Splitting Delay Research Articles

Shallow sources of upper mantle seismic anisotropy in East Africa

The East African rift overlies one or more mantle upwellings and it traverses heterogeneous Archaean-Paleozoic lithosphere rifted in Mesozoic and Cenozoic time. We re-analyze XKS shear wave splitting at publicly available stations to evaluate models for rifting above mantle plumes. We use consistent criteria to compare and contrast both splitting direction and strength, infilling critical gaps with new data from the Turkana Depression and North Tanzania Divergence sectors of the East African rift system. Our results show large spatial variations in the amount of splitting (0.1–2.5 s), with fast axes predominantly sub-parallel to the orientation of Cenozoic rifts underlain by thinned lithosphere with and without surface magmatism. The amount of splitting increases with lithospheric thinning and magmatic modification. Nowhere are fast axes perpendicular to the rift, arguing against the development of extensional strain fabrics. Thick cratons are characterized by small amounts of splitting (≤0.5 s) with a variety of orientations that may characterize mantle plume flow. Splitting rotates to rift parallel and increases in strength over short distances into rift zones, implying a shallow depth range for the anisotropy in some places. The shallow source and correlation between splitting direction and the shape of upper mantle thin zones suggests that the combination of channel flow and oriented melt pockets contribute > 1 s to the observed splitting delays. Enhanced flow, metasomatism, and melt intrusion at the lithosphere-asthenosphere boundary suggest that fluid infiltration to the base of the lithosphere may facilitate rifting of cratonic lithosphere.

Constraints on the Lithospheric Kinematics in the Aegean and Western Anatolia Unveiled by SKS Splitting Observations

AbstractThe present study investigates azimuthal anisotropy and its relation to the geodynamical processes beneath the back‐arc of the Hellenic subduction zone in the eastern Aegean and western Anatolia where surface tectonics is dominated by the right‐lateral strike‐slip North Anatolian Fault Zone (NAFZ) in the north and E‐W oriented normal fault systems. We obtained apparent SKS splitting parameters from 1,660 good quality and 137 null measurements extracted from 542 events recorded at 40 permanent broadband seismic stations. Overall, the station‐averaged splitting parameters indicate NNE‐SSW oriented fast directions (∼N20°E) and splitting delays around ∼1.5 s. The large splitting delays, particularly observed beneath the northern Aegean can be explained by either an enlarged mantle wedge thickness or increased strength of upper mantle anisotropy. We constrain complex anisotropy structures within two layer models from notable backazimuthal variations in individual splitting measurements observed beneath a few stations at the north located in a close proximity to the NAFZ and central‐western Anatolia. At the western end of the NAFZ, our estimated upper layer anisotropy direction (at ∼120 km) is rather parallel to the NAFZ reflecting the imprint of a lithospheric petrofabric formed by recent deformation while in central‐western Anatolia they correlate well with maximum shear directions and small splitting delays (∼0.6 s) appear to further support relatively thin lithosphere (∼90 km). An overall pattern of extension‐parallel fast directions (N10°E) within lower layer can be attributed to the slab rollback‐induced mantle flow that is highly oblique with respect to the WSW‐ward motion of the Anatolian lithosphere.

Varying Shear Wave Splitting Parameters Suggest Interaction Between Lithosphere and Asthenosphere in Arxan‐Chaihe Volcanic Field, NE China

Over 2 years of broadband seismic data from 27 temporary stations deployed in the Arxan-Chaihe Volcanic Field (ACVF) provides us a valued opportunity to study mantle deformation and related geodynamic processes in NE China intraplate region. We estimated 153 pairs of high-quality teleseismic shear wave splitting parameters at 22 stations and 98 non-splitting measurements at 19 stations. Most of the stations in ACVF are characterized by NNW–SSE trending fast directions with variable splitting delay times, suggesting the fossilized extensional deformation within the lithosphere is locally thermal erased by the hot mantle upwellings associated with the volcanism. In the region south of the 46.5°N, a sharp swing of fast polarization occurs, and both nearly NW-SE and W-E oriented fast directions are observed, which could be explained by small local mantle convection caused by the lithospheric delamination.

Anomalous elasticity of talc at high pressures: Implications for subduction systems

Talc is a layered hydrous silicate mineral that plays a vital role in transporting water into Earth’s interior and is crucial for explaining geophysical observations in subduction zone settings. In this study, we explored the structure, equation of state, and elasticity of both triclinic and monoclinic talc under high pressures up to 18 GPa using first principles simulations based on density functional theory corrected for dispersive forces. Our results indicate that principal components of the full elastic constant tensor C11 and C22, shear components C66, and several off-diagonal components show anomalous pressure dependence. This non-monotonic pressure dependence of elastic constant components is likely related to the structural changes and is often manifested in a polytypic transition from a low-pressure polytype talc-I to a high-pressure polytype talc-II. The polytypic transition of talc occurs at pressures within its thermodynamic stability. However, the bulk and shear elastic moduli show no anomalous softening. Our study also shows that talc has low velocity, extremely high anisotropy, and anomalously high VP/VS ratio, thus making it a potential candidate mineral phase that could readily explain unusually high VP/VS ratio and large shear wave splitting delays as observed from seismological studies in many subduction systems.

Constraints on Olivine Deformation From SKS Shear‐Wave Splitting Beneath the Southern Cascadia Subduction Zone Back‐Arc

AbstractShear‐wave splitting observations of SKS and SKKS phases have been used widely to map azimuthal anisotropy, as caused by the occurrence of olivine, to constrain the dominant directions of upper mantle deformation. While SK(K)S splitting measurements at individual seismic stations are often averaged before interpretation, it is useful to consider additional information, for example, based on the variation of splitting parameters with azimuth due to the non‐vertical incidence of core‐phases. These constraints in theory enable a differentiation between various types of olivine and may allow us to infer otherwise poorly known upper mantle parameters such as stress, temperature, and water content. In this study, we predict the azimuthal variation of splitting parameters for A‐, C‐, and E‐type olivine fabrics and match them with observations from the High Lava Plains, Northwestern Basin and Range, and Western Yellowstone Snake River Plain in the Pacific Northwest US. This helps to constrain the amount of water in the upper mantle in the back‐arc of the Cascadia subduction zone, known for its consistent E‐W oriented seismic anisotropy, and particularly large splitting delay times. Our investigation renders the C‐type olivine mechanism improbable for this location; A‐ and E‐type fabrics match the observations, although differentiating between them is difficult. However, the agreement of the amplitude of backazimuthal variation of the fast orientation, plus the potential to explain large splitting delay times, suggest the occurrence of E‐type olivine, and thus the likely presence of a moderately hydrated upper mantle beneath Cascadia's back‐arc.

Control of Timing Stability, and Suppression in Delayed Feedback Induced Frequency-Fluctuations by Means of Power Split Ratio and Delay Phase-Dependent Dual-Loop Optical Feedback

We experimentally demonstrated a power split ratio and optical delay phase dependent dual-loop optical feedback to investigate the suppression of frequency-fluctuations induced due to delayed optical feedback. The device under investigation is self-mode-locked (SML) two-section quantum-dash (QDash) laser operating at ∼21 GHz and emitting at ∼1.55 μm. The effect of two selective combinations of power split ratios (Loop-I: −23.29 dB and Loop-II: −28.06 dB, and Loop-I and Loop-II: −22 dB) and two optical delay phase settings ((i) stronger cavity set to integer resonance and fine-tuning the weaker cavity, (ii) weaker cavity set to integer resonance and fine-tuning of stronger cavity) on the suppression of cavity side-bands have been studied. Measured experimental results demonstrate that delayed optical feedback induced frequency-fluctuations can be effectively suppressed on integer resonance as well as on full delay range tuning (0–84 ps) by adjusting coupling strength −22 dB through Loop-I and Loop-II, respectively. Our findings suggest that power split ratio and delays phase-dependent dual-loop optical feedback can be used to maximize the performance of semiconductor mode-locked lasers.

Seismic anisotropy and mantle deformation in NW Iran inferred from splitting measurements of SK(K)S and direct S phases

SUMMARY We present the results of a shear wave splitting analysis performed on the teleseismic SK(K)S and direct S wave recordings of 68 temporary broad-band stations to investigate the mantle deformation on the northern side of the Arabia–Eurasia collision zone in NW Iran. We used the Reference Station Technique to overcome potential contamination from the source-side anisotropy on the direct S wave signals. This method enabled us to expand our splitting measurement database beyond the usual SK(K)S phases. The average splitting delay time over the entire region was found to be 1.14 ± 0.42 s for the SK(K)S wave and 1.36 ± 0.26 s for the direct S wave. In most parts of the study area, the fast polarization directions for both shear phases are consistent and show a uniform NE–SW direction with an average of 36° and 37° for SK(K)S and S wave-derived results, respectively. This direction is in close agreement with the direction of the absolute plate motion vector in NW Iran (N39°E). The fast directions are associated with neither the surface geological trends, nor the geodetic strain fields. We propose that the observed anisotropy is mainly controlled by the LPO fabric developed due to the shearing of the asthenospheric layer in response to the motion of the lithosphere relative to the deeper mantle. Only in a narrow region near the tectonic boundaries of central Iran with NW Iran and the Alborz, NW–SE oriented SK(K)S fast directions tend to align with the major geological structures. Fast directions obtained from direct S wave indicate significantly smoother variations in the same regions and mostly continue to be aligned in the NE–SW direction. We attribute these differences to the change in the structure of the lithosphere in the tectonic boundary zone. The western margins of central Iran possess a strong deformational fabric as evidenced by the major active strike-slip zones there. Considering that the depth extent of this fabric expands over a relatively narrow zone in the mantle, it can locally influence the SK(K)S phases. The direct S waves, on the other hand, have a larger footprint and therefore average over a larger region, and relative to the SK(K)S phases, are influenced more strongly by the asthenospheric fabric due to their larger angles of incidence, which results in a larger zone of influence for station average anisotropy parameters.

Seismic anisotropy and mantle deformation beneath Eastern Ghats Mobile Belt using direct-S waves

Seismic anisotropy observations provide a potential tool to constrain the signatures of various continental-scale deformations that have shaped the lithospheric structure of various mobile belts and ancient cratons across the world. They shed light on various aspects of evolutionary tectonics in any region. The current study primarily aims at examining the direct S-wave derived seismic anisotropy beneath the Eastern Ghats Mobile Belt (EGMB) and the adjacent Archean Singhbhum and Bastar Cratons. The granulite Terrane of EGMB represents the complex deformational history and is a crucial link in the reconstruction of Rodinia and Gondwana supercontinents. The Terrane underwent collision and rifting in Mesoproterozoic, followed by thermal overprint of the mid-Neoproterozoic to early Phanerozoic orogeny modifying its lithospheric structure. We have implemented the Reference Station Technique and obtained 854 well-constrained individual splitting measurements. The observations are based on 185 earthquake events (with Mw⩾5.5, and epicentral distance ranging 30° to 90°) recorded at 27 seismic stations deployed in the study area. The large splitting delay times (1.2–2.3 s) suggest that the lithospheric mantle is highly anisotropic. The NNE-SSW oriented Fast Polarization Directions (FPDs) observed at the Singhbhum Craton can be devoted to the evolutionary tectonic regime of the craton. The strained minerals at the deeper depths, result in FPD patterns sub-parallel to the Kerajung Fault Zone (~70°) in the Angul Domain. The FPDs are predominantly oriented along the Absolute Plate Motion (APM) direction of the Indian plate (~39°) for the Bastar Craton. However, the varying FPD patterns in the Eastern Ghats Boundary Shear Zone (Khariar Domain) signify the influence of the lithospheric deformation along with APM. The imprints of the continental building orogenies, that have modified the lithospheric structure of the study area over the Mesoproterozoic, mid-Neoproterozoic, and early Phanerozoic periods, are preserved as frozen anisotropic signatures. Our observations highlight these signatures. The study also investigates seismic anisotropic patterns in the hitherto uncharted regions close to Chilka Lake using the core-mantle refracted (SKS, SKKS, and PKS) and direct S phases. The markedly distinct E-W (average ~95°) oriented fast wave azimuths, against the ~47° average FPD of the adjacent western Phulbani Domain, distinguishes it as a separate block amongst the collage of domains forming the Eastern Ghats Province.

Mantle dynamics in the SE Tibetan Plateau revealed by teleseismic shear-wave splitting analysis

The tectonic evolution of the southeastern Tibetan Plateau is key to understanding the mechanism of growth of the whole Tibetan Plateau. Its imprint on lithospheric and asthenospheric mantle rock fabrics is expected to have a strong signature on seismic anisotropy. In this study, we measure the splitting delays and fast polarization directions (FPD) with the minimum-transverse-energy and the splitting intensity methods using core-refracted phases (e.g., SKS, SKKS, PKS) recorded by the ChinArray experiment. The potential complex anisotropic structures, i.e., the frequency-dependent splitting and two-layer anisotropy, are investigated in particular. The FPDs are consistent in different frequency bands, but the delay times measured at low frequency (8–20 s) are often larger than those measured at high frequency (2–8 s). The measurements can be better explained by a two-layer anisotropy model at many stations. Splitting intensities are also inverted to constrain the depth distribution of seismic anisotropy in the upper mantle. Combined with the two-layer anisotropy models, these results suggest different patterns of seismic anisotropy in the lithosphere and asthenosphere. In the lithosphere, anisotropy shows distinct patterns in the different tectonic blocks, while the FPDs are oriented along the major tectonic boundaries. In the asthenosphere, continuous NW-SE FPDs spread from the Tibetan plateau to the Youjiang Orogen along the southwestern margin of the Yangtze craton, implying asthenospheric extrusion from the high plateau. In the northern Indochina block, the dominant E-W FPDs may indicate asthenospheric flow driven by the eastern subduction and subsequent rollback of the Indian plate.

Seismic anisotropy beneath the eastern margin of the Tibetan Plateau from SKS splitting observations

The eastern margin of the Tibetan Plateau is one of the areas of the entire region with the most complex deformation. Seismic anisotropy reveals the imprints of past and present deformation in the lithosphere and asthenosphere, which could help elucidate the deformation mechanism. This study investigated the characteristics of seismic anisotropy beneath 512 seismic stations based on SKS phases from ~200 earthquake. The measured splitting delay time varied from 0.2–1.4 s (average: 0.7 s). In the central Sichuan basin (SCB), the average delay time is only ~0.35 s, consistent with the characteristics of the relatively thick and rigid lithosphere of the stable SCB. However, the northeastern SCB has relatively stronger anisotropy with a fast polarization direction (FPD) with NE-SW orientation, which is markedly different from the weak anisotropy with NW–SE orientation in the central SCB. We speculate that owing to E–W compressive stress caused by eastward escape of material from the Tibetan Plateau, the lithosphere in this area has undergone considerable deformation along the right-lateral maximum shear direction. In the Chuandian diamond block, the FPD of several adjacent stations was approximately E-W, i.e., almost perpendicular to the FPD in the northern and southern regions, which might be related to partial melting in the lithosphere and to compressive stress. The FPD in the west Qinling orogenic belt is approximately parallel to the strike of the surface faults and the average delay time is approximately 1.3 s. As the lithosphere is insufficient to produce such strong anisotropy in this region, we infer that the lithosphere and asthenosphere both contribute to the seismic anisotropy. Delay time in the southern Songpan-Ganzi block is much smaller than in the north, which might be attributable to low-velocity mantle upwelling and complex tectonic stress in this area.

Upper mantle deformation signatures of craton–orogen interaction in the Carpathian–Pannonian region from SKS anisotropy analysis

SUMMARYSince the Mesozoic, central and eastern European tectonics have been dominated by the closure of the Tethyan Ocean as the African and European plates collided. In the Miocene, the edge of the East European Craton and Moesian Platform were reworked in collision during the Carpathian orogeny and lithospheric extension formed the Pannonian Basin. To investigate the mantle deformation signatures associated with this complex collisional-extensional system, we carry out SKS splitting analysis at 123 broad-band seismic stations in the region. We compare our measurements with estimates of lithospheric thickness and recent seismic tomography models to test for correlation with mantle heterogeneities. Reviewing splitting delay times in light of xenolith measurements of anisotropy yields estimates of anisotropic layer thickness. Fast polarization directions are mostly NW–SE oriented across the seismically slow West Carpathians and Pannonian Basin and are independent of geological boundaries, absolute plate motion direction or an expected palaeo-slab roll-back path. Instead, they are systematically orthogonal to maximum stress directions, implying that the indenting Adria Plate, the leading deformational force in Central Europe, reset the upper-mantle mineral fabric in the past 5 Ma beneath the Pannonian Basin, overprinting the anisotropic signature of earlier tectonic events. Towards the east, fast polarization directions are perpendicular to steep gradients of lithospheric thickness and align along the edges of fast seismic anomalies beneath the Precambrian-aged Moesian Platform in the South Carpathians and the East European Craton, supporting the idea that craton roots exert a strong influence on the surrounding mantle flow. Within the Moesian Platform, SKS measurements become more variable with Fresnel zone arguments indicating a shallow fossil lithospheric source of anisotropy likely caused by older tectonic deformation frozen in the Precambrian. In the Southeast Carpathian corner, in the Vrancea Seismic Zone, a lithospheric fragment that sinks into the mantle is sandwiched between two slow anomalies, but smaller SKS delay times reveal weaker anisotropy occurs mainly to the NW side, consistent with asymmetric upwelling adjacent to a slab, slower mantle velocities and recent volcanism.

BRAIN ACTIVATION CHANGES DURING THE DELAY PERIOD IN A WORKING MEMORY TASK: NORMAL AGING-RELATED EFFECTS

Brain activation is assumed to be constant during the delay period of a delay match to sample task (DMTS). The objective was to examine the dynamic changes in brain activation (using fMRI) during a DMTS in young and older healthy subjects. There were 33 young participants (under 40 years old) and 23 older participants (over 55 years old). The study was approved by the Research Ethics Committee of the Faculty of Health Sciences in Trinity College Dublin. All participants signed informed consent before participating in study. The DMS task had 3 trial types: face-name, face and a control (scrambled images). Trials consisted of an encode phase (2s), a delay (18s) and a probe phase (4s), with a jittered inter-trial interval (6–14s). There were 75 trials (25 face-name pairs, 25 face and 25 control stimuli). The regressors for the analysis included in the first case regressors for the encoding, delay and probe periods but these were expanded to subdivide the delay period into three 6 sec period: initial, middle, and late periods. Statistical significance was set at voxel-wise threshold of p≤0.005. To correct for multiple comparisons, each cluster had a minimum size of 683μl (23 voxels) which was calculated using AlphaSim (AFNI) to get a family wise error corrected statistical significance level of p<0.05. For the young group the statistical parametric map (SPM) of the face vs control are shown in Figure 1 and with the delay period sub-divided into 3 parts in Figure 2. Similarly the face-name DMTS results are shown in Figure 3 and Figure 4 (split delay periods). In Figure 5, face vs control activation patterns seen in the older healthy groups (split delay) are seen, and Figure 6 contain the results from the face-name vs control task. The statistical maps show that during the delay period with either face or face-name stimuli, the activation pattern varied among the 3 different periods in both young and older cohorts.

Complicated seismic anisotropy beneath south-central Mongolia and its geodynamic implications

Two years of high-quality broadband seismic data from 69 temporary stations deployed in south-central Mongolia provide an opportunity to study the anisotropy-forming mechanisms in this area. The majority of shear wave splitting observations determined from the analysis of teleseismic SKS phase are characterized by NW–SE trending fast directions with large splitting delay times (greater than 2.0 s at six stations), which is inferred to be generated by active asthenospheric flow. The variation of the fast direction may be associated with deflection of asthenosphere around the deep Siberian cratonic keel at the base of the lithosphere. Several of the NE–SW trending fast directions with relatively small delay times observed in the Gobi Desert are parallel to the strike of the main faults and sutures, which may represent lithospheric deformation. In addition, it is inferred that small-scale hot mantle upwelling is responsible for generating a cluster of null measurements observed on the south of the Hentiy Mountain.

Minimizing Blocking Probability in Elastic Optical Networks by Varying the Bandwidth Granularity Based on Optical Path Fragmentation

Elastic optical networks (EONs) based on orthogonal frequency-division multiplexing (OFDM) are considered a promising solution for the next optical network’s generation. It basically makes it possible to choose an adequate portion of the available spectrum to satisfy the requested capacity. In this paper, we consider the impact of spectrum fragmentation along the optical single/multipath routing transmission on the efficiency of the EONs. This involves reducing the fragmentation effects by dynamically updating and controlling the minimum bandwidth allocation granularity (g).We adopt linear and nonlinear dynamic mechanisms, which are denoted as LDAEi and NLDAEi, respectively, to choose proper band width granularities that are proportional to the optical link/path bandwidth fragmentation status. In order to avoid neither splitting the capacity request over many routing paths, which would increase the management complexity, nor encouraging single path transmission, the proposed schemes aim to choose a proper bandwidth allocation granularity (g) of a predefined set of suggested values. Simulation results show that varying the bandwidth granularity based on the optical path fragmentation status can offer improved performance over fixed granularity with respect to the bandwidth blocking probability, the number of path splitting, the throughput, and differential delay constraint issue in terms of: the network bandwidth utilization and multipath distribution.

Azimuthal anisotropy beneath north central Africa from shear wave splitting analyses

AbstractThis study represents the first multistation investigation of azimuthal anisotropy beneath the interior of north central Africa, including Libya and adjacent regions, using shear wave splitting (SWS) analysis. Data used in the study include recently available broadband seismic data obtained from 15 stations managed by the Libyan Center for Remote Sensing and Space Science, and those from five other stations at which data are publicly accessible. A total of 583 pairs of high‐quality SWS measurements utilizing the PKS, SKKS, and SKS phases demonstrate primarily N‐S fast orientations with an average splitting delay time of approximately 1.2 s. An absence of periodic azimuthal variation of the observed splitting parameters indicates the presence of simple anisotropy, and lack of correlation between surficial features and the splitting parameters suggests that the origin of the observed anisotropy is primarily asthenospheric. This conclusion is enhanced by nonperiodic azimuthal variation of the splitting parameters observed at one of the stations located near the boundary of areas with different anisotropic properties. We interpret the observed anisotropy to be the consequence of northward movement of the African plate relative to the asthenosphere toward the Hellenic and Calabrian subduction zones. Local variance in fast orientations may be attributable to flow deflection by the northern edge of the African continental root. The observations provide critical and previously lacking constraints on mantle dynamic models in the vicinity of the convergent boundary between the African and Eurasian plates.

Assessing the Effect of Velocity Model Accuracy on Microseismic Interpretation at the In Salah Carbon Capture and Storage Site

Injection of carbon dioxide (CO2) to be stored at depth at the In Salah Carbon Capture and Storage (CCS) site began in 2004 with the subsequent installation of a pilot microseismic monitoring well in 2009. The project is one of only two industrial-scale storage projects to have been monitored for microseismicity. Such projects are vital to demonstrate the validity of CCS technology and the role of microseismic monitoring, a technology that could be used in real-time to regulate the geomechanical response of a site to CO2 injection. Substantial microseismicity (over 9000 events) was recorded by a single three-component geophone situated at 80m deep almost directly above one of the In Salah injection wells. The events occur in two main clusters with estimated locations of wellrecorded events within one of these clusters to be within 1km horizontal distance from the geophone and between 2.1km and 2.7km deep, at least 200m below the injection depth and CO2 storage interval. Errors in the depth range of event locations are investigated using modified velocity models, revealing that 10% slower velocities create uncertainties up to 450m in depth. Alternatively, 20% slower velocities in the shallow sub-surface or an anisotropic model have a similar effect. Independent of the absolute depth, there is no migration of event locations to shallower depths with time. Evidence from the analysis of shear-wave splitting delay times implies that, between 2009 and 2011, CO2 injection is opening pre-existing fractures that then close as pressure decreases, rather that creating new fractures. The estimated dominant fracture orientation is approximately NW-SE, in agreement with fracture orientations inferred from logging data, and the observed maximum moment magnitude, MW = 1.7, is also consistent with estimated pre-existing fracture dimensions at the injection depth. This work demonstrates the value of microseismic monitoring of CCS projects, even with a limited array, but an accurate velocity model is critical to allow reliable interpretation of the data. We recommend that microseismic monitoring is conducted prior to CO2 injection at future CCS sites to enable baseline and comparative studies. Real-time microseismic monitoring would help inform injection decision and contribute to the safe operation of a project. © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of GHGT.

Seismic Anisotropy from SKS Splitting beneath Northeastern Tibet

The northeastern boundary of the Tibetan high plateau is marked by a 2 km topographic drop and a coincident rapid change in crustal thickness. Surface tectonics are dominated by the Kunlun strike?slip fault system and adjacent Kunlun concealed thrust. The main objective of the current study is to map lateral variations of seismic anisotropy parameters in this region along the linear INDEPTH IV array in order to investigate the link between surface and internal deformation in the context of crust and mantle structure. To achieve this aim, we performed Minimum?Transverse?Energy based SKS splitting measurements using 23 stations of the INDEPTH IV array deployed across the northeastern margin of Tibet. Average fast polarization directions and splitting time delays are obtained by averaging stacked misfit surfaces of all analyzed events at each station. The agreement of fast directions with the strikes of major active strike?slip faults and strike?slip focal mechanisms, but not with fossil structures such as the Jinsha suture, implies that the anisotropy records lithospheric petrofabric formed by recent deformation within the lithosphere rather than representing frozen?in anisotropy or shear within the asthenosphere due to absolute plate motion. The distribution of large splitting delays throughout the northern plateau suggests that deformation is distributed rather than focused onto narrow shear zones associated with the Kunlun strike?slip faults. The drop in splitting delays toward the Qaidam is then a natural consequence of the much lower degree of deformation there.

Crust–mantle coupling at the northern edge of the Tibetan plateau: Evidence from focal mechanisms and observations of seismic anisotropy

The Tarim basin is distinct from the Tibetan plateau by the apparently low degree of internal deformation. Relative motion between the lithospheres of Tibet and Tarim likely results in coherent mantle deformation, with a diagnostic signature of anisotropic seismic wave speed. Birefringence (splitting) in core-refracted shear phases (SKS, PKS) observed along the Tibet–Tarim border is indicative of seismic anisotropy along their path. The mantle deformation direction inferred from shear wave splitting under the assumption of a single source of anisotropy is approximately E–W. The size of the splitting delays inferred in our analysis suggests that most of the anisotropic signature is coming from the mantle. Furthermore, our data suggest vertical stratification of rock fabric.Focal mechanisms of regional earthquakes, both published and derived from new data, provide evidence for ~NNE–SSW compression within the Tarim basin north of the border with Tibet. This type of internal deformation in the lithosphere is consistent with its underthrusting (subduction?) beneath Tibet. Within the crust of the Tibetan plateau we find a more complex pattern suggestive of the ~E–W extension, and generally consistent with the sense of motion on the Altyn Tagh fault that extends along the Tibet–Tarim boundary. Therefore, at the Tibet–Tarim border we find close similarity between deformation directions within the crust of the plateau and within the upper mantle on both sides of the boundary. A plausible explanation of such similarity would be the coupling of the crust and the upper mantle, with no weak zone being present in the lower crust. One scenario for such coupling would involve an extension of a deformation zone associated with the Altyn Tagh fault into the uppermost mantle, making this fault zone very similar to a plate boundary.

Relationship between topography, rates of extension and mantle dynamics in the actively-extending Italian Apennines

To investigate the mechanism driving active extension in the central and southern Italian Apennines and the geography of seismic hazard, we compare spatial variations in upper crustal strain-rate measured across exposed fault scarps since 15±3ka with data on cumulative upper-crustal strain and topographic elevation, and free-air gravity, P-wave tomography and SKS splitting delay times that are a proxy for strain in the mantle. High extensional strain-rates across the Apennines since 15±3ka (0.4–3.1mm/yr along 90km transects) occur in two areas (Lazio-Abruzzo; SE Campania and Basilicata) where values for finite extensional strains that have developed since 2–3Ma are highest (2–7km cumulative throw), and where mean elevation in 5×90km NE–SW boxes is >600m; the intervening area (NW Campania and Molise) with<600m mean elevation in 5×90km boxes has extension-rates<0.4mm/yr and lower values for finite extensional strains (<2km cumulative throw). These two areas with high upper-crustal strain-rates overlie mantle that has relatively-long spatially-interpolated SKS delay times (1.2–1.8s) indicating relatively-high mantle strains and free-air gravity values (140–160mGals); the intervening area of lower extension-rate has shorter spatially-interpolated SKS delay times (0.8–1.2s) and lower free-air gravity values (120mGals). The two areas with high upper crustal strain-rates and strain, mean elevation, and mantle strain, coincide with the northern and southern edges of a slab window in the Tyrrhenian–Apennines subducting plate that has been inferred from published P-wave tomography. Together these correlations suggest that dynamic support of the topography by mantle flow through the slab window may control the present day upper crustal strain-rate field in the Apennines and the geography of seismic hazard in the region.

Shear wave splitting at the Hawaiian hot spot from the PLUME land and ocean bottom seismometer deployments

We examine upper mantle anisotropy across the Hawaiian Swell by analyzing shear wave splitting of teleseismic SKSwaves 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.