SKS Waveforms Research Articles

PREM-like velocity structure in the outermost core from global SKS and ScS waveform modeling

Seismic structure in the Earth's outermost core is important for our understanding of thermochemical stratification in the outer core, and is yet heavily debated. Here we study the compressional velocity structure in the Earth's outermost core based on waveform modeling of a unique untapped SKS and ScS dataset near bifurcation distances, collected from global seismic arrays for earthquakes occurring from 2000 to 2020. Using the SKS-ScS array dataset minimizes the effects of many uncertainties associated with earthquake source parameters and seismic heterogeneities in the mantle, and affords an opportunity to study and assess the seismic structure in the outermost core. We study outer core structure by testing two end-member models: 1) the D″ model that attributes any anomalous seismic observations to the effects of the lowermost mantle structure and is paired with a PREM (the Preliminary Reference Earth's Model) structure in the outermost core and 2) the outer core model that is paired with either a PREM or a tomographic structure in the lowermost mantle and attributes other unexplained seismic signals to an outer core structure. The results of the outer core models present unreasonable large lateral variations of >3.1% in the outermost core, while the inferred D″ models exhibit large-scale seismic anomalies that are consistent with the tomographic models and small-scale anomalies that are confirmed by further analysis of the seismic array data. Our analyses suggest a PREM-like seismic velocity structure and a lack of strong thermochemical anomalies in the topmost ∼200 km of the outer core, placing bounds on possible thermal and compositional conditions in the region of the Earth's outermost core. Our study also identifies the existence of small-scale seismic anomalies and sharp velocity variations in the lowermost mantle beneath the south coast of Alaska, northwestern Atlantic and the middle of Central America.

Reproducing complex anisotropy patterns at subduction zones from splitting intensity analysis and anisotropy tomography

SUMMARYMeasurements of seismic anisotropy provide a lot of information on the deformation and structure as well as flows of the Earth's interior, in particular of the upper mantle. Even though the strong and heterogeneous seismic anisotropic nature of the upper mantle has been demonstrated by a wealth of theoretical and observational approaches , most of standard teleseismic body-wave tomography studies overlook P- and S-wave anisotropy, thus producing artefacts in tomographic models in terms of amplitude and localization of heterogeneities. Conventional methods of seismic anisotropy measurement have their limitations regarding lateral and mainly depth resolution. To overcome this problem much effort has been done to develop tomographic methods to invert shear wave splitting data for anisotropic structures, based on finite-frequency sensitivity kernels that relate model perturbations to splitting observations. A promising approach to image the upper mantle anisotropy is the inversion of splitting intensity (SI). This seismic observable is a measure of the amount of energy on the transverse component waveform and, to a first order, it is linearly related to the elastic perturbations of the medium through the 3-D sensitivity kernels, that can be therefore inverted, allowing a high-resolution image of the upper mantle anisotropy. Here we present an application of the SI tomography to a synthetic subduction setting. Starting from synthetic SKS waveforms, we first derived high-quality SKS SI measurements; then we used the SI data as input into tomographic inversion. This approach enables high-resolution tomographic images of upper-mantle anisotropy through recovering vertical and lateral changes in anisotropy and represents a propaedeutic step to the real cases of subduction settings. Additionally this study was able to detect regions of strong dipping anisotropy by allowing a 360° periodic dependence of the splitting vector.

Slab tear and rotation imaged with core-refracted shear wave anisotropy

We investigate the complex flow field around the Vrancea slab in Romania, a steeply sinking seismogenic lithospheric block that experienced lateral tear-off and possible rotation. The Vrancea slab is located beneath the South-East Carpathians and generates frequent seismicity despite its remote location from active collisional boundaries. We analyse core-refracted shear wave (SKS) splitting recorded by permanent broadband seismic stations from the Romanian Seismic Network for periods up to 10 years, and compare our results with seismic tomography of the upper mantle. We identify several stations with large backazimuthal variations of SKS fast axis polarization and delay times both in the slab hinge zone, the back-arc and the circum-slab region, indicating complex mantle deformation patterns. To investigate the effect of a two-level rotated slab we invert SKS waveforms using cross-convolutional misfit combined with a neighbourhood search algorithm to model two layers of anisotropy. In the shallow mantle, anisotropy aligns with the upper slab strike and reorients along the strike of the lower slab at depths below the hinge zone. In the back-arc trench-perpendicular anisotropy switches to trench-parallel, due to the recent retreat and roll-back of the slab. Our results have important implications for understanding SKS interference from subducted slab fragments and provide evidence of the recent retreat, break-off and rotation of two Vrancea slab levels sinking into the upper mantle.

Quantification of Small-Scale Heterogeneity at the Core–Mantle Boundary Using Sample Entropy of SKS and SPdKS Synthetic Waveforms

Qualitative and quantitative analysis of seismic waveforms sensitive to the core–mantle boundary (CMB) region reveal the presence of ultralow-velocity zones (ULVZs) that have a strong decrease in compressional (P) and shear (S) wave velocity, and an increase in density within thin structures. However, understanding their physical origin and relation to the other large-scale structures in the lowermost mantle are limited due to an incomplete mapping of ULVZs at the CMB. The SKS and SPdKS seismic waveforms is routinely used to infer ULVZ presence, but has thus far only been used in a limited epicentral distance range. As the SKS/SPdKS wavefield interacts with a ULVZ it generates additional seismic arrivals, thus increasing the complexity of the recorded wavefield. Here, we explore utilization of the multi-scale sample entropy method to search for ULVZ structures. We investigate the feasibility of this approach through analysis of synthetic seismograms computed for PREM, 1-, 2.5-, and 3-D ULVZs as well as heterogeneous structures with a strong increase in velocity in the lowermost mantle in 1- and 2.5-D. We find that the sample entropy technique may be useful across a wide range of epicentral distances from 100° to 130°. Such an analysis, when applied to real waveforms, could provide coverage of roughly 85% by surface area of the CMB.

Sensitivity of SK(K)S and ScS phases to heterogeneous anisotropy in the lowermost mantle from global wavefield simulations

SUMMARY Observations of seismic anisotropy at the base of the mantle are abundant. Given recent progress in understanding how deformation relates to anisotropy in lowermost mantle minerals at the relevant pressure and temperature conditions, these observations can be used to test specific geodynamic scenarios, and have the potential to reveal patterns of flow at the base of the mantle. For example, several recent studies have sought to reproduce measurements of shear wave splitting due to D″ anisotropy using models that invoke specific flow and texture development geometries. A major limitation in such studies, however, is that the forward modelling is nearly always carried out using a ray theoretical framework, and finite-frequency wave propagation effects are not considered. Here we present a series of numerical wave propagation simulation experiments that explore the finite-frequency sensitivity of SKS, SKKS and ScS phases to laterally varying anisotropy at the base of the mantle. We build on previous work that developed forward modelling capabilities for anisotropic lowermost mantle models using the AxiSEM3D spectral element solver, which can handle arbitrary anisotropic geometries. This approach enables us to compute seismograms for relatively short periods (∼4 s) for models that include fully 3-D anisotropy at moderate computational cost. We generate synthetic waveforms for a suite of anisotropic models with increasing complexity. We first test a variety of candidate elastic tensors in laterally homogeneous models to understand how different lowermost mantle elasticity scenarios express themselves in shear wave splitting measurements. We then consider a series of laterally heterogeneous models of increasing complexity, exploring how splitting behaviour varies across the edges of anisotropic blocks and investigating the minimum sizes of anisotropic heterogeneities that can be reliably detected using SKS, SKKS and ScS splitting. Finally, we apply our modelling strategy to a previously published observational study of anisotropy at the base of the mantle beneath Iceland. Our results show that while ray theory is often a suitable approximation for predicting splitting, particularly for SK(K)S phases, full-wave effects on splitting due to lowermost mantle anisotropy can be considerable in some circumstances. Our simulations illuminate some of the challenges inherent in reliably detecting deep mantle anisotropy using body wave phases, and point to new strategies for interpreting SKS, SKKS and ScS waveforms that take full advantage of newly available computational techniques in seismology.

Upper mantle anisotropy in the northwest Himalaya and Ladakh-Karakoram zone based on SKS splitting analysis

Seismic anisotropy of the upper mantle beneath the northwest Himalaya and eastern part of the Ladakh-Karakoram zone has been investigated using shear-wave splitting of core-mantle refracted SKS waveforms from 94 teleseismic earthquakes recorded at 28 broadband seismic stations. The SKS splitting parameters (Φ and δt) show a significant strength of anisotropy with a wide range of delay time of split waves (∼0.75−2.94 s). The distinct variations in Fast Polarization Directions (FPDs) along the study section suggest a complex source of anisotropy that cannot be explained by a simple model. In the frontal part of the Himalaya, the FPDs are mostly parallel or sub-parallel to the strike of the Himalayan orogeny suggesting deformation in the shallow lithospheric mantle under compression owing to the India-Asia collision. On the other hand, FPDs observed in the Lesser, Higher, and Tethyan Himalaya largely follow NE-oriented absolute plate motion (APM) of the Indian plate which can be attributed to basal shear as the Indian plate moves over the asthenospheric mantle with a minor contribution from shallow lithospheric deformation. A complex anisotropy pattern is observed in the Indus Suture Zone. The FPDs near the Karakoram Fault Zone (KFZ) are parallel or sub-parallel to the strike of the KF. The strike-slip or transpressional deformation in the lithospheric mantle in the KFZ is considered as the major source of anisotropy beneath the KFZ. The study envisaged that the KF is a lithospheric scale fault that largely accommodates the India-Asia collision and extrusion in the Tibetan Plateau.

Lateral Variations in SKS Splitting Across the MAGIC Array, Central Appalachians

AbstractThe eastern margin of North America has been shaped by several cycles of supercontinent assembly. These past episodes of orogenesis and continental rifting have likely deformed the lithosphere, but the extent, style, and geometry of this deformation remain poorly known. Measurements of seismic anisotropy in the upper mantle can shed light on past lithospheric deformation, but may also reveal contributions from present‐day mantle flow in the asthenosphere. Here we examine SKS waveforms and measure splitting of SKS phases recorded by the MAGIC experiment, a dense transect of seismic stations across the central Appalachians. Our measurements constrain small‐scale lateral variations in azimuthal anisotropy and reveal distinct regions of upper mantle anisotropy. Stations within the present‐day Appalachian Mountains exhibit fast splitting directions roughly parallel to the strike of the mountains and delay times of about 1.0 s. To the west, transverse component waveforms for individual events reveal lateral variability in anisotropic structure. Stations immediately to the east of the mountains exhibit complicated splitting patterns, more null SKS arrivals, and a distinct clockwise rotation of fast directions. The observed variability in splitting behavior argues for contributions from both the lithosphere and the asthenospheric mantle. We infer that the sharp lateral transition in splitting behavior at the eastern edge of the Appalachians is controlled by a change in anisotropy in the lithospheric mantle. We hypothesize that beneath the Appalachians, SKS splitting reflects lithospheric deformation associated with Appalachian orogenesis, while just to the east this anisotropic signature was modified by Mesozoic rifting.

The origin of shear wave splitting beneath Iceland

The origin of shear wave splitting (SWS) in the mantle beneath Iceland is examined using numerical models that simulate 3-D mantle flow and the development of seismic anisotropy due to lattice-preferred orientation (LPO). Using the simulated anisotropy structure, we compute synthetic SKS waveforms, invert them for fast polarization directions and split times, and then compare the predictions with the results from three observational studies of Iceland. Models that simulate a mantle plume interacting with the Mid-Atlantic Ridge in which the shallow-most mantle has a high viscosity due to the extraction of water with partial melting, or in which C-type olivine LPO fabric is present due to high water content in the plume, produce the largest chi-squared misfits to the SWS observations and are thus rejected. Models of a low-viscosity mantle plume with A-type olivine fabric everywhere, or with the added effects of E-type fabric in the plume below the solidus produce lower misfits. The lowest misfits are produced by models that include a rapid (∼50 kmMyr −1 ) northward regional flow (NRF) in the mid-upper mantle, either with or without a plume. NRF was previously indicated by a receiver function study and a regional tomography study, and is shown here to be a major cause of the azimuthal anisotropy beneath Iceland. The smallest misfits for the models with both a plume and NRF are produced when LPO forms above depths of 300–400 km, which, by implication, also mark the depths above which dislocation creep dominates over diffusion creep. This depth of transition between dislocation and diffusion creep is greater than expected beneath normal oceanic seafloor, and is attributed to the unusually rapid strain rates associated with an Iceland plume and the NRF.

Anisotropic lithosphere under the Fennoscandian shield from P receiver functions and SKS waveforms of the POLENET/LAPNET array

Seismic azimuthal anisotropy is the key evidence of the past and present strains in the upper mantle. The standard analysis of shear-wave splitting with the SKS techniques is useful in mapping lateral variations but it is insensitive to depth of anisotropy and to variations of anisotropy with depth. To retrieve the depth localized anisotropy under the Fennoscandian shield, we inverted P-wave receiver functions of the POLENET/LAPNET array in northern Finland jointly with SKS recordings. Shear-wave anisotropy of ~2.5% with the fast direction of 40°–60° in a depth range from the Moho to ~110km is a robust result of the inversion. The obtained direction is nearly normal to the azimuth of the maximum horizontal compressional stress in the lithosphere, but a recent origin of this anisotropy is in doubt. This anisotropy may be frozen since the Precambrian, but it shows no clear relation with the trends of the Precambrian tectonics. The upper anisotropic layer accounts for ~40% of shear-wave splitting in SKS, and to explain the rest another anisotropic layer is required. The top of the second layer with a practically similar fast direction is found at a depth of 200–240km. This direction is close to the current APM direction of the lithosphere with implication that the inferred anisotropy may be related with the current plate motion, and the anisotropic layer belongs to the asthenosphere. The bottom of this layer is uncertain, but it is at least 320km deep. In a depth range from 160km to 200–240km the fast anisotropy direction is 110–150°. Origin of this direction is unclear.

Stratified seismic anisotropy and the lithosphere‐asthenosphere boundary beneath eastern North America

AbstractLong records of teleseismic observations accumulated at permanent seismic stations Harvard, MA; Palisades, NY; and Standing Stone, PA, in eastern North America are inverted for vertical distribution of anisotropic parameters. High‐resolution anisotropy‐aware P wave receiver function analysis and multiple‐layer core‐refracted SKS waveform modeling favor more than one layer of anisotropy beneath all sites. Our analyses suggest that the depth sensitivity to stratified anisotropic seismic velocity in converted phases and the SKS waveforms are complementary and confirm that these two approaches yield consistent lithospheric anisotropic fast axis directions. We illustrate the feasibility of the lithosphere‐asthenosphere boundary detection on a regional scale through anisotropy‐aware receiver functions. Joint interpretation of receiver functions and SKS waveforms beneath eastern North America suggests a thin (~100 km) anisotropic lithosphere with fast axis orientation nearly orthogonal to the strike of major tectonic units and an underlying anisotropic asthenosphere with fast axis directions that favor the HS3‐NUVEL 1A plate motion model. Consistent lithospheric anisotropy inferred from both techniques suggests broad presence of coherent fabric in the lower lithosphere, possibly developed in a regional scale delamination event after the assembly of Appalachians.

Upper and lower mantle anisotropy inferred from comprehensive SKS and SKKS splitting measurements from India

In this study, we investigate the upper mantle anisotropy beneath India using high quality SKS and SKKS waveforms from 382 teleseismic earthquakes recorded at 119 broadband seismic stations. In addition, we present evidence for anisotropy in the D″ layer beneath southeast Asia using SKS and SKKS splitting discrepancies on the same seismogram. During this exercise, we obtain 200 new splitting measurements from 35 stations recently deployed in the Indo-Gangetic plains (IGP), central India and northeast India. While the delay times between the fast and slow axes of anisotropy (δt) range from 0.3 to 1.7 s, the fast polarization azimuths (Φ) at a majority of stations in the IGP and central India coincide with the absolute plate motion of India implying shear at the base of the lithosphere as the dominant mechanism for forging anisotropy. However, stations in NE India reveal fast polarization azimuths mainly in the ENE–WSW direction suggestive of lithospheric strain induced by the ongoing Indo-Eurasian collision. Our analysis for D″ anisotropy yielded a total of 100 SKS–SKKS pairs, which can be categorized into those exhibiting (I) null measurements for one phase and significant splitting for the other phase, (II) null measurement for both the phases, (III) significant splitting for both the phases. A pair is considered to be anomalous if the splitting difference between SKS and SKKS is ⩾0.5 s and the individual split time is ⩾0.5 s. Using this criterion, we obtain 12 measurements under category III and 9 under category I that show a null measurement for SKS and large splitting for the SKKS phase. Further, we quantify the strength of the lower mantle anisotropy by correcting the SKKS measurement for the upper mantle anisotropy obtained by the SKS phase on the same seismogram. The SKS delay times are found to be consistently less than SKKS times, suggesting that the SKS phases do not capture the lower mantle anisotropy in comparison to their SKKS counterparts. Seven coherent measurements thus obtained reveal measurable D″ anisotropy, with fast polarization azimuths oriented mainly in the ENE–WSW direction. These results suggest presence of a large region of deformation in the lowermost mantle beneath southeast Asia. A possible model for anisotropy in these regions could be the presence of slab material that pounded upon the core mantle boundary (CMB) and is experiencing large shear deformation, resulting in lattice preferred orientation (LPO) of the lower mantle (Van der Hilst and Kárason, 1999; Long, 2009). The other possibility is the phase transformation from MgSiO3 perovskite to a more stable post-perovskite phase under favorable conditions, which results in LPO of the lower mantle.

Upper mantle seismic anisotropy beneath a convergent boundary: SKS waveform modeling in central Tibet

This study used SKS waveforms from the International Deep Profiling of Tibet and the Himalayas (INDEPTH) III dataset and a new 2D method for modeling seismic waves in anisotropic media to construct an image of anisotropic structures beneath central Tibet. A preferred model revealed three-segment anisotropic structures in the upper mantle beneath the study region. Waveform modeling demonstrated that the anisotropy was mainly generated by the lithosphere but not the asthenosphere, and that an anisotropic model with a flatter axis of symmetry provides a more consistent interpretation of the observations than models having steeply dipping symmetry axes. A relatively low velocity zone may underlie or intermingle with the anisotropic structures in the northern portion of the region. Synthetic tests also indicate that variations in the elastic constants and depth extent of the anisotropy assumed by the calculations do not affect the general conclusions, although trade-offs exist among certain model parameters. The modeling results suggest that the complex seismic structures in central Tibet were associated with underthrusting of the Indian lithosphere beneath the Asian lithosphere; the inferred flat symmetry axis of the anisotropy was likely generated during this collision process. If this were not the case, the inherited anisotropy would exhibit a steeply dipping axis of symmetry, parallel to the direction of underthrusting.

Waveform modeling of shear wave splitting from anisotropic models in Iceland

No quantitative model of seismic anisotropy beneath Iceland has yet explained the observed shear wave splitting results in Iceland. In this study we explore the structure of mantle seismic anisotropy associated with plume ridge interaction beneath Iceland by modeling synthetic waveforms using a pseudo‐spectral method. First, predicted SKS waveforms are shown to produce shear wave splitting results for two anisotropic layers that are in generally agreement with analytic solutions. Next, simple models in which anisotropy are imposed at different depths and geographic regions around Iceland are used to examine different hypothesized effects of plume‐ridge interaction. The model that is designed to represent crystallographic fabric due to mantle deformation associated with lithosphere spreading in western Iceland and along‐axis, channeled asthenospheric flow in eastern Iceland can predict the distinct shear wave splitting observations on the east and west sides of the island. Last, we analyze geodynamic models that simulate 3‐D mantle flow and evolution of mineral fabric. The predicted fast directions of shear wave splitting from the models with realistic spreading rate (10 km/Myr) and broad range of plume viscosities are consistent with the observations in central and eastern Iceland. However, because these models are symmetric about the ridge, none of them can predict the observed shear wave splitting results in western Iceland.

Frozen and active seismic anisotropy beneath southern Africa

P receiver functions from 23 stations of the SASE experiment in southern Africa are inverted simultaneously with SKS waveforms for azimuthal anisotropy in the upper mantle. Our analysis resolves the long‐standing issue of depth dependence and origins of anisotropy beneath southern Africa. In the uppermost mantle we observe anisotropy with a nearly E‐W fast direction, parallel to the trend of the Limpopo belt. This anisotropy may be frozen since the Archean. At a depth of 160 km the fast direction of anisotropy changes to 40° and becomes close to the recent plate motion direction. This transition is nearly coincident in depth with activation of dominant glide systems in olivine and with a pronounced change in other properties of the upper mantle. Another large change in the fast direction of anisotropy corresponds to the previously found low‐S‐velocity layer atop the 410‐km discontinuity.

Seismic Anisotropy of Upper Mantle Beneath the Ordos Block and Adjacent Regions

AbstractBased on the polarization analysis of teleseismic SKS waveform data recorded at 90 broadband stations in the Ordos block and adjacent regions, the splitting parameters of fast polarization direction and delay time between the fast and slow shear waves are determined for each station using the minimum transverse energy method, yielding the images of upper mantle anisotropy. Combined with the GPS horizontal velocity field with respect to the Ordos block during 1999~2007, we analyze the seismic anisotropy as well as the crust‐mantle coupling beneath the Ordos block and its adjacent regions. At last, a new understanding of the block movement and the dynamic causes of the adjacent fault zones is achieved. The results reveal that there are nulls in the block which show an obvious isotropic characteristic. The delay time of the adjacent stations is obviously greater than those in the central block, which indicates an obvious anisotropy. The fast polarization directions are compatible with the tectonic strike direction and the horizontal velocity direction, which shows the crust‐mantle coupling. The differences between the directions at some stations may be associated with the early block activities. According to the SKS shear wave splitting results and the current GPS velocity field, we suggest that the Ordos block has no rotation. Meanwhile, the horizontal and ascending intrusion and the transverse extension of the plastic material in the upper mantle lead to the bifurcated flow around the Liupan Shan Mountains in the southwestern margin of Ordos. The generation and development of the Fenwei graben are the result from upward intrusion and lateral spreading of upper mantle material along deep faults.

Mantle flow beneath northwestern Venezuela: Seismic evidence for a deep origin of the Mérida Andes

We measured shear wave splitting from SKS data recorded by the national seismic network of Venezuela and a linear broadband PASSCAL/Rice seismic array across the Mérida Andes. The linear array was installed in the second phase of the passive seismic component of the BOLIVAR project to better understand the complicated regional tectonics in western Venezuela. Using the method proposed by Wolfe and Silver (1998), SKS waveforms from 2 to 36 earthquakes, mostly from the Tonga subduction zone, were selected for each of the 23 stations in the region in order to do splitting analysis. The fast polarization directions can be divided into 3 zones, all in agreement with local GPS data: The first zone comprises the stations north of the dextral strike–slip Oca–Ancon fault. These stations show the largest split times (1.6–3.2s), oriented in a roughly EW direction, and are similar to splitting observations made further to the east along the strike slip plate boundary (Growdon et al., 2009). We attribute this to trench-parallel mantle flow that passes around the northwest corner of the subducting Caribbean plate and along the northern edge of South America as proposed by Russo and Silver (1994), forming an eastward flow beneath the southern Caribbean plate. Zone two is the Mérida Andes, with the right lateral Bocono fault in the center, where split orientations are at ~N45°E, suggesting that the observed seismic anisotropy is likely caused by lithospheric deformation parallel to the Bocono fault. Zone three is east of the Bocono fault inside the Barinas–Apure Basin, where the measured split times are smaller (~0.8s) with an EW fast direction that is consistent with those observed at the Guarico Basin, Maturin Basin and the Guayana Shield in the east, and are interpreted as orientation with the motion of the continent.

Characterization of the D″ beneath the Galapagos Islands using SKKS and SKS waveforms

SKS and SKKS waveforms from 16 events occurring between 2003 and 2005 in the Tonga Trench that were recorded by the BOLIVAR array are analyzed to determine the structure of the D″ layer beneath the Galapagos Islands. 248 differential travel-time residuals of SKKS-SKS are measured and reveal a region of positive residuals of differential travel times in the northeast portion of the sampled region. Analyzing correlation statistics between the measured SKKS-SKS residuals and the observed absolute travel time delay of the individual SKS and SKKS phases for two events with high data quality, we determine that the residual differential travel time is due to excess low velocity along the SKKS raypaths. First order modeling of three potential low velocity structures, ultra-low velocity zones (ULVZ), plume conduit, D″ structure, indicates that the observed SKKS-SKS residuals can be best explained by a low velocity anomaly within the D″ layer. To determine the presence of lower mantle anisotropy, amplitude ratios of the radial and transverse component of SKS and SKKS waveform are calculated and compared. Regions with significant presence of seismic anisotropy are interpreted as the edge of the flow field associated with a hypothetical mantle upwelling.

Short-scale variations of shear-wave splitting across the Dead Sea basin: Evidence for the effects of sedimentary fill

[1] We examine shear-wave splitting of SKS waveforms collected by a temporary array of 68 stations in the region of the Dead Sea basin. The observed splitting parameters exhibit systematic variations along a dense, EW-trending 60 km profile across the basin. The delay times vary significantly between 1.0 and 2.8 seconds with smaller values in the very center of the profile. The fast polarizations are oriented more-or-less parallel to the strike of the Dead Sea transform fault and vary between −10 and 20 degrees with respect to North. Finite-frequency waveform modeling reveals that the source-region of the small-scale lateral variations is likely located within the crust. The modeling further shows that purely isotropic velocity variations affect shear-wave splitting: To a large degree, the observed variations of splitting parameters can be explained by the sedimentary fill of the basin and its low isotropic seismic velocities, whereas the mantle is uniformly anisotropic. Our study indicates that precaution must be taken when interpreting short-scale lateral variations of shear wave splitting in terms of anisotropic structures in the crust or upper mantle.

Anisotropic stratification beneath Africa from joint inversion of SKS and P receiver functions

The analysis of rock anisotropy revealed by seismic waves provides fundamental constraints on stress‐strain field in the lithosphere and asthenosphere. Nevertheless, the anisotropic models resolved for the crust and the upper mantle using seismic waves sometimes show substantial discrepancies depending on the type of data analyzed. In particular, at several permanent stations located in Africa, previous studies revealed that the observations of SKS splitting are accounted for by models with a single and homogeneous anisotropic layer whereas 3‐D tomographic models derived from surface waves exhibit clear anisotropic stratification. Here we tackle the issue of depth‐dependent anisotropy by performing joint inversion of receiver functions (RF) and SKS waveforms at four permanent broadband stations along the East African Rift System (EARS) and also on the Congo Craton. For three out of the four stations studied, stratified models allow for the best fit of the data. The vertical variations in the anisotropic pattern show interesting correlations with changes in the thermomechanical state of the mantle associated with the lithosphere‐asthenosphere transition and with the presence of hot mantle beneath the Afar region and beneath the EARS branches that surround the Tanzanian Craton. Our interpretation is consistent with the conclusion of earlier studies that suggest that beneath individual stations, multiple sources of anisotropy, chiefly olivine lattice preferred orientation and melt pocket shape preferred orientation in our case, exist at different depths. Our study further emphasizes that multiple layers of anisotropy must often be considered to obtain realistic models of the crust and upper mantle.

Interpreting spatial variations in anisotropy: insights into the Main Ethiopian Rift from SKS waveform modelling

Seismic anisotropy is a common feature in the upper mantle and measuring shear wave splitting in core phases is a common approach in estimating its characteristics. Large lateral variations in estimated splitting parameters are observed over small spatial distances in many differing tectonic regions, including areas of continental break-up such as the Main Ethiopian Rift (MER). We investigate the ability of shear wave splitting analysis to constrain spatial variations in anisotropy using a one-way wave equation modelling scheme to generate bandlimited waveforms for a suite of models representing regions with rapidly changing anisotropy. We show that shear wave splitting can identify lateral variation in anisotropy on the order of 20–50 km, where a change in fast direction demarcates the transition in anisotropy. In addition, variation in the amount of splitting is complicated close to the transition, and is sensitive to the vertical thickness of anisotropy. We have used these modelling results to interpret shear wave splitting measurements for the MER. The model that best fits the observations has a 100- km wide rift zone with a fast direction of 30° outside and 20° inside the rift. The model has 9 per cent anisotropy close to the western margin, with 7 per cent anisotropy elsewhere. In all regions of the model we constrain the anisotropy to begin at a depth of 90 km. The depth of anisotropy is consistent with geochemical estimates of the depth of melt initiation beneath the region. Also the elevated splitting beneath the western margin supports evidence of low velocities and highly conductive zones from seismic tomography and magneto-tellurics, suggesting melt is more focused along the western margin. This study shows how observations of SKS-wave splitting from dense seismic networks can be used to map sharp lateral changes and constrain the depth of the anisotropy.