Spatially dependent seismic anisotropy in the Tonga subduction zone: A possible contributor to the complexity of deep earthquakes (original) (raw)
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On the origin of radial anisotropy near subducted slabs in the midmantle
Geochemistry, Geophysics, Geosystems, 2019
Recent seismic studies indicate the presence of seismic anisotropy near subducted slabs in the transition zone and uppermost lower mantle (mid-mantle). In this study, we investigate the origin of radial anisotropy in the mid-mantle using 3-D geodynamic subduction models combined with mantle fabric simulations. These calculations are compared with seismic tomography images to constrain the range of possible causes of the observed anisotropy. We consider three subduction scenarios: (i) slab stagnation at the bottom of the transition zone; (ii) slab trapped in the uppermost lower mantle; and (iii) slab penetration into the deep lower mantle. For each scenario, we consider a range of parameters, including several slip systems of bridgmanite and its grain-boundary mobility. Modeling of lattice-preferred orientation shows that the upper transition zone is characterized by fast-SV radial anisotropy anomalies up to −1.5%. For the stagnating and trapped slab scenarios, the uppermost lower mantle is characterized by two fast-SH radial anisotropy anomalies of ∼+2% beneath the slab's tip and hinge. On the other hand, the penetrating slab is associated with fast-SH radial anisotropy anomalies of up to ∼+1.3% down to a depth of 2,000 km. Four possible easy slip systems of bridgmanite lead to a good consistency between the mantle modeling and the seismic tomography images: [100](010), [010](100), [001](100), and < 110 > { ̄ 110}. The anisotropy anomalies obtained from shape-preferred orientation calculations do not fit seismic tomography images in the mid-mantle as well as lattice-preferred orientation calculations, especially for slabs penetrating into the deep lower mantle. Plain Language Summary Seismology studies reveal that subducting slabs show different characteristics across the Earth; some flatten in the upper mantle (at 660-km depth), others are trapped in the uppermost lower mantle (660-to 1,200-km depth), and a few penetrate into the deep lower mantle. Subducting slabs cause the surrounding mantle to deform, but the way in which the minerals deform in the mid-mantle (410-to 1,200-km depth) remains poorly understood. Geodynamic modeling can help us to infer how the mantle flows and deforms around subduction zones. However, the pattern and evolution of mantle flow around the full range of subduction scenarios has yet to be studied in such detail. Therefore, in this study, geodynamic modeling is used to explore a range of mid-mantle parameters that best fit observations around subduction zones from seismology studies. Deformation in the mid-mantle induced by subducting slabs, including deeply penetrating slabs, is found to be consistent with a mechanism known as dislocation creep, which involves the movement of defects in the crystal lattice of rocks in the deep Earth, and agrees with recent seismic, geodynamic, and laboratory studies.
Some remarks on the origin of seismic anisotropy in the D” layer
Earth, Planets and Space, 1998
Physical mechanisms of seismic anisotropy in the D" layer are examined based on seismological and mineral physics observations. The results of body-wave seismology on the fine structure of the D" layer and of mineral physics studies on the elastic constants and the lattice preferred orientation in lower mantle minerals as well as the shape preferred orientation of melt pockets are taken into account. Evidence of large but depth (pressure)-dependent elastic anisotropy of lower mantle minerals, particularly (Mg,Fe)O, and of tilted shape preferred orientation of sheared partial melts is summarized. It is shown that both shape preferred orientation of partial melts (or iron-rich secondary phases) and lattice preferred orientation of minerals with well-documented slip systems are difficult to reconcile with seismological observations. However, lattice preferred orientation of highly anisotropic mineral, (Mg,Fe)O, is consistent with most of the seismic observations if the dominant glide plane under the D" layer conditions is {100} rather than {110} as observed at lower pressures. Such a change in glide plane in MgO (or (Mg,Fe)O) is likely to occur as a result of pressure-induced change in elastic anisotropy and/or in the nature of chemical bonding (and possibly due to high temperatures). Both solid-state and partial melt mechanisms of anisotropy imply that the V SH > V SV (V SV > V SH) polarization anisotropy means horizontal (vertical) flow. In the solid-state mechanism, significant V SH > V SV in the D" layer beneath the circum-Pacific (Alaska and the Caribbean) implies horizontal shear at high stress caused presumably by the collision of subducting materials with the core-mantle boundary. Highly variable anisotropy beneath the central-Pacific can be attributed to solid-state fabrics caused by a complicated threedimensional flow presumably related to the upwelling of plumes, but anisotropy in this region could also be attributed to the shape preferred orientation of melt pockets the presence of which is suggested by very low average velocities.
Ubiquitous lower-mantle anisotropy beneath subduction zones
Nature Geoscience, 2019
The Earth's upper and lower mantle have quite distinct physical properties, with the characteristics of material exchange between them being a long-debated issue. Progress in global seismic tomography in the 1990s 1,2 showed that the upper and lower mantle interact mainly via subducting slabs and mantle plumes, albeit subject to the presence of strong resistance along the upper-lower mantle boundary at ~660 km depth. More recently, enhanced tomography images showed that among the slabs that penetrate into the lower mantle, many of them stagnate down to about ~1,000 km depth 3. Conversely, mantle plumes rising from the deep lower mantle seem to deflect laterally when they reach this region 4. However, the uppermost lower mantle, located at depths of ~660-1,000 km, remains an enigmatic part of the Earth. It has been suggested that compositional layering 5,6 or a viscosity increase 7,8 may cause flow stagnation in this region, but its rheology and role in mantle convection are poorly understood. The stagnation of subducting slabs at ~660 km depth and their penetration into the lower mantle lead to intense strain and deformation around the slabs, which in turn can align mineral aggregates. As the most abundant lower-mantle mineral (bridgmanite) is anisotro-pic, observable seismic anisotropy should develop when considering a dislocation creep deformation mechanism 9-11. However, apart from the D" region in the lowermost mantle 12 , the presence of seismic anisotropy in the lower mantle is uncertain and debated 13-15 , with most previous seismological models suggesting that the bulk of the uppermost lower mantle is radially isotropic in shear wavespeed 16. To resolve this paradox, it has been proposed that the dominant deformation mechanisms in the lower mantle, such as superplastic flow 17 or a pure climb creep mechanism 18 , may not produce anisotropy. Observations of anisotropy in the uppermost lower mantle Some recent regional shear-wave splitting studies suggest the presence of anisotropy in the transition zone and uppermost lower mantle near some subduction zones 19-21. However, the limited depth resolution and azimuthal coverage in regional studies, together with the difficulty in isolating lower-mantle anisotropy from upper-mantle effects, can restrict the interpretation of these studies. While illuminating mostly large-scale features, global anisotropy tomography overcomes these issues by mapping the whole mantle, which is key to interpreting large-scale processes and global mantle flow in a unified way 22,23. Nevertheless, several issues such as the use of different data and modelling approaches, notably when handling crustal effects 24-26 , led to poor agreement between past global mantle anisotropy models. SGLOBE-rani is a recent whole-mantle shear-wave radially anisotropic model that is based on a large seismic dataset of over 43 million seismic measurements with complementary sensitivity to the entire Earth's mantle. It simultaneously models crustal thickness and mantle structure to reduce artefacts in the retrieved anisotropic structure 27,28. The use of a huge set of over 10 million surface-wave overtone measurements, which have sensitivity down to ~1,000 km depth (Supplementary Fig. 1), enables good data coverage in the transition zone (Supplementary Figs. 2-4). Below that, a large set of body-wave travel-time measurements assures good data coverage in the remainder of the lower mantle (Supplementary Fig. 3). However, the poor balance between SV-and SH-sensitive travel-time data in existing body-wave datasets leads to poorly resolved lowermost-mantle anisotropy and leakage effects 28 , in agreement with the findings from other previous whole-mantle anisotropy studies 29,30. Thus, we take the conservative approach of not interpreting any anisotropic structures below ~1,400 km depth. Chang et al. 28 compared SGLOBE-rani with other recent global anisotropy models and, as expected, found better correlations between the iso-tropic part of the models than between the anisotropic structure. Yet, a correlation of about 0.5 was found between the anisotropic structure in SGLOBE-rani and in the recent model Savani 31 , which Seismic anisotropy provides key information to map the trajectories of mantle flow and understand the evolution of our planet. While the presence of anisotropy in the uppermost mantle is well established, the existence and nature of anisotropy in the transition zone and uppermost lower mantle are still debated. Here we use three-dimensional global seismic tomography images based on a large dataset that is sensitive to this region to show the ubiquitous presence of anisotropy in the lower mantle beneath subduction zones. Whereas above the 660 km seismic discontinuity slabs are associated with fast SV anomalies up to about 3%, in the lower mantle fast SH anomalies of about 2% persist near slabs down to about 1,000-1,200 km. These observations are consistent with 3D numerical models of deformation from subducting slabs and the associated lattice-preferred orientation of bridgmanite produced in the dislocation creep regime in areas subjected to high stresses. This study provides evidence that dislocation creep may be active in the Earth's lower mantle, providing new constraints on the debated nature of deformation in this key, but inaccessible, component of the deep Earth.
Seismic anisotropy in the lowermost mantle beneath the Pacific
Geophysical Research Letters, 1998
Onset time differences of up to 3 s are ceding S s•t in S wave recordings of Tonga-Fiji earthobserved between transverse (S s•t) and longitudinal quakes at station HKT (Hockley, Texas), while record-(S sv) components of broadband S waves at distances ings at more distant stations in the northeastern United of 85 ø to 120 ø for paths traversing the lowermost States indicate a transition to TI [e.g., Vinnik et al., mantle (D") beneath the Pacific. After correction 1995]. We further explore the spatial variations of seisfor upper mantle anisotropy, S ss usually arrives ear-mic anisotropy beneath the Pacific using an extensive lief than S sv with the splitting increasing with dis-data set of S and Sdi•r wave recordings from broadband tance from 100 ø to 120 ø. The data yield two possible stations in North America. models of anisotropy: (1) anisotropy may vary laterally, with transverse isotropy existing in higher-than-2. Measurement of Seismic Anisotropy average shear velocity regions beneath the northeastern Pacific, or (2) anisotropy may vary with depth, with Anisotropy in D" produces subtle effects on S wavetransverse isotropy concentrated in a thin (100 km) forms [Maupin, 1994], These effects are not easily modthermal boundary layer at the base of D". A few record-eled because the presence of radial velocity gradients in ings at distances less than 105 ø show that S sv arrives D" and the strong velocity contrast at the core-mantle earlier than S sH, indicating that general anisotropy boundary (CMB) result in non-linear S and Sdia polarlikely exists in shallower regions of D". izations that are model dependent. This is illustrated in However, lateral vm:iafion of •he symmetry and m•gni•ude of seismic aniso•rop¾ in D" is also evident.
Variable Azimuthal Anisotropy in Earth's Lowermost Mantle
Science, 2004
A persistent reversal in the expected polarity of the initiation of vertically polarized shear waves that graze the Dµ layer (the layer at the boundary between the outer core and the lower mantle of Earth) in some regions starts at the arrival time of horizontally polarized shear waves. Full waveform modeling of the split shear waves for paths beneath the Caribbean requires azimuthal anisotropy at the base of the mantle. Models with laterally coherent patterns of transverse isotropy with the hexagonal symmetry axis of the mineral phases tilted from the vertical by as much as 20-are consistent with the data. Small-scale convection cells within the mantle above the Dµ layer may cause the observed variations by inducing laterally variable crystallographic or shapepreferred orientation in minerals in the Dµ layer.
Annual Review of Earth and Planetary Sciences, 2008
Seismic anisotropy is caused mainly by the lattice-preferred orientation of anisotropic minerals. Major breakthroughs have occurred in the study of lattice-preferred orientation in olivine during the past~ 10 years through large-strain, shear deformation experiments at high pressures. The role of water as well as stress, temperature, pressure, and partial melting has been addressed. The influence of water is large, and new results require major modifications to the geodynamic interpretation of seismic anisotropy in tectonically active ...
Development of texture and seismic anisotropy during the onset of subduction
Geochemistry Geophysics Geosystems, 2014
How reliable are shear wave splitting measurements as a means of determining mantle flow direction? This remains a topic of debate, especially in the context of subduction. The answer hinges on whether our current understanding of mineral physics provides enough to accurately translate between seismic observations and mantle deformation. Here, we present an integrated model to simulate strainhistory-dependent texture development and estimate resulting shear wave splitting in subduction environments. We do this for a mantle flow model that, in its geometry, approximates the double-sided Molucca Sea subduction system in Eastern Indonesia. We test a single-sided and a double-sided subduction case. Results are compared to recent splitting measurements of this region by Di Leo et al. (2012a). The setting lends itself as a case study, because it is fairly young and, therefore, early textures from the slab's descent from the near surface to the bottom of the mantle transition zone-which we simulate in our models-have not yet been overprinted by subsequent continuous steady state flow. Second, it allows us to test the significance of the double-sided geometry, i.e., the need for a rear barrier to achieve trench-parallel subslab mantle flow. We demonstrate that although a barrier amplifies trenchparallel subslab anisotropy due to mantle flow, it is not necessary to produce trench-parallel fast directions per se. In a simple model of A-type olivine lattice-preferred orientation and one-sided subduction, trench-parallel fast directions are produced by a combination of simple shear and extension through compression and pure shear in the subslab mantle.
The sensitivity of seismic free oscillations to upper mantle anisotropy 1. Zonal symmetry
Journal of Geophysical Research, 1993
t•ayleigh-Love coupling is a diagnostic tool for detecting anisotropic lateral structure in the upper mantle. The inference of mantle anisotropy, however, is complicated by the 21 independent parameters to be constrained in the fouxth-order elastic tensor. We simplify the elastic tensor to possess hexagonal symmetry about an arbitrary axis, a reasonable first-order model for deformed peridotire. The elastic tensor in this case is determined by the orientation of the symmetry axis and five linear coefficients that can be related to the Backus/Crampin equations for the azimuthal dependence of head wave velocity in marine refraction. Three anisotropic parameters B, C, and E govern the P wave cos 2f, cos 4f, and SV wave cos 2f terms, respectively, where/• is the angle from the symmetry axis. We estimate coupling interaction between pairs of seismic free oscillations using zonally symmetric lateral structuxe models with angular wavenumber s. Zonally symmetric models afford computational simplicity and emphasize forward scattering interaction. For mode,, pairs with angular degrees l, I t, couplinginteraction tends to peak near s = [l-ltl and s -l +l t. For mode pairs with f •20 mHz from distinct dispersion branches, particularly fundamental spheroidal and toroidal modes, the sensitivity io smooth isotropic lateral structuxe is very weak relative to anisotropic lateral structuxe. Sensitivity to the P wave cos 2f coefficient B tends to peak near the crust-mantle interface, suggesting poor potential for depth resolution. Sensitivity to/7 and E for fundamental and first overtone modes, however, often peaks at 100-to 250-kin depth and is often maximal for symmetry axes oriented between the vertical and horizontal. The parameter C•0.01 is small and weakly constrained in most estimates from seismic data and mineralogical studies. The parameter E --0.01 -0.03 in most estimates, and shear wave splitting studies argue for significant shear anisotropy in the uppermost mantle. Therefore it appears that S wave anisotropy is the likely too1 for constraining global-scale mantle deformation in the deep lithosphere and asthenosphere with coupled-mode surface wave anomalies. anomalous polarizations [Kirkwood and Crampin, 19818, b; Tajima and Kawasaki, 1989; Kawasaki and ]. In early studies, the surface waves were short period (< 35 s), largely crustal, and predominantly associated with the overtone Love and Rayleigh dispersion branches, which possess nearly identical phase and group ve-