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The Turkish-Iranian Plateau and the Zagros highlands are among the most prominent physiographic features in the Middle East and were formed as a result of continental collision between the Arabian and Eurasian plates. To better understand the nature of the lithospheric mantle and the origin of the observed seismic anomalies in this region, we investigated seismic attenuation of the uppermost mantle by detailed measurements of the quality factor of the Sn seismic phase (Sn Q). To that end, we collected a large data set consisting of 30 years (1990–2020) of waveforms recorded by 1266 permanent and temporary seismic stations, applying both the two-station method (TSM) and reverse two-station method (RTM) to measure path-averaged Sn Q. Finally, we performed a tomographic inversion on the path-averaged Sn Q to map the lateral variations of the upper-mantle attenuation across the northern Middle East. Our Sn attenuation maps show moderately low Q (350) beneath the Zagros and northern edge of the Arabian plate. Furthermore, our Sn Q model is broadly consistent with seismic velocity models in the region suggesting that most of the seismic anomalies are the result of thermal rather than compositional effects.

The obduction of the Semail Ophiolite onto the Arabian continental margin during the convergence of the Arabian and Eurasian plates has left a significant impact on the lithospheric structure beneath the Oman Mountains. However, there remains a degree of uncertainty concerning the extent to which the inherited structures (pre-existing features of the lithosphere) contribute to the obduction of ophiolites. To gain a deeper understanding of the impact of the obduction process on the mantle structure beneath northern Oman, we analyze seismic anisotropy beneath this region using splitting analysis of teleseismic shear wave data collected from a dense network of 40 seismic stations that have been operational for approximately 3 years since 2013. Based on azimuthal distribution of the shear wave splitting (SWS) parameters, φ and δt, we are able to divide the study area into two subregions. The stations located to the west of the Semail gap exhibit relatively azimuthally invariant SWS parameters suggesting a single anisotropic layer. On the other hand, at most of the stations located in the central and eastern regions we observe a 90-degree periodicity versus back-azimuth, indicative of a depth-dependent anisotropic medium. In the western part, the fast axes are aligned with the strike of the collision between the continental and oceanic plates, where the oceanic lithosphere is believed to be obducted over the continental lithosphere. We also invert the azimuthal variation of the SWS parameters from the central and eastern stations for two layers of anisotropy. The fast axes of the upper layer exhibit a predominantly NW-SE trend, in good agreement with the anisotropy directions of the one-layer models obtained in the western region. The fast axes of the lower layer display a NE-SW trend, possibly representative of the large-scale mantle flow resulting from the present-day plate motion.

Seismic anisotropy provides a unique link between directly observable surface structures and the more elusive dynamic processes in the mantle below. The ability to infer the vertically- and laterally-varying anisotropic structures is of great significance for the geodynamic interpretation of surface-recorded waveform effects.In the first part of this presentation, we assess the capabilities of different observables for the inversion XKS phases to uniquely resolve the anisotropic structure of the upper mantle. For this purpose, we perform full-waveform calculations for simple models of upper-mantle anisotropy. In addition to waveforms, we consider the effects on apparent splitting parameters and splitting intensity. The results show that, generally, it is not possible to fully constrain the anisotropic parameters of a given model, even if complete waveforms are considered. We also discuss advantages and disadvantages of using the different observables.Recent technological advances have prompted implementations of large-scale seismic experiments producing huge amounts of seismic data. Standard processing procedures, thus, require automatization to facilitate fast and objective data processing. This also applies to the analysis of shear-wave splitting. A recent extension of the SplitRacer software code allows for an automatization of the analysis by choosing a time window based on spectral analyses and by categorization of results based on different splitting methods.Finally, we will present new results from the application of Neural Networks to the analysis of shear-wave splitting. Our initial approach involves training based on synthetic data and deconvolution of the real waveforms. Current limitations and possibilities for extension will be discussed.

We investigate the flow field and deformation in the mantle wedge and subslab mantle beneath the Kuril-Kamchatka subduction zone using seismological data from a recently deployed seismic network around the Klyuchevskoy Volcanic Group (KVG) complemented by data from previous temporary deployments and permanent stations to reach a total number of 145 seismic stations covering a region defined in the geographic coordinates 150°-167°E and 50°-61°N.We perform splitting analysis of both local and core-refracted (SKS) shear waves to study mantle seismic anisotropy as a proxy for the pattern of the mantle flow field and deformation. Anisotropy in the mantle wedge is studied by shear splitting analysis (SWS) of waveform data from local mantle events that occurred along the subducting slab (Wadati-Benioff-Zone) and in the mantle wedge. Crustal anisotropy is also studied by SWS analysis of crustal events. The combined data set (SKS and local) allows us to discriminate the source of mantle anisotropy (sub-slab, mantle wedge, or crust). Shear-wave splitting measurements from the local shear waves give small delay times independent of the depth of the events suggesting that the mantle wedge is characterized by a weak anisotropic fabric. The fast directions of mantle wedge anisotropy are predominantly parallel to the strike of the slab indicating either a trench-parallel flow or B-type seismic anisotropy in the mantle wedge. The relatively small delay times from local shear waves suggest that SKS waves are less affected by potential anisotropy in the mantle wedge and that the results of the SKS-splitting analysis are mainly representative of the sub-slab anisotropy. Our SKS-splitting measurements indicate a trench-normal mantle flow beneath the eastern edge of the Kamchatka peninsula that converts to a more complex pattern beneath the KVG region. We argue that this pattern of fast polarization direction suggests the rotational mantle flow beneath the slab that may be related to the change in slab geometry at the junction between the Kuril-Kamchatka and Aleutian arcs. The observation of relatively strong sub-slab anisotropy against weak mantle-wedge anisotropy suggests that slab termination causes some disturbance in mantle flow; however, no significant component of an around-slab flow occurs in the mantle wedge.

SUMMARY We present a technique to derive robust estimates for the crustal thickness and elastic properties, including anisotropy, from shear wave splitting of converted phases in receiver functions. We combine stacking procedures with a correction scheme for the splitting effect of the crustal converted Ps-phase and its first reverberation, the PpPs-phase, where we also allow for a predefined dipping Moho. The incorporation of two phases stabilizes the analysis procedure and allows to simultaneously solve for the crustal thickness, the ratio of average P- to S-wave velocities, the percentage of anisotropy and the fast-axis direction. The stacking is based on arrival times and polarizations computed using a ray-based algorithm. Synthetic tests show the robustness of the technique and its applicability to tectonic settings where dip of the Moho is significant. These tests also demonstrate that the effects of a dipping layer boundary may overprint a possible anisotropic signature. To constrain the uncertainty of our results we perform statistical tests based on a bootstrapping approach. We distinguish between different model classes by comparing the coherency of the stacked amplitudes after moveout correction. We apply the new technique to real-data examples from different tectonic regimes and show that coherency of the stacked receiver functions can be improved, when anisotropy and a dipping Moho are included in the analysis. The examples underline the advantages of statistical analyses when dealing with stacking procedures and potentially ambiguous solutions.

Oldoinyo Lengai in the North Tanzanian Divergence is one of the few highly active volcanoes in Africa. Its eruptive cycle is characterized by effusions of carbonatite lava and severe explosions. The most recent of these occurred in 2007 and left a circular crater nearly 400 wide and approximately 100 m deep. The crater is currently being filled with new lava which solidifies and has formed several characteristic hornitos. In 2019, we set up a temporary seismic network of 10 short-period stations, equipped with 4.5 Hz geophones, surrounding the crater area at altitudes between about 1990 and 2885 m to monitor the eruptive activity of the volcano. Seven of the stations were recovered in February 2020. The retrieval of the remaining stations was delayed due travel restrictions caused by the pandemic. However, in Sept. 2021, two of the missing stations were returned from the volcano. Due to the limited battery capacity, recordings were restricted to a period of about five weeks between 14/09 and 23/10/2019. Analysis of the data shows tremor activity and more than 80 distinct recordings of high-frequency seismic signals in the immediate vicinity of the network. However, the recordings lack clear S-wave arrivals, and the station configuration is unfavorable for the application of classical localization techniques based on iterative inversion. We, therefore, apply a grid-search approach based on a Bayesian formulation which also accounts for the topography and shape of the volcanic edifice. The results show that the events are located within or close to the circular crater rim. We argue that the events are caused by sliding segments of the crater wall which have become gravitationally unstable, possibly due to magmatic undermining. The interpretation is supported by surface observations of opening cracks at the outer base of the crater rim.

We perform an adjoint waveform tomography using Rayleigh-wave empirical Green‘s functions (EGFs) at periods 10-50 s to improve a pre-existing 3-D velocity model of the crust and uppermost mantle beneath the Iranian plateau. The starting model was derived from a conventional surface-wave dispersion tomography based on high-frequency ray-theory assumption to invert of a quasi-3-D shear-wave velocity model. We use the EGFs from the same study that were derived from cross correlation of continuous seismic noise. Adjoint tomography refines the initial model by iteratively minimizing the frequency-dependent travel-time misfits between synthetic and observed EGFs measured in different period bands. Our new model covers the known tectonic units such as the Central Iranian Block, Zagros fold-and-thrust belt, Sanandaj-Sirjan metamorphic zone and Urumieh-Dokhtar magmatic arc.Overall, the adjoint tomography provides images with better lateral resolution and depth sensitivity and more realistic absolute velocity values due to the inclusion of finite-frequency waveforms. The use the numerical spectral-element solver in adjoint tomography provides accurate structural sensitivity kernels, which helps to obtain more robust images rather than those generated by ray-theory tomography. The final model adjusts the shapes of velocity anomalies at crustal depth more specifically for the eastern Zagros. The final model significantly improves the initial model at the upper mantle depths and provides a higher resolution for the shape of velocity anomalies.

We use seismological data collected from a recently deployed seismic network around the Klyuchevskoy Volcanic Group (KVG) in Kamchatka to study the crustal structure and mantle anisotropy beneath the region. In order to improve and extend the data coverage, we combined this data set with data from previous temporary deployments and permanent stations to reach a total number of 145 stations covering a region defined in the geographic coordinates 150°-167°E and 50°-61°N.We use receiver function (RF) analysis to study the crustal structure beneath the study area. P-RFs are migrated to depth and stacked to image seismic interfaces beneath the network of seismic stations. We used a recently published three-D seismic tomography model to migrate the RFs from time to space domain. The RF amplitudes are stacked in the space domain using the Common Conversion Point (CCP) approach. The stacked RF amplitudes provide a 3-D image of seismic interfaces. The use of the 3-D velocity model helps migrate the RF amplitude to correct depths so that the depth and geometry of subsurface interfaces are constrained more correctly. Furthermore, we are able to better compare the 3-D CCP images with the 3-D tomography model. In addition, at stations with a sufficient number of RFs, we also tried to calculate Moho depth and mean Vp/Vs ratio using the single-station H-k stacking approach. This analysis provides a way to better identify the interfaces beneath different locations and verify and adjust the depths obtained using the CCP stacking. We found a relatively complex crustal structure in the entire region of the KVG that laterally merges to a simpler structure to the west. Seismic tomography images provide better lateral resolution of velocity anomalies while RF analysis provides better vertical resolution of vertical velocity constants. Our RF-CCP images reveal two main interfaces beneath the active volcanic region. The shallow interfaces with a limited lateral extent have depths varying between 20 and 30 km. The deeper interface occurs at depths 50-60 km with an east-to-west dipping direction. In comparison with the seismic tomography model, we infer that the shallow interface is related to a velocity increase from 7 km/s, implying the presence of a shallow low-velocity zone beneath the volcanic group. The deeper interface that correlates with a velocity increase from In addition to the RF analysis for the crustal structure, we also perform splitting analysis of core-refracted shear waves (SKS) to study mantle seismic anisotropy as a proxy for the pattern of the mantle flow field. Our SKS-splitting analysis indicates a trench-normal mantle flow beneath the eastern edge of the Kamchatka peninsula that converts to a more complex pattern beneath the KVG region. We argue that this pattern of fast polarization direction suggests the rotational mantle flow that may be related to a slab gap at the junction between the Kuril-Kamchatka and Aleutian arcs.

We assess the capabilities of different observables for the inversion of core-refracted shear waves (XKS phases) to uniquely resolve the anisotropic structure of the upper mantle. For this purpose, we perform full-waveform calculations for relatively simple, canonical models of upper-mantle anisotropy. The models are characterized by two and four domains of different anisotropic properties. Specifically, we assume hexagonal symmetry with arbitrarily chosen strength of the anisotropy and orientation of the horizontal fast axis. XKS waveforms, generated from plane-wave initial conditions, traverse through anisotropic models and are recorded at the surface by a single station (in case of vertical variations) and by a dense station profile across the laterally and vertically varying structure. In addition to waveforms, we consider the effects of anisotropic variations on apparent splitting parameters and splitting intensity. The results show that, generally, it is not possible to fully resolve the anisotropic parameters of a given model, even if complete waveforms (under noisefree conditions and for the complete azimuthal range) are considered. This is because waveforms for significantly different anisotropic models can be indistinguishable. However, inversions of both waveforms and apparent splitting parameters lead to similar models that exhibit systematic variations of anisotropic parameters. These characteristics may be exploited to better constrain the inversions. The results also show that splitting intensity holds some significant drawbacks: First, even from measurements over a wide range of back-azimuth, there is no characteristic signature that would indicate depth variations of anisotropy. Secondly, identical azimuthal variations of splitting intensity for different anisotropic structures do not imply that the corresponding split waveforms are also similar. Thus, fitting of observed and calculated splitting intensities could lead to anisotropic models that are incompatible with the observed waveforms. We conclude that (bandlimited) XKS-splitting inversions and related tomographic schemes, even if based on complete waveforms, are not sufficient to fully resolve the heterogeneous anisotropic structures of the upper mantle and that combinations with alternative methods, based on e.g., receiver-function splitting, P-wave travel-time deviations, or surface waves, are required.

A common deviation from typical subduction models occurs when thrust sheets of oceanic crust and upper‐mantle rocks are emplaced over more buoyant continental lithosphere. The archetypal example of ophiolite obduction is the Semail ophiolite in the United Arab Emirates (UAE)‐Oman orogenic belt, formed and obducted onto the Arabian continental margin during the Late Cretaceous. The Strait of Hormuz syntaxis, the northern extent of the UAE‐Oman mountains, marks the transition from ocean‐continent convergence in the Gulf of Oman to continental collision along the Zagros Mountains. Based on new seismic data from a focused recording network, we infer continental crustal and mantle deformation in the northeastern corner of the Arabian plate (including the southern Zagros and the UAE‐Oman mountains), using observations from anisotropic tomography and shear‐wave splitting (SWS) measurements. We recover a change of ∼90° (from approximately WNW to nearly NS) in the axis of fast‐anisotropic orientations in the crust from the Zagros to the UAE‐Oman mountain belt, consistent with the dominant strike of the orogenic belts. We also find evidence in our SWS parameters for localized fossil deformation in the lithospheric mantle underlying the UAE‐Oman mountain range, possibly related to stress‐induced tectonism triggered by north‐east oriented underthrusting of the proto‐Arabian continental margin beneath the overriding Semail ophiolite. Shear‐wave‐splitting anisotropy orientations along two transects across the northern Musandam peninsula, averaging 15° anticlockwise from the north, provide the first geophysical verification of previous geological evidence that suggests a NE polarity of the Late Cretaceous Oman subduction zone system.

We used more than one decade of core-refracted teleseismic shear (SKS) waveforms recorded at more than 160 broadband seismic stations across the Iranian plateau and Zagros to investigate seismic anisotropy beneath the region. Splitting analysis of SKS waveforms provides two main parameters, i.e., fast polarization direction and split delay time, which serve as proxies for the trend and strength of seismic anisotropy beneath the stations. Our observation revealed a complex pattern of splitting parameters with variations in the trend and strength of anisotropy across the tectonic boundaries. We also verified the presence of multiple layers of anisotropy in conjunction with the lithosphere deformation and mantle flow field. Our observation and modeling imply that a combined system of lithosphere deformation and asthenospheric flow is likely responsible for the observed pattern of anisotropy across the Iranian Plateau and Zagros. The rotational pattern of the fast polarization directions observed locally in Central Zagros may indicate the diversion of mantle flow around a continental keel beneath the Zagros. The correlation between the variation in lithosphere thickness and the trend of anisotropy in the study area implies that the topography of the base of lithosphere is also a determining factor for the pattern of mantle flow inferred from the observations.

We perform an adjoint waveform tomography using a combined data set consisting of regional earthquake waveforms and Rayleigh wave ambient-noise Green's function to construct a new 3-D wave velocity model of the crust and uppermost mantle beneath the Iranian plateau. The earthquake waveforms come from 250 regional events with magnitudes of 4.5-6.5 recorded by 136 broadband seismic stations. The EGFs are derived from cross correlations of more than three years of continuous seismic noise. The inversion starts with an initial model derived from the global Crust1 model locally modified using information from previous studies. Adjoint tomography refines the initial model by iteratively minimizing the frequency-dependent travel-time misfits between real and synthetic earthquake data and EGFs and synthetic Green’s functions measured in different period bands. Our new model covers the known tectonic units such as the Central Iranian Block, Zagros fold-and-thrust belt, Sanandaj-Sirjan metamorphic zone and Urumieh-Dokhtar magmatic arc. Overall, the adjoint tomography provides images with more real resolutions and amplitudes due to the finite-frequency consideration. Using the numerical spectral-element solver in adjoint tomography provides accurate structural sensitivity kernels, which help generate more robust images rather than those generated by ray-theory tomography. Our study also demonstrates improvement of lateral resolution and depth sensitivity using combined data set instead of only earthquake data.

Over the last two decades, there has been significant progress in the development of tomographic schemes to infer the anisotropic properties of the upper mantle from teleseismic XKS phases. The methods are based on linking the anisotropic material properties and their changes with surface observables and waveform proxies, such as splitting parameters and intensities, through finite-frequency sensitivity kernels. These approaches are supported by increasingly dense seismic networks that allow for a more precise characterization of short-scale waveform variations due to lateral variations of anisotropy.Here we focus on the general capability of the tomographic schemes to uniquely resolve the anisotropic structure of the upper mantle from surface observations. For this purpose, we perform full-waveform calculations for relatively simple, canonical models of upper-mantle anisotropy. Our approach involves checkerboard-style tests similar to those typically used to assess the resolving power of tomographic schemes. The models are characterized by four zones of different anisotropic properties. Specifically, we assume orthorhombic symmetry with arbitrarily chosen strength of the anisotropy and orientation of the horizontal a-axis. XKS waveforms, generated from plane-wave initial conditions, traverse through anisotropic models and are recorded at the surface by a dense station profile. In addition to waveforms, we also consider the effects of different anisotropic models on splitting parameters and splitting intensities.The results show that it is, generally, not possible to uniquely resolve the eight anisotropic parameters (a-axis orientation and strength of anisotropy in four zones) of a given model, even if complete waveforms (under noise-free conditions) are considered. This is related to the fact that waveforms for significantly different anisotropic models, often, are indistinguishable. We conclude that finite-frequency XKS-splitting tomography, alone, is not suited to resolve the anisotropic structures of the upper mantle and that combinations with alternative methods, based on e.g. receiver-function splitting or surface waves, are required.

International audience; Imaging and resolving the lateral continuity of 3-D crustal structures enhance our ability to interpret seismicity and to understand how orogens are created. We apply a Bayesian, hierarchical inversion approach based on a transdimensional trees-structured wavelet parameterization to recover phase velocity maps at 2-40 s periods. We then invert phase velocity dispersion to constrain a 3-D shear velocity model of the crust beneath south-central Iran. Together with accurate earthquake centroid depths and focal mechanisms, the pattern of 3-D velocity variations supports recent suggestions that most large earthquakes in the Zagros occur within the lower sedimentary cover or close to the sediment-basement interface. Furthermore, we find evidence for Arabian basement underthrusting beneath central Iran, although only in one location does it appear to generate earthquakes. Our new 3-D tomographic model clarifies and throws new light on the crustal structure of the SE Zagros and its relation to seismicity and active faulting.

SUMMARY We have constructed a 3-D shear wave velocity (Vs) model for the crust and uppermost mantle beneath the Middle East using Rayleigh wave records obtained from ambient-noise cross-correlations and regional earthquakes. We combined one decade of data collected from 852 permanent and temporary broad-band stations in the region to calculate group-velocity dispersion curves. A compilation of >54 000 ray paths provides reliable group-velocity measurements for periods between 2 and 150 s. Path-averaged group velocities calculated at different periods were inverted for 2-D group-velocity maps. To overcome the problem of heterogeneous ray coverage, we used an adaptive grid parametrization for the group-velocity tomographic inversion. We then sample the period-dependent group-velocity field at each cell of a predefined grid to generate 1-D group-velocity dispersion curves, which are subsequently inverted for 1-D Vs models beneath each cell and combined to approximate the 3-D Vs structure of the area. The Vs model shows low velocities at shallow depths (5–10 km) beneath the Mesopotamian foredeep, South Caspian Basin, eastern Mediterranean and the Black Sea, in coincidence with deep sedimentary basins. Shallow high-velocity anomalies are observed in regions such as the Arabian Shield, Anatolian Plateau and Central Iran, which are dominated by widespread magmatic exposures. In the 10–20 km depth range, we find evidence for a band of high velocities (>4.0 km s–1) along the southern Red Sea and Arabian Shield, indicating the presence of upper mantle rocks. Our 3-D velocity model exhibits high velocities in the depth range of 30–50 km beneath western Arabia, eastern Mediterranean, Central Iranian Block, South Caspian Basin and the Black Sea, possibly indicating a relatively thin crust. In contrast, the Zagros mountain range, the Sanandaj-Sirjan metamorphic zone in western central Iran, the easternmost Anatolian plateau and Lesser Caucasus are characterized by low velocities at these depths. Some of these anomalies may be related to thick crustal roots that support the high topography of these regions. In the upper mantle depth range, high-velocity anomalies are obtained beneath the Arabian Platform, southern Zagros, Persian Gulf and the eastern Mediterranean, in contrast to low velocities beneath the Red Sea, Arabian Shield, Afar depression, eastern Turkey and Lut Block in eastern Iran. Our Vs model may be used as a new reference crustal model for the Middle East in a broad range of future studies.

Reliable moment magnitude estimates for seismic events in the Middle East region can be difficult to obtain due to the uneven distribution of stations, the complex tectonic structure, and regions of high attenuation. In this study, we take advantage of the many new broadband seismic stations that have become available through improved national networks and numerous temporary deployments. We make coda envelope‐amplitude measurements for 2247 events recorded by 68 stations over 13 narrow frequency bands ranging between 0.03 and 8 Hz. The absolute scaling of these spectra was calculated based on independent waveform modeling solutions of the moment magnitudes for a subset of these events to avoid circularity. Using our 1D path calibrations, we determined coda‐based magnitudes for a majority of the events. We obtain fairly good agreement with waveform‐modeled seismic moments for the larger events ( M w >4.5) at low frequencies ( 0.7 Hz) because of unaccounted 2D path effects, as well as mixing of both Sn coda and Lg coda, which have different attenuation behavior. This scatter leads to increased variance in the magnitudes estimated for smaller events in which low‐frequency amplitudes are below the noise levels and the higher frequencies are the only signals available. We quantify the expected variance in coda envelope amplitudes as a function of frequency using interstation scatter as our metric. The net results of this study provide thousands of new 1D coda magnitude estimates for events in the broad region, as well as the necessary initial starting model for use in a new related 2D coda study (Pasyanos et al. , 2016). Online Material: Table of site terms and moment magnitudes.

The continental collision between the Arabian and Eurasian plates plays an important role in the geodynamical evolution of the Zagros and Iranian plateau. In order to investigate the upper mantle structure across the collision zone, we calculated the absolute and relative shear-wave velocity structure along a dense and long temporary seismic profile in NW Iran. The probabilistic 1-D absolute shear-wave velocity (Vs) depth profiles beneath 23 seismic stations were calculated using a trans-dimensional joint inversion approach of P-wave coda together with the Rayleigh wave phase and group velocity dispersion data. These 1-D models are then interpolated to obtain a 2-D Vs model along the profile. In addition, we used a nonlinear teleseismic tomography method to determine shear wave velocity variations beneath 44 seismic stations. Both the absolute and relative velocity models, obtained by different data sets and methods, show a thick (>150 km) high-velocity anomaly beneath the northern part of the Zagros, which could be the thickened Zagros lithosphere associated with continental shortening. A relatively thin lithosphere (~80 km) and lower S-wave velocity beneath Central Iran may suggest a weakened Iranian mantle lithosphere by upwelling of the hot asthenosphere following slab break-off. The presence of a thickened lithosphere beneath the Zagros implies that the Arabian-Eurasian convergence is accommodated mainly as shortening in the mantle lithosphere beneath the Zagros, rather than underthrusting beneath Central Iran. A narrow low-velocity corridor between the Zagros lithosphere and overriding plate (Central Iran), south of the Sanandaj-Sirjan Zone (SSZ), probably due to delamination of the subducting Arabian lithosphere from the Zagros lower crust, likely prohibits transferring stress from the Zagros lithosphere to Central Iran, causing less deformation in the SSZ which is the edge of the overriding plate.

We investigate the effect of anisotropy on estimates of crustal thickness H and average bulk VP/VS-ratio κ. Here we extend the stacking approach of Zhu and Kanamori (2000) to include all 20 P-to-S converted waves and their crustal reverberations that are generated in the anisotropic case—instead of only five phases in the isotropic case. The ray-based algorithm of Frederiksen and Bostock (2000) is used to calculate the amplitude and arrival time of each phase. Synthetic tests are performed to investigate the feasibility and robustness of the stacking approach. For simplicity, we assume hexagonal symmetry and a horizontal symmetry axis, but more general anisotropy may be considered. The tests reveal that the estimates of H and κ can be significantly affected by the presence of crustal anisotropy. We verify the feasibility of the stacking approach for real data by applying the method to examples from three different tectonic regions. The results show that the anisotropic stacking scheme presented here can provide a much better constraint on the estimation of H and κ than is achieved using isotropic stacking. Anisotropic stacking can also help resolve the ambiguity in determination of H and κ that arises when several maxima from stacking amplitudes of receiver functions occur in the case of complex crustal structure.

We present a new approach for understanding the origin and nature of seismic anomalies in the continental crust of the Northern Middle East. We have created detailed attenuation (QLg) and velocity (VLg) models for the Northern Middle East based on the analysis of waveforms of regional seismic phase Lg from 3171 regional earthquakes recorded at 578 stations in Turkish and Iranian Plateaus and surrounding regions. The attenuation and velocity models are assumed to serve as proxies for the bulk average crustal shear wave attenuation (Qβ) and velocities (Vs). 31 232 reliable Lg spectra were collected and used to measure the two-station method (TSM) and reverse two-station/event method (RTM) Lg Q at 1 Hz (Q0) and its frequency-dependence factor (η). The Lg Q0 and η values are measured over the individual TSM and RTM paths and are then used to perform an LSQR tomographic inversion for lateral variations in Q0 and η. The Lg Q0 and η models both correlate well with the major tectonic boundaries in the region. The tomographic models as well as the individual TSM and RTM measurements show lower values of Lg Q0 over the Turkish-Anatolian Plateau (

[1] Shear waveforms from core-refracted (SKS) phases recorded at 105 portable stations belonging to the DESERT and DESIRE campaigns and nine permanent broadband stations of the Israel Seismological Network are analyzed to study polarization seismic anisotropy beneath the region of the Dead Sea Transform fault in the Middle East. Shear wave splitting parameters exhibit variations with back azimuth (initial polarization) of the incoming SKS waves. The pattern of this variation is nearly constant along the strike of the fault suggesting a laterally uniform anisotropic structure beneath the Dead Sea region. The modeling of the azimuthal variations of the shear wave splitting parameters and split waveforms yields two-layered anisotropic models consisting of an upper layer with nearly N-S symmetry axis and a deeper layer with around N25°E symmetry axis. The split time is almost equally partitioned between the upper and lower layers allotting a value of 0.6–0.8 s to each layer. 2-D finite difference modeling across the southern segment of the Dead Sea Transform fault demonstrates that anisotropic structure in the strike-normal direction is relatively uniform. The Dead Sea Transform fault appears not to have a significant role in the development of the regional anisotropic fabric. The upper anisotropic layer is possibly related to a fossil fabric in the lithosphere, inherited from the Precambrian Pan-African Orogeny. The lower layer may be related to the mantle deformation due to the relative motion between the lithospheric plates and the asthenosphere and possibly affected by the local flow field due to mantle plumes as inferred by other studies.

We invert 3775 relative P wave arrival times using the ACH damped least square method of Aki et al. (1977) to study upper mantle structure beneath the NE Iran continental collision zone. The data for this study were recorded by 17 three component broad-band stations operated from August 2006 to February 2008 along a profile from the center of Iranian Plateau, near Yazd, to the northeastern part of Iran on the Turan Platform just north of the Kopeh Dagh Mountains. The results confirm the previously known low velocity upper mantle beneath Central Iran. Our tomographic model reveals a deep high velocity anomaly. The surficial expressions of this anomaly are between the Ashkabad and Doruneh Faults, where the resolution and ray coverage are good. A transition zone in uppermost mantle is recognized under the Binalud foreland that we interpreted as suture zone between Iran and Turan platform. Our results indicate that Atrak Valley which is the boundary between the Binalud and Kopeh Dagh Mountains can be considered as the northeastern suture of the Iranian Plateau where Eurasia and Turan Platform under-thrust beneath the Binalud range and Central Iran.

Non-linear teleseismic S-phase tomography across the Zagros collision zone in southwestern Iran is used to determine a high-resolution image of the upper-mantle structure. The inversion was done using 41 high-quality earthquakes recorded by 19 broad-band and medium-band stations along a 620 km long profile across the collision zone. Smearing from strong crustal velocity anomalies into the upper-mantle is suppressed by travel-time corrections calculated based on a 3-D crustal model for the study area. Our results show that the relatively old and cold Arabian shield has a higher velocity (up to 6% faster, at depths between 70 and 300 km) than the younger lithosphere farther north in Central Iran. These two upper-mantle domains are separated by a sharp near-vertical transition whose surface expression coinciding with the Main Zagros Thrust.

We present a synthesis and a comparison of the results of two temporary passive seismic experiments installed for a few months across Central and Northern Zagros. The receiver function analysis of teleseismic earthquake records gives a high-resolution image of the Moho beneath the seismic transects. On both cross-sections, the crust has an average thickness of 43±2 km beneath the Zagros fold-and-thrust belt and the Central domain. The crust is thicker beneath the hanging wall of the Main Zagros Reverse Fault (MZRF), with a larger maximum Moho depth in Central (69±2 km) than in Northern Zagros (56±2 km). The thickening affects a narrower region (170 km) beneath the Sanandaj-Sirjan zone of Central Zagros and a wider region (320 km) in Northern Zagros. We propose that this thickening is related to overthrusting of the crust of the Arabian margin by the crust of Central Iran along the MZRF considered as a major thrust fault cross-cutting the whole crust. The fault is imaged as a lowvelocity layer in the receiver function data of the Northern Zagros profile. Moreover, the crustal-scale thrust model reconciles the imaged seismic Moho with the Bouguer anomaly data measured on the Central Zagros transect. At upper mantle depth, P-wave tomography confirms the previously observed strong contrast between the faster velocities of the Arabian margin and the lower velocities of the Iranian micro-blocks. Our higher-resolution tomography combined with surface-wave analysis locates the suture in the shallow mantle of the Sanandaj-Sirjan zone beneath Central Zagros. The Arabian upper-mantle has shield-like shear-wave velocities, while the lower velocities of the Iranian upper mantle are likely due to higher temperature. But these velocities are not low enough and the low-velocity layer not thick enough to conclude on a delamination of the lithospheric mantle lid beneath Iran. The lack of a high-velocity anomaly in the mantle beneath Central Iran suggests that the Neotethyan oceanic lithosphere is probably detached from the Arabian margin.

International audience; Records of core-refracted phases from permanent and temporary seismological stations are analysed to provide a map of seismic anisotropy covering most tectonic provinces of Iran. Anisotropy is coherent at the scale of a single province, but it differs from one province to another. No significant shear-wave splitting is observed in the mountain ranges of Zagros, Alborz, and Kopeh-Dagh. Anisotropy with average time lags of 1.1 ± 0.3 s between fast and slow shear-waves with axes of the faster polarization oriented mostly NW–SE is observed in Central Iran, but the faster orientation is NE–SW in NE Iran. The orientations of fast axes do not coincide with the directions of absolute plate motion in the no-net rotation frame or in the hot-spot frame, suggesting that the observed anisotropy cannot be explained by a simple model of asthenospheric flow. The heterogeneous orientation of fast axes with differences in length scales of a few hundreds km suggests that the anisotropy originates in the lithospheric mantle. In the area where strike-slip shear dominates, the fast axes lie approximately parallel to the orientations of maximum shear estimated from GPS measurements, either with right-lateral shear associated with the NS-trending right-lateral faults in Central Iran, or left-lateral shear associated with EW-trending left-lateral faults in Northeast Iran. In the mountain belts, the null measurements may be related to horizontal shortening and thickening by pure shear, as proposed from synthetic models of olivine fabric which both imply a large amount of vertical strain, not seen by SKS splitting. Our data therefore do not agree with previous explanations of shear-wave splitting in the neighbouring regions of Eastern Anatolia and Saudi Arabia in terms of simple asthenospheric flow. They rather support the view that Iran deforms over a broad area, with vertically-coherent deformation at a lithospheric scale, that includes both shear between blocks in Central Iran and shortening at the mountain belts.

International audience; Surface wave dispersion measurements are interpreted jointly with the inversion of teleseismic P-wave traveltime residuals along a dense 620-km long temporary seismic profile across the Zagros to investigate its upper-mantle structure. The S-wave model determined from Rayleigh wave dispersion in the Zagros fold and thrust belt has high velocities from 4.5 ± 0.2 km s−1 below the Moho to 4.9 ± 0.25 km s−1 at 200 km depth, which are comparable to a shield-like structure. Beneath the suture region from the Main Zagros Thrust (MZT) to the Urumieh-Dokhtar volcanic arc, S-wave velocities are lower than beneath the Zagros in the top 50 km of the upper mantle, with a minimum of 4.4 ± 0.2 km s−1 at 80 km depth. From 150 km and deeper, S velocities are as high as beneath the Zagros. We suggest that part of the velocity difference at shallow depth is due to higher mantle temperatures and/or higher fluid content beneath the northern half of the profile, but that velocities are too high to support the hypothesis of mantle lid delamination under the suture zone. Teleseismic P traveltime relative residuals display a long-wavelength variation along the transect, with a difference of 1.1 s between negative residuals in the Zagros Simple Folded Belt and positive residuals in Central Iran. This difference backprojects into a 6–7 per cent lateral variation of P-wave velocity in the shallow upper mantle, with higher VP beneath Zagros and lower VP beneath Central Iran. The main short wavelength variation of the residual is located in the suture region, with late P arrivals in the region of the MZT and early arrivals in the Sanandaj-Sirjan zone (SSZ). Using synthetic models of VP perturbations, we show that the high velocities of the Arabian platform have to extend laterally at least to the SSZ to fit the observed P delays. This model also predicts Rayleigh wave phase velocities, which are within the error bars of the observed dispersion. It supports the model of crustal-scale overthrusting at the MZT.

The objective of this project is to construct regional wave (Pg, Sn, and Lg) attenuation models for the crust and upper mantle throughout the Iranian plateau and surrounding regions. This research should increase ray path coverage of critical areas and help to create robust regional phase amplitude and attenuation models. We have combined our Iranian data set with our existing Middle Eastern Lg waveform database. These two data sets, combined, give us reasonably good coverage of the northern Middle East. We have created a catalog of seismic events in and around Iran with reliable source spectra as well as tested regional phase attenuation maps for the Zagros and Iranian plateau. We have developed an Sn attenuation model using two station measurements of Q for regional phase Sn (0.1 to 0.5 Hz) and for high frequency Sn (0.5 to 2 Hz). We have also processed Pg waveforms in order to obtain robust estimates of Qp in the crust within the Iranian plateau. For most of the phases we see moderately high attenuation within the plateau and less attenuation along the Zagros mountains. This project is the first to use two station and reverse two station methods to measure attenuation of phases such as Pg and Sn.

Development of high-resolution seismic attenuation models of the Iranian Plateau and the Zagros Mountains is undertaken. Previous studies have suggested a complexity in crustal and uppermost mantle seismic attenuation structure as well as regional phase propagation characteristics beneath much of the Iranian Plateau and the surrounding regions. Regional seismic phases show strong lateral variability in amplitude and frequency content due to this very complex structure. Furthermore, due to a lack of two-dimensional seismic array data, much of the three-dimensional seismic structure and propagation characteristics of the crust, lithosphere and upper mantle remain unknown. Data collected by a series of deployments which consisted of approximately 140 temporary and permanent seismic stations in the Iranian Plateau and the Zagros Mountains is being used with a comprehensive set of techniques to develop and calibrate high-resolution regional phase attenuation models of the crust and upper mantle for a major portion of Arabian-Eurasian plate boundary. Validation of these results will include calibration with selected ground-truth events and comparison with existing amplitude models. A catalog of well located events that have reliable source spectra determined using coda will be created.

[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.

International audience; Crustal receiver functions computed from the records of 45 temporary seismological stations installed on a 620-km long profile across central Zagros provide the first direct evidence for crustal thickening in this mountain belt. Due to a rather short 14-km average station spacing, the migrated section computed from radial receiver functions displays the Moho depth variations across the belt with good spatial resolution. From the coast of the Persian Gulf to 25 km southwest of the Main Zagros Thrust (MZT), the Moho is almost horizontal with slight depth variations around 45 km. Crustal thickness then increases abruptly to a maximum of ~70 km beneath the Sanandaj-Sirjan metamorphic zone, between 50 and 90 km northeast of the surface exposure of the MZT. Further northeast, the Moho depth decreases to ~42 km beneath the Urumieh-Dokhtar magmatic assemblage and the southern part of the Central Iranian micro-continent. The region of thickest crust is located ~75 km to the northeast of the Bouguer anomaly low at –220 mgals. Gravity modelling shows that the measured Moho depth variations can be reconciled with gravity observations by assuming that the crust of Zagros underthrusts the crust of central Iran along the MZT considered as a crustal-scale structure. This hypothesis is compatible with shortening estimates by balanced cross-sections of the Zagros folded belt, as well as with structural and petrological studies of the metamorphic Sanandaj-Sirjan zone.

International audience; We present a synthesis and a comparison of the results of two temporary passive seismic experiments installed for a few months across Central and Northern Zagros. The receiver function analysis of teleseismic earthquake records gives a high-resolution image of the Moho beneath the seismic transects. On both cross-sections, the crust has an average thickness of 43±2 km beneath the Zagros fold-and-thrust belt and the Central domain. The crust is thicker beneath the hanging wall of the Main Zagros Reverse Fault (MZRF), with a larger maximum Moho depth in Central (69±2 km) than in Northern Zagros (56±2 km). The thickening affects a narrower region (170 km) beneath the Sanandaj-Sirjan zone of Central Zagros and a wider region (320 km) in Northern Zagros. We propose that this thickening is related to overthrusting of the crust of the Arabian margin by the crust of Central Iran along the MZRF considered as a major thrust fault cross-cutting the whole crust. The fault is imaged as a lowvelocity layer in the receiver function data of the Northern Zagros profile. Moreover, the crustal-scale thrust model reconciles the imaged seismic Moho with the Bouguer anomaly data measured on the Central Zagros transect. At upper mantle depth, P-wave tomography confirms the previously observed strong contrast between the faster velocities of the Arabian margin and the lower velocities of the Iranian micro-blocks. Our higher-resolution tomography combined with surface-wave analysis locates the suture in the shallow mantle of the Sanandaj-Sirjan zone beneath Central Zagros. The Arabian upper-mantle has shield-like shear-wave velocities, while the lower velocities of the Iranian upper mantle are likely due to higher temperature. But these velocities are not low enough and the low-velocity layer not thick enough to conclude on a delamination of the lithospheric mantle lid beneath Iran. The lack of a high-velocity anomaly in the mantle beneath Central Iran suggests that the Neotethyan oceanic lithosphere is probably detached from the Arabian margin.