scholarly journals Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations

2021 ◽  
Vol 225 (3) ◽  
pp. 2097-2119
Author(s):  
Brandon P VanderBeek ◽  
Manuele Faccenda
Keyword(s):  
Subduction Zone ◽  
Upper Mantle ◽  
Large Scale ◽  
Seismic Anisotropy ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Zone Model ◽  
Upper Mantle Anisotropy ◽  
Delay Times ◽  
Mantle Anisotropy

SUMMARY Despite the well-established anisotropic nature of Earth’s upper mantle, the influence of elastic anisotropy on teleseismic P-wave imaging remains largely ignored. Unmodelled anisotropic heterogeneity can lead to substantial isotropic velocity artefacts that may be misinterpreted as compositional heterogeneities. Recent studies have demonstrated the possibility of inverting P-wave delay times for the strength and orientation of seismic anisotropy. However, the ability of P-wave delay times to constrain complex anisotropic patterns, such as those expected in subduction settings, remains unclear as synthetic testing has been restricted to the recovery of simplified block-like structures using ideal self-consistent data (i.e. data produced using the assumptions built into the tomography algorithm). Here, we present a modified parametrization for imaging arbitrarily oriented hexagonal anisotropy and test the method by reconstructing geodynamic simulations of subduction. Our inversion approach allows for isotropic starting models and includes approximate analytic finite-frequency sensitivity kernels for the simplified anisotropic parameters. Synthetic seismic data are created by propagating teleseismic waves through an elastically anisotropic subduction zone model created via petrologic-thermomechanical modelling. Delay times across a synthetic seismic array are measured using conventional cross-correlation techniques. We find that our imaging algorithm is capable of resolving large-scale features in subduction zone anisotropic structure (e.g. toroidal flow pattern and dipping fabrics associated with the descending slab). Allowing for arbitrarily oriented anisotropy also results in a more accurate reconstruction of isotropic slab structure. In comparison, models created assuming isotropy or only azimuthal anisotropy contain significant isotropic and anisotropic imaging artefacts that may lead to spurious interpretations. We conclude that teleseismic P-wave traveltimes are a useful observable for probing the 3-D distribution of upper mantle anisotropy and that anisotropic inversions should be explored to better understand the nature of isotropic velocity anomalies particularly in subduction settings.

Can Teleseismic Travel-Times Constrain 3D Anisotropic Structure in Subduction Zones? Insights from Realistic Synthetic Experiments

2020 ◽  
Author(s):  
Brandon VanderBeek ◽  
Manuele Faccenda
Keyword(s):  
Upper Mantle ◽  
Subduction Zones ◽  
Seismic Anisotropy ◽  
Joint Inversion ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Elastic Model ◽  
Anisotropic Structure ◽  
Travel Times ◽  
Delay Times

<p>Despite the well-established anisotropic nature of Earth’s upper mantle, the influence of elastic anisotropy on teleseismic tomographic images remains largely ignored. In subduction zones, unmodeled anisotropic heterogeneity can lead to substantial isotropic velocity artefacts that may be misinterpreted as compositional heterogeneities (e.g. Bezada et al., 2016). Recent studies have demonstrated the possibility of inverting P-wave delay times for the strength and orientation of seismic anisotropy assuming a hexagonal symmetry system (e.g. Huang et al., 2015; Munzarová et al., 2018). However, the ability of P-wave delay times to constrain complex anisotropic patterns, such as those expected in subduction settings, remains unclear as the aforementioned methods are tested using ideal self-consistent data (i.e. data produced using the assumptions built into the tomography algorithm) generated from simplified synthetic models. Here, we test anisotropic P-wave imaging methods on data generated from geodynamic simulations of subduction. Micromechanical models of polymineralic aggregates advected through the simulated flow field are used to create an elastic model with up to 21 independent coefficients. We then model the teleseismic wavefield through this fully anisotropic model using SPECFEM3D coupled with AxiSEM. P-wave delay times across a synthetic seismic array are measured using conventional cross-correlation techniques and inverted for isotropic velocity and the strength and orientation of anisotropy using travel-time tomography methods. We propose and validate approximate analytic finite-frequency sensitivity kernels for the simplified anisotropic parameters. Our results demonstrate that P-wave delays can reliably recover horizontal and vertical changes in the azimuth of anisotropy. However, substantial isotropic artefacts remain in the solution when only inverting for azimuthal anisotropy parameters. These isotropic artefacts are largely removed when inverting for the dip as well as the azimuth of the anisotropic symmetry axis. Due to the relative nature of P-wave delay times, these data generally fail to reconstruct anisotropic structure that is spatially uniform over large scales. To overcome this limitation, we propose a joint inversion of SKS splitting intensity with P-wave delay times. Preliminary results demonstrate that this approach improves the recovery of the magnitude and azimuth of anisotropy. We conclude that teleseismic P-wave travel-times are a useful observable for probing the 3D distribution of upper mantle anisotropy and that anisotropic inversions should be explored to better understand the nature of isotropic velocity anomalies in subduction settings.</p>

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>

Northwest U.S. crustal seismic anisotropy suggests crustal flow driven by vertical loads in the underlying mantle

2020 ◽  
Author(s):  
Eugene Humphreys ◽  
Jorge Castellanos ◽  
Robert Clayton ◽  
Jonathan Perry-Houts ◽  
YoungHee Kim ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Anisotropy Field ◽  
Azimuthal Anisotropy ◽  
Mantle Flow ◽  
Crustal Anisotropy ◽  
Vertical Loading ◽  
Close Proximity ◽  
Mantle Anisotropy ◽  
Tectonic Forces

&lt;p&gt;Azimuthal anisotropy in the NW U.S. crust is derived using 3-17 s Rayleigh waves derived using ambient noise from about 300 broadband stations. Velocity is resolved between all station pairs in close proximity, and velocity as a function of azimuth is determined for each station. Azimuthal anisotropy orientations point strongly toward tomographically-imaged high-velocity structures in the underlying mantle, but show no relation to the underlying mantle anisotropy field. We suggest that the crustal anisotropy is decoupled from lateral tectonic forces and is created by upper mantle vertical loading, which in turn generates lateral pressure gradients that drive channelized flow in the ductile mid and lower crust. This idea is tested with geodynamic modeling. Using reasonable values for crustal viscosity and mantle buoyancy structure, we find that the local buoyancy sources within the upper mantle will drive the viscous crustal flow in a manner that reproduces well the imaged crustal anisotropy. We conclude that mantle vertical loading, acting independently from mantle flow, can actively control crustal deformation on a scale of several hundred kilometers.&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>

Investigation of the Upper Mantle Anisotropy Beneath the Anatolian Plate and Surroundings by Shear Wave Splitting Analysis

2021 ◽  
Author(s):  
Ceyhun Erman ◽  
Seda Yolsal-Çevikbilen ◽  
Tuna Eken ◽  
Tuncay Taymaz
Keyword(s):  
Shear Wave ◽  
Upper Mantle ◽  
Large Scale ◽  
Shear Wave Splitting ◽  
Transform Faults ◽  
Complex Deformation ◽  
Lithospheric Deformation ◽  
Upper Mantle Anisotropy ◽  
Wave Splitting ◽  
Mantle Anisotropy

&lt;p&gt;The Anatolia, one of the most actively deforming continental regions of the Earth, is considered to be a natural laboratory for studying tectonic structures, complex deformation patterns, and intense seismicity at various scales. Active tectonics of this plate has been shaped by complex interactions between the Arabian, African and Eurasian plates. In the region, there are several suture zones associated with the closure of Tethys Ocean, large-scale transform faults (e.g. North Anatolian Fault) and geological structures developed in relation to extensional and compressional tectonics. Seismic anisotropy studies are needed to better understand the relationship between surface deformation and mantle dynamics, and to establish a connection between the involved deformation models and anisotropic structures in the lithosphere and asthenosphere layers beneath Anatolia. To evaluate lateral and vertical variations in the upper mantle anisotropy and thus underlying geodynamic processes, we apply teleseismic shear wave splitting (e.g. SKS, PKS, SKKS) analyses using about 500 broad-band seismic stations located throughout Anatolia, which belong to AFAD, KOERI and NOA seismic networks. Splitting intensities (SI) were calculated for the entire data set to compare piercing parameters obtained from both SI and SWS techniques. Overall, the NE-SW fast directions were observed for the entire Anatolia. Local changes in FPDs and DTs should be interpreted with caution as they will give important clues about the correlation between existing tectonic forces and upper mantle deformation. In particular, complex anisotropy signature along the large-scale transform faults (NAF and EAF) was investigated by using multisplit approach (e.g., Eken and Tilmann, 2014) that uses a grid search over four splitting parameters of two-layer anisotropy. A bootstrap-based analysis was performed to statistically evaluate the possible variations in two-layer models. Preliminary results reveal that a two-layer anisotropy exists at the western part of the Anatolia along the NAF. The obtained two-layer anisotropy models imply that signatures of lithospheric deformation and of asthenospheric flow driven shearing remarkably differ in NW Anatolia. In this part of the Anatolian plate, we observed large time delays up to ~2.2 sec, and fast polarization directions: i) mainly consistent with the strike of NAF in the lithosphere, ii) N-S oriented in the asthenosphere that is likely attributed to the mantle flow regime under the influence of slab roll-back and trench retreat along the Hellenic subduction zone.&lt;/p&gt;

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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