Interpretation Of Seismic Anisotropy Research Articles

Normal mode observability of radial anisotropy in the Earth’s mantle

SUMMARYObservations of seismic anisotropy provide useful information to infer directions of mantle flow. However, existing global anisotropic tomography models are not consistent, particularly in the lower mantle. Therefore, the interpretation of seismic anisotropy in terms of mantle dynamics and evolution remains difficult. While surface and body waves are commonly used to build radially anisotropic tomography models, they provide heterogeneous data coverage and the radial anisotropy structure retrieved using these data may be biased by the use of imperfect crustal corrections. Normal modes, the free oscillations of the Earth, automatically provide global data coverage and their sensitivity to shear wave (vs) and compressional wave (vp) velocity makes them suitable to study both vs and vp anisotropy in the mantle. In this study, we assess whether current normal mode splitting data have sufficient sensitivity to lower mantle anisotropy to potentially constrain it. We consider the uncertainties in the data and the effect of inaccuracies in crustal thickness corrections and the assumed scaling between vp and vs. We perform forward modelling of normal mode data using six different 3-D global radially anisotropic tomography models to document how strong and widespread anisotropy has to be to be observable in current normal mode data. We find that, on average 50% of the spheroidal and 55% of the toroidal modes investigated show significant sensitivity to vs anisotropy, while roughly 57% of the spheroidal modes also have strong sensitivity to vp anisotropy. Moreover, we find that the normal mode data fit varies substantially for the various anisotropic tomography models considered, with the addition of anisotropy not always improving the data fit. While we find that crustal thickness corrections do not strongly impact modes that are sensitive to the lower mantle, we observe a trade-off between radial anisotropy and vp scaling for these modes. As long as this is taken into consideration, our findings suggest that existing normal mode data sets can provide valuable information on both vs and vp anisotropy in the mantle.

The effect of deformation history on the evolution of olivine CPO

Olivine crystallographic preferred orientation (CPO) is the primary cause of seismic anisotropy in the upper mantle. In tectonic environments with complex flow patterns, for example corner flow near mid-ocean ridges or subducting slabs, the interpretation of seismic anisotropy may be complicated by evolving thermochemical deformation conditions and the integrated deformation history. To understand how deformation history influences CPO evolution, deformation experiments were conducted on samples of Åheim dunite, which has a strong pre-existing texture. Experiments were performed in a triaxial geometry using a Griggs apparatus at P=1 GPa, T=1473 K, up to a maximum strain of ∼0.7. To simulate different deformation histories, samples were deformed in three different configurations, with the pre-existing foliation perpendicular, oblique, and parallel to the shortening axis of deformation. Distinct patterns of CPO development are observed for each experimental configuration. Likewise, texture strength, symmetry, and orientation evolved differently in each set of experiments. These data are interpreted as evidence that CPO did not reach steady state and that achieving steady state texture requires larger strains than previously thought. It is concluded that the integrated deformation history plays a significant role in CPO evolution and the consequent interpretation of seismic anisotropy in Earth's mantle.

CONSTRAINTS ON SUBDUCTION GEODYNAMICS FROM SEISMIC ANISOTROPY

AbstractMuch progress has been made over the past several decades in delineating the structure of subducting slabs, but several key aspects of their dynamics remain poorly constrained. Major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology, slab hydration and dehydration, mechanical coupling between slabs and the ambient mantle, the geometry of mantle flow above and beneath slabs, and the interactions between slabs and deep discontinuities such as the core‐mantle boundary. Observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy: when mantle rocks are deformed, a preferred orientation of individual mineral crystals or materials such as partial melt often develops, resulting in the directional dependence of seismic wave speeds. Measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems. Here I review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge, slab, and sub‐slab anisotropy. I also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations. I evaluate different models in light of constraints from seismology, geodynamics, and mineral physics. Finally, I discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress, particularly in frontier areas such as understanding slab dynamics in the deep mantle.

The influence of deformation history on the interpretation of seismic anisotropy

Seismic anisotropy in the mantle is primarily caused by the lattice‐preferred orientation (LPO) of olivine, which forms and evolves in response to progressive plastic deformation. The style of LPO and its rate of evolution depend on the effects of physiochemical conditions on the microphysics of deformation and recovery. Here, we integrate results from laboratory experiments and naturally deformed shear zones to evaluate how variability in the evolution of olivine LPO modifies existing strategies for interpreting seismic anisotropy. We concentrate on two particular effects. First, we show that olivine A‐ and E‐type LPOs evolve with opposite senses of rotation, producing a wide spectrum of angular relationships between the orientation of fast seismic wave propagation and the direction of flow. Second, we show that the presence of a pre‐existing LPO introduces a lag between changes in deformation kinematics and alignment of the seismic fast direction with the shear direction. The combination of these two factors affects the inference of deformation kinematics from seismic anisotropy and the interpretation of shear wave splitting parameters.

Impact of Fractures on CO2Storage Monitoring: Keys for an Integrated Approach

The monitoring of CO2 storage in fractured reservoirs (depleted hydrocarbon fields or brine aquifers) requires the study of the impact of fracturation and fluid substitution on seismic data. Seismic data can provide information about the additional compliance due to the fractures and the fluids through the analysis of seismic azimuthal anisotropy with an appropriate rock physics model. We introduce a rock physics model built in collaboration with geologists, providing a realistic description of fractured media. This model concerns fractured geological media in the presence of fluids characterized by some degree of matrix porosity, the presence of pore fluids, connected and/or non-connected fractures, the presence of several fracture sets, and an inherent seismic anisotropy. The direct application of this rock model shows that the P -wave anisotropy value measured through seismic data can be explained by several sets of different parameters such as the fracture density, the pore fluid compliance or the porosity. The presence of inherent layer-induced anisotropy can also modify the P -wave anisotropy and thus the interpretation of this value in terms of fluid substitution in a fractured porous medium. As far as fluid substitution monitoring is concerned, if seismic data are acquired before and after this substitution, a change in the P -wave anisotropy value can be linked to the modification of the compliance of the fluid content in the same medium exhibiting the same fracture network and the same porosity. This relative value can only be correctly interpreted in terms of fluid substitution provided we have some constraints on a few of the parameters involved in the P -wave anisotropy value such as the porosity, and a rough idea of the level of normalized fracture compliance. Then, a multidisciplinary approach is mandatory to constrain these parameters. For instance, borehole and outcrop geological information can give the upper limit of the fracture density expected at depth in the same formation. Furthermore, rock mechanics helps in understanding the fracturation state at depth to identify the predominant fractures in regard to the interpretation of seismic anisotropy in terms of fluid substitution inside the fracture network.

Anisotropy of the Indian continental lithospheric mantle

SUMMARY Due to the paucity of seismological data available in the public domain, the structure of the Indian lithosphere is still little known. We investigate the lithospheric structure and potential mechanical coupling between the crust and upper mantle along the Himalayan arc and underneath peninsular India using seismic anisotropy. Shear wave splitting measurements are performed on core-refracted phases. For each event recorded at a given seismological station we measured the orientation of the polarization plane of the fast S wave (phi), assumed to be a proxy for the orientation of the a axis of olivine, and the delay (dt) between the arrival time of the fast and slow S waves. We present a very comprehensive data set recorded at 86 seismological stations, deployed from the Himalayas to the southern tip of the Indian peninsula, in a joint effort by the National Geophysical Research Institute, Hyderabad, India, the University of Cambridge and the Indian Institute of Astrophysics. The unprecedented data set we present sheds light on the mechanisms involved in the India–Eurasia continental collision in a region along the Himalayan arc, south of the Indus-Tsangpo suture zone. At the scale of the Indian plate, the majority of the stations show a NNE–SSW orientation of phi over hundreds of kilometres, from Sri Lanka to the northern part of the Dharwar craton. This direction closely parallels the trend of the Indian plate motion, with respect to a fixed Eurasian plate, as defined through the NUVEL1A plate model. Along the Himalayan arc, from Ladakh in the northwest, to Bhutan and the Shillong plateau in the east, the orientation of phi rotates to become ∼EW, perpendicular to the plate motion as defined through NUVEL1A. Unlike previous studies, we do find strong evidence for seismic anisotropy south of the Indus Tsangpo suture zone. A large number of null results have been computed, with consistent orientation of the two fast polarization directions (phi) across the subcontinent. We demonstrate the potential value of the too often neglected null measurements in the interpretation of seismic anisotropy. From these results, we infer the dominance, beneath the Indian lithosphere, of the asthenospheric flow in aligning minerals in the sheared lithosphere–asthenosphere boundary layer, masking any compression induced anisotropy expected to be normal to this direction. Closer to the collision front in northern India, the anisotropy may in part, be due to the foliation planes of the Himalayan fold and thrust belt aligning the a axis of olivine perpendicular to the compression axis, but more likely to the turning of the relative asthenospheric flow along the strike caused by the downthrusting Indian lithosphere acting as a barrier. The continent-wide consistency of results strengthens the understanding that the Indian lithosphere has distinct anisotropic signatures, contrary to the hitherto assumed isotropy and allows one to interpret the results in a coherent framework of Indo-Eurasian convergence.

Stress aligned cracks in the upper crust of the Val d'Agri region as revealed by shear wave splitting

SUMMARY Shear wave splitting is measured at 19 seismic stations of a temporary network deployed in the Val d’Agri area to record low-magnitude seismic activity. The splitting results suggest the presence of an anisotropic layer between the surface and 15 km depth (i.e. above the hypocentres). The dominant fast polarization direction strikes NW–SE parallel to the Apennines orogen and is approximately parallel to the maximum horizontal stress in the region, as well as to major normal faults bordering the Val d’Agri basin. The size of the normalized delay times in the study region is about 0.01 s km −1 , suggesting 4.5 per cent shear wave velocity anisotropy (SWVA). On the south-western flank of the basin, where most of the seismicity occurs, we found larger values of normalized delay times, between 0.017 and 0.02 s km −1 . These high values suggest a 10 per cent of SWVA. These parameters agree with an interpretation of seismic anisotropy in terms of the Extensive-Dilatancy Anisotropy (EDA) model that considers the rock volume pervaded by fluid-saturated microcracks aligned by the active stress field. Anisotropic parameters are consistent with borehole image logs from deep exploration wells in the Val d’Agri oil field that detect pervasive fluid saturated microcracks striking NW–SE parallel to the maximum horizontal stress in the carbonatic reservoir. However, we cannot rule out the contribution of aligned macroscopic fractures because the main Quaternary normal faults are parallel to the maximum horizontal stress. The strong anisotropy and the seismicity concentration testify for active deformation along the SW flank of the basin.

Ab initio lattice dynamics calculations on the combined effect of temperature and silicon on the stability of different iron phases in the Earth's inner core

The Earth's solid inner core consists mainly of iron (Fe), alloyed with lighter elements, such as silicon (Si). Interpretation of seismic anisotropy and layering requires knowledge of the stable crystal structure in the inner core. We report ab initio density functional theory calculations on the free energy and vibrational stability of pure iron and Fe–Si alloys at conditions representative of the Earth's inner core. For pure Fe the stable phase is already known to be hexagonal close-packed (hcp). However, with the addition of ∼7 wt.% Si at high temperatures, we observe a transition to the face-centred cubic (fcc) phase. We also produce a phase diagram for the Fe–Si system and show that the inner core may exist in the two-phase region, with coexisting fcc and hcp. This may also explain the low S-velocities observed in the inner core.

Seismic anisotropy and its relation with crust structure and stress field in the Reggio Emilia Region (Northern Italy)

SUMMARY Shear wave splitting is measured at 14 seismic stations in the Reggio Emilia region above local background seismicity and two sequences of seismic events. The good quality of the waveforms together with the favourable distribution of earthquake foci allows us to place strong constraints on the geometry and the depth of the anisotropic volume. It is about 60 km 2 wide and located between 6 and 11 km depth, inside Mesozoic age carbonate rocks. The splitting results suggest also the presence of a shallower anisotropic layer about 1 km thick and few km wide in the Pliocene‐Quaternary alluvium above the Mesozoic layer. The fast polarization directions (N30 ◦ E) are approximately parallel to the maximum horizontal stress (σ 1 is SSW‐NNE) in the region and also parallel to the strike of the main structural features in the Reggio Emilia area. The size of the delay times suggests about 4.5 per cent shear wave velocity anisotropy. These parameters agree with an interpretation of seismic anisotropy in terms of the extensivedilatancy anisotropy model which considers the rock volume to be pervaded by fluid-saturated microcracks aligned by the active stress field. We cannot completely rule out the contribution of aligned macroscopic fractures as the cause of the shear wave anisotropy even if the parallel shear wave polarizations we found are diagnostic of transverse isotropy with a horizontal axis of symmetry. This symmetry is commonly explained by parallel stress-aligned microcracks.

Interpretation of seismic anisotropy in terms of mantle flow when melt is present

The use of seismic anisotropy to image upper mantle flow is usually based on the assumption that the direction of fast propagation of seismic waves is the same as the flow direction. Laboratory experiments showed that when melt is present, it is rather the direction of slow propagation that aligns with the flow direction. This paper presents a modeling of the effect of melt on the development of lattice preferred orientation (LPO) and its implications for the interpretation of anisotropy. Taking into account strain partitioning associated with melt segregation in the modeling yields a good agreement with experimental results. Implementing strain partitioning in a test flow shows that the effect of melt is mainly local and has a limited effect on seismic anisotropy. Water‐induced texture, melt‐filled cracks or channelized flow, might better account for seismic observations of direction of fast propagation normal to plate motion on a regional scale.

Water-induced fabric transitions in olivine.

The interpretation of seismic anisotropy in Earth's upper mantle has traditionally been based on the fabrics (lattice-preferred orientation) of relatively water-poor olivine. Here we show that when a large amount of water is added to olivine, the relation between flow geometry and seismic anisotropy undergoes marked changes. Some of the puzzling observations of seismic anisotropy in the upper mantle, including the anomalous anisotropy in the central Pacific and the complicated anisotropy in subduction zones, can be attributed to the enrichment of water in these regions.

A kinematic model for recrystallization and texture development in olivine polycrystals

The interpretation of seismic anisotropy in the mantle requires a knowledge of the relationship between the lattice preferred orientation (LPO) of crystals and the convective flow field. In order better to understand this link, we present a model for the evolution of LPO in olivine aggregates that deform by both intracrystalline slip and dynamic recrystallization. Dynamic recrystallization depends on the dislocation density of the grains, which is a function of the applied local stress. Grains with a large density of dislocations lower their bulk strain energy by nucleating strain-free sub-grains at a rate proportional to a dimensionless nucleation parameter λ*. Grains with high energy are then invaded by grains with low energy by grain-boundary migration, at a rate proportional to a dimensionless grain-boundary mobility M*. The value of λ* is constrained by observed LPO patterns in experimentally deformed olivine aggregates, and M* is constrained by the temporal evolution of the strength of the LPO. For M*=125±75 and λ*>3, the model predictions agree well with the experimental results. Numerical calculations of LPO using our model are significantly faster than those based on viscoplastic self-consistent or equilibrium-based theories, making the model especially suitable for applications for complex convective flows.