Complex seismic anisotropy beneath western Tibet and its geodynamic implications
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In this article, we study the possibility that Ceres has, or had in the past, a crust heavier than a pure or muddy ice mantle, in principle gravitationally unstable. Such a structure is not unusual in the Solar system: Callisto is an example. In this work, we test how the composition (i.e. the volumetric quantity of ice) and the size of the crust can affect its survival during thermo-physical evolution after differentiation. We have considered two different configurations: the first characterized by a dehydrated silicate core and a mantle made of pure ice, the second with a hydrated silicate core and a muddy mantle (ice with silicate impurities). In both cases, the crust is composed of a mixture of ice and silicates. These structures are constrained by a recent measurement of the mean density by Park et al. The Rayleigh–Taylor instability, which operates in such an unstable structure, could reverse all or part of the crust. The whole unstable crust (or part of it) can interact chemically with the underlying mantle and what is currently observed could be a partially/totally new crust. Our results suggest that, in the case of a pure ice mantle, the primordial crust has not survived until today, with a stability timespan always less than 3 Gyr. Conversely, in the case of a muddy mantle, with some 'favourable' conditions (low volumetric ice percentage in the crust and small crustal thickness), the primordial crust could be characterized by a stability timespan compatible with the lifetime of the Solar system.
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The dependence of seismic wavespeeds on propagation or polarization direction, called seismic anisotropy, is a relatively direct indicator of mantle deformation and flow. Mantle seismic anisotropy is often inferred from measurements of shear-wave splitting. A number of standard techniques to measure shear-wave splitting have been applied globally; for example, *KS splitting is often used to measure upper mantle anisotropy. In order to obtain robust constraints on anisotropic geometry, it is necessary to sample seismic anisotropy from different directions, ideally using different seismic phases with different incidence angles. However, many standard analysis techniques can only be applied for certain epicentral distances and source-receiver ge-ometries. In this work, we apply a “wavefield differencing” approach to (systematically) understand what parts of the seismic wavefield are most affected by seismic anisotropy in the mantle. We systematically analyze differences between synthetic global wavefields calculated for isotropic and anisotropic input models, incorporating seismic anisotropy at different depths. Our results confirm that the seismic phases that are commonly used in splitting techniques are indeed strongly influenced by mantle anisotropy. However, we also identify less commonly used phases whose waveforms reflect the effects of anisotropy. For example, PS is strongly affected by upper mantle seismic anisotropy. We show that PS can be used to fill in gaps in global coverage in shear wave splitting datasets (for example, beneath ocean basins). We find that PcS is also a promising phase, and present a proof-of-concept example of PcS splitting analysis across the contiguous United States using an array processing approach. Because PcS is recorded at at much shorter distances than *KS phases, PcS splitting can therefore fill in gaps in backazimuthal coverage. The insights provided by a wavefield differencing approach provide promising new strategies for improving our ability to detect and characterize seismic anisotropy in the mantle.
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We examine upper mantle anisotropy across the Hawaiian Swell by analyzing shear wave splitting of teleseismic SKS waves recorded by the PLUME broadband land and ocean bottom seismometer deployments. Mantle anisotropy beneath the oceans is often attributed to flow‐induced lattice‐preferred orientation of olivine. Splitting observations may reflect a combination of both fossil lithospheric anisotropy and anisotropy due to present‐day asthenospheric flow, and here we address the question whether splitting provides diagnostic information on possible asthenospheric plume flow at Hawaii. We find that the splitting fast directions are coherent and predominantly parallel to the fossil spreading direction, suggesting that shear wave splitting dominantly reflects fossil lithospheric anisotropy. The signature of anisotropy from asthenospheric flow is more subtle, although it could add some perturbation to lithospheric splitting. The measured delay times are typically 1 s or less, although a few stations display larger splitting delays of 1–2 s. The variability in the delay times across the different stations indicates differences in the degree of anisotropy or in the thickness of the anisotropic layer or in the effect of multilayer anisotropy. Regions with smaller splitting times may have experienced processes that modified the lithosphere and partially erased the fossil anisotropy; alternatively, asthenospheric splitting may either constructively add to or destructively subtract from lithospheric splitting to produce the observed variability in delay times.
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Shear wave splitting is a robust tool to infer the direction and strength of seismic anisotropy in the lithosphere and underlying asthenosphere. Previous shear wave splitting studies in the Afar Depression and adjacent areas concluded that either Precambrian sutures or vertical magmatic dikes are mostly responsible for the observed anisotropy. Here we report results of a systematic analysis of teleseismic shear wave splitting using all the available broadband seismic data recorded in the Afar Depression, Main Ethiopian Rift (MER), and Ethiopian Plateau. We found that while the ∼450 measurements on the Ethiopian Plateau and in the MER show insignificant azimuthal variations with MER‐parallel fast directions and thus can be explained by a single layer of anisotropy, the ∼150 measurements in the Afar Depression reveal a systematic azimuthal dependence of splitting parameters with a π /2 periodicity, suggesting a two‐layer model of anisotropy. The top layer is characterized by a relatively small (0.65 s) splitting delay time and a WNW fast direction that can be attributed to magmatic dikes within the lithosphere, and the lower layer has a larger (2.0 s) delay time and a NE fast direction. Using the spatial coherency of the splitting parameters obtained in the MER and on the Ethiopian Plateau, we estimated that the optimal depth of the source of anisotropy is centered at about 300 km, i.e., in the asthenosphere. The spatial and azimuthal variations of the observed anisotropy can best be explained by a NE directed flow in the asthenosphere beneath the MER and the Afar Depression.
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We developed a three‐dimensional (3D) shear‐wave splitting tomography method to image the spatial anisotropy distribution by back projecting shear wave splitting delay times along ray paths derived from a 3D shear velocity model, assuming the delay times are accumulated along the ray paths. The local strength of the anisotropy is indicated by a parameter of anisotropy percentage, K. Using the shear‐wave splitting delay times for 575 earthquakes measured at PASO and HRSN stations, we imaged a detailed 3D anisotropy percentage model around the San Andreas Fault Observatory at Depth (SAFOD). The anisotropy percentage model shows strong heterogeneities, consistent with the strong spatial variations in both measured delay times and fast polarization directions. The San Andreas Fault (SAF) zone is highly anisotropic down to a depth of ∼4 km and then becomes less anisotropic at greater depths. Outside the fault zone, the highly anisotropic zone extends as deep as ∼7 km, consistent with the systematic depth dependence of the average time delays. To the southwest of the SAF, the Salinian granitic block shows relatively strong anisotropic anomalies that are presumably caused by aligned microcracks consistent with the direction of the regional maximum compressive horizontal stress. To the northeast of the fault zone, a strong anisotropic anomaly between depths ∼2 and ∼4 km corresponds to a serpentinite body sandwiched between Franciscan rocks.
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This special issue of Annals of Geophysics “Seismic anisotropy and shear-wave splitting: Achievements and perspectives” originates from a session (S10) of the 37th General assembly of the European Seismological commission ESC 2021 Conference which was planned to take place on 21 September 2021, in Corfu Greece, but due to the Covid19 pandemic was Virtual. The main theme of the session and of this special issue was the crucial role of seismic anisotropy in investigating the Earth’s interior from the upper crust to the inner core. Shear-wave splitting, one of the most effective ways to study seismic anisotropy, can identify the properties and the geodynamics of the upper mantle, and identify the presence of fluid-saturated microcracks, oriented according to the stress regime, in the upper crust. Azimuthal anisotropy and radial anisotropy can be assessed from earthquake or ambient noise recordings to detect the seismic layered features and to rebuild the 3D seismic structure
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SUMMARY Determinations of seismic anisotropy, or the dependence of seismic wave velocities on the polarization or propagation direction of the wave, can allow for inferences on the style of deformation and the patterns of flow in the Earth’s interior. While it is relatively straightforward to resolve seismic anisotropy in the uppermost mantle directly beneath a seismic station, measurements of deep mantle anisotropy are more challenging. This is due in large part to the fact that measurements of anisotropy in the deep mantle are typically blurred by the potential influence of upper mantle and/or crustal anisotropy beneath a seismic station. Several shear wave splitting techniques are commonly used that attempt resolve seismic anisotropy in deep mantle by considering the presence of multiple anisotropic layers along a raypath. Examples include source-side S-wave splitting, which is used to characterize anisotropy in the deep upper mantle and mantle transition zone beneath subduction zones, and differential S-ScS and differential SKS-SKKS splitting, which are used to study anisotropy in the D″ layer at the base of the mantle. Each of these methods has a series of assumptions built into them that allow for the consideration of multiple regions of anisotropy. In this work, we systematically assess the accuracy of these assumptions. To do this, we conduct global wavefield modelling using the spectral element solver AxiSEM3D. We compute synthetic seismograms for earth models that include seismic anisotropy at the periods relevant for shear wave splitting measurements (down to 5 s). We apply shear wave splitting algorithms to our synthetic seismograms and analyse whether the assumptions that underpin common measurement techniques are adequate, and whether these techniques can correctly resolve the anisotropy incorporated in our models. Our simulations reveal some inaccuracies and limitations of reliability in various methods. Specifically, explicit corrections for upper mantle anisotropy, which are often used in source-side direct S splitting and S-ScS differential splitting, are typically reliable for the fast polarization direction ϕ but not always for the time lag δt, and their accuracy depends on the details of the upper mantle elastic tensor. We find that several of the assumptions that underpin the S-ScS differential splitting technique are inaccurate under certain conditions, and we suggest modifications to traditional S-ScS differential splitting approaches that lead to improved reliability. We investigate the reliability of differential SKS-SKKS splitting intensity measurements as an indicator for lowermost mantle anisotropy and find that the assumptions built into the splitting intensity formula can break down for strong splitting cases. We suggest some guidelines to ensure the accuracy of SKS-SKKS splitting intensity comparisons that are often used to infer lowermost mantle anisotropy. Finally, we suggest a new strategy to detect lowermost mantle anisotropy which does not rely on explicit upper mantle corrections and use this method to analyse the lowermost mantle beneath east Asia.
Shear wave splitting
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Core–mantle boundary
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