &amp;#160;The spatial variation of splitting delay times is particularly interesting. On one hand, there is a clear positive correlation with Alpine topography, suggesting that part of the seismic anisotropy (deformation) is caused by the Alpine orogeny. On the other hand, anisotropic strength around the mountain chain shows a distinct contrast between western and eastern Alps. This difference is best explained by the more active mantle flow around the Western Alps. We discuss earlier concepts of Alpine geodynamics in the light of these new observational constraints.&amp;#160;&lt;/p&gt;

scholarly journals Deep Anisotropic Structure under the Central Volcanic Region, New Zealand

2021 ◽  
Author(s):  
◽  
Sonja Melanie Greve
Keyword(s):  
Subduction Zone ◽  
Shear Wave ◽  
Large Scale ◽  
Mantle Wedge ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Small Scale ◽  
Mantle Flow ◽  
Delay Times ◽  
Wave Splitting

<p>Seismic anisotropy across the Hikurangi subduction zone measured from shear-wave splitting exhibits strong lateral changes over distances of about 250 km. Teleseismic S-phases show trench-parallel fast polarisations with increasing delay times across the forearc and arc region. In the arc region, delay times reach up to 4.5 s, one of the largest delay times measured in the world. Such large delay times suggest strong anisotropy or long travel paths through the anisotropic regions. Delay times decrease systematically in the backarc region. In contrast, local S-phases exhibit a distinct change from trench-parallel fast orientations in the forearc to rench-perpendicular in the backarc, with average delay times of 0.35 s. In the far backarc, no apparent anisotropy is observed for teleseismic S-phases. The three different anisotropic regions across the subduction zone are interpreted by distinct anisotropic domains at depth: 1) In the forearc region, the observed "average" anisotropy (about 4%) is attributed to trench-parallel mantle flow below the slab with possible contributions fromanisotropy in the slab. 2) In the arc region, high (up to 10%) frequency dependent anisotropy in the mantle wedge, ascribed to melt, together with the sub-slab anisotropy add up to cause the observed high delay times. 3) In the far backarc region, the mantle wedge dynamic ends. The apparent isotropy must be caused by different dynamics, e.g. vertical mantle flow or small-scale convection, possibly induced by convective removal of thickened lithosphere. The proposed hypothesis is tested using anisotropicwave propagation in two-dimensional finite difference models. Large-scale models of the subduction zone (hundreds of kilometres) incorporating the proposed anisotropic domains of the initial interpretation result in synthetic shear-wave splittingmeasurements that closely resemble all large-scale features of real data observations across the central North Island. The preferred model constrains the high (10%) anisotropy to the mantle wedge down to about 100 kmunder the CVR, bound to the west by an isotropic region under the western North Island; the slab is isotropic and the subslab region has average (3.5%) anisotropy, down to 300 km. This model succeeds in reproducing the constant splitting parameters in the forearc region, the strong lateral changes across the CVR and the apparent isotropy in the far backarc region, as well as the backazimuthal variations. The influence of melt on seismic anisotropy is examined with different small-scale (tens of kilometres) analytical modelling approaches calculating anisotropy due to melt occurring in inclusions, cracks or bands. Conclusions are kept conservative with the intention not to over-interpret the data due to model complexities. The models show that seismic anisotropy strongly depends on the scale of inclusions and wavelengths. Frequency dependent anisotropy for local and teleseismic shear-waves, e.g. for frequency ranges of 0.01-1Hz can be observed for aligned inclusions on the order of tens of meters. To test the proposed frequency dependence in the recorded data, two different approaches are introduced. Delay times exhibit a general trend of -3 s/Hz. A more detailed analysis is difficult due to the restricted frequency content of the data. Future studies with intermediate frequency waves (such as regional S-phases) are needed to further investigate the cause of the discrepancy between local and teleseismic shear-wave splitting. An additional preliminary study of travel time residuals identifies a characteristic pattern across central North Island. Interpretation highlights the method as a valuable extension of the shear-wave splitting study and suggests a more detailed examination to be conducted in future.</p>

Radially and azimuthally anisotropic shear-wave velocity model of the Earth’s upper mantle

2021 ◽  
Author(s):  
François Lavoué ◽  
Sergei Lebedev ◽  
Nicolas Celli ◽  
Andrew Schaeffer
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Global Scale ◽  
Horizontal Shear ◽  
Crust And Upper Mantle ◽  
Slip Mechanism ◽  
Radial Anisotropy

&lt;p&gt;We present new models of shear-wave velocity and of its radial and azimuthal anisotropy in the crust and upper mantle at global scale. Seismic anisotropy is the consequence of the preferential orientation of minerals due to deformation. The reconstruction of both its radial and azimuthal components provides insights into past and present deformation and flow in the lithosphere and asthenosphere. The full consideration of anisotropy also makes possible to accurately determine the isotropic shear-velocity average, and therefore to isolate the effects of thermal or compositional variations from those of anisotropic fabric.&amp;#160;&lt;/p&gt;&lt;p&gt;Our model is constrained by a large compilation of waveform fits for more than 750,000 vertical-component and 250,000 transverse-component seismograms. We follow a two-step procedure that comprises the Automated Multimode Inversion of surface, S, and multiple-S waveforms in a period range from 10 to 450 s, followed by a 3D tomographic inversion that reconstructs &lt;em&gt;dV&lt;sub&gt;SH&lt;/sub&gt;&lt;/em&gt; and &lt;em&gt;dV&lt;sub&gt;SV&lt;/sub&gt;&lt;/em&gt; velocity perturbations and their 4-&amp;#968; and 2-&amp;#968; azimuthal dependencies. The joint inversion of vertical and transverse components is regularised in terms of linear isotropic average perturbations &lt;em&gt;dV&lt;sub&gt;S0&lt;/sub&gt; = (dV&lt;sub&gt;SH&lt;/sub&gt;&lt;/em&gt; + &lt;em&gt;dV&lt;sub&gt;SV&lt;/sub&gt;&lt;/em&gt;)/2 and of radial anisotropy &amp;#948; &lt;em&gt;= dV&lt;sub&gt;SH&lt;/sub&gt;&lt;/em&gt; - &lt;em&gt;dV&lt;sub&gt;SV&lt;/sub&gt;&lt;/em&gt;.&lt;/p&gt;&lt;p&gt;We compare our model with other published anisotropic models. The different models show good agreement on major isotropic structures but relatively poor agreement on anisotropic features. We identify different patterns of anisotropy for different tectonic regions. At shallow depths (&lt; 60 km), there is a clear difference between oceanic and continental regions of different ages. While radial anisotropy is consistently negative (&lt;em&gt;V&lt;sub&gt;SH&lt;/sub&gt;&lt;/em&gt; &lt; &lt;em&gt;V&lt;sub&gt;SV&lt;/sub&gt;&lt;/em&gt;) in the top 50 km of oceanic lithosphere, it is positive (&lt;em&gt;V&lt;sub&gt;SH&lt;/sub&gt;&lt;/em&gt; &gt; &lt;em&gt;V&lt;sub&gt;SH&lt;/sub&gt;&lt;/em&gt;) under continents, with a thick layer of slightly positive anisotropy under cratons and a shallower layer of stronger anisotropy under phanerozoic crust, subject to more recent deformation. The largest anisotropy &amp;#8212;positive and exceeding 2% in our and most other models&amp;#8212; occurs between 70 and 150 km depth. This pattern is observed in both continents and oceans, and depends on their age and lithospheric thickness, which is indicative of the anisotropic fabric developed in the asthenosphere and frozen in the lithosphere. Finally, we observe a remarkable reversal from positive to negative anisotropy between 200 and 330 km depth over the entire globe. Again, the depth at which this reversal occurs depends on the tectonic settings: it is deeper under cratons and old oceans than under young continents and oceans. Synthetic tests demonstrate the robustness of this observation. While it could be interpreted as a transition from dominantly horizontal to dominantly vertical deformation in the mantle, this anisotropy reversal is also consistent with mineralogic experiments that suggest a transition in olivine slip mechanism which causes horizontal shear to induce negative seismic anisotropy below a certain depth. In lack of a satisfying scenario that could explain a global trend to vertical mantle flow between 260 and 410 km depth, we favour the second interpretation. If this interpretation is correct, our anisotropic model provides global-scale evidence for the transition in the olivine slip mechanism documented in the mineralogic literature.&lt;/p&gt;

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