Fast Axis Research Articles

Independent Amplitude and Phase Control of Two Orthogonal Linearly Polarised Light and Its Applications

A technique to provide independent control on the amplitude and phase of two orthogonal linearly polarised light is presented. It is based on operating a commercial dual-polarisation dual-drive Mach Zehnder modulator (DPol-DDMZM) in reverse direction that routes the input light in slow and fast axis into two different DDMZMs. The technique has the advantage of controlling the phase of the slow-axis light and the amplitude of the fast-axis light has no effect on the light travelling in the orthogonal polarisation state. It has applications in linearised microwave photonic links, frequency multipliers and microwave phase shifters. A new microwave photonic Hilbert transformer based on using the reverse operating DPol-DDMZM to alter the phase of an RF modulation sideband without affecting the orthogonally polarised optical carrier is developed. Experimental results demonstrate 24.3 dB attenuation in the fast-axis light with no change in the slow-axis light, and 0°–360° RF phase shift for only 3.3 V change in DC voltage. The new Hilbert transformer with less than 2.5° phase imbalance and 0.4 dB amplitude ripples over 4–18 GHz frequency range is also demonstrated.

Imaging structure and geometry of slabs in the greater Alpine area – a P-wave travel-time tomography using AlpArray Seismic Network data

Abstract. We perform a teleseismic P-wave travel-time tomography to examine the geometry and structure of subducted lithosphere in the upper mantle beneath the Alpine orogen. The tomography is based on waveforms recorded at over 600 temporary and permanent broadband stations of the dense AlpArray Seismic Network deployed by 24 different European institutions in the greater Alpine region, reaching from the Massif Central to the Pannonian Basin and from the Po Plain to the river Main. Teleseismic travel times and travel-time residuals of direct teleseismic P waves from 331 teleseismic events of magnitude 5.5 and higher recorded between 2015 and 2019 by the AlpArray Seismic Network are extracted from the recorded waveforms using a combination of automatic picking, beamforming and cross-correlation. The resulting database contains over 162 000 highly accurate absolute P-wave travel times and travel-time residuals. For tomographic inversion, we define a model domain encompassing the entire Alpine region down to a depth of 600 km. Predictions of travel times are computed in a hybrid way applying a fast TauP method outside the model domain and continuing the wave fronts into the model domain using a fast marching method. We iteratively invert demeaned travel-time residuals for P-wave velocities in the model domain using a regular discretization with an average lateral spacing of about 25 km and a vertical spacing of 15 km. The inversion is regularized towards an initial model constructed from a 3D a priori model of the crust and uppermost mantle and a 1D standard earth model beneath. The resulting model provides a detailed image of slab configuration beneath the Alpine and Apenninic orogens. Major features are a partly overturned Adriatic slab beneath the Apennines reaching down to 400 km depth still attached in its northern part to the crust but exhibiting detachment towards the southeast. A fast anomaly beneath the western Alps indicates a short western Alpine slab whose easternmost end is located at about 100 km depth beneath the Penninic front. Further to the east and following the arcuate shape of the western Periadriatic Fault System, a deep-reaching coherent fast anomaly with complex internal structure generally dipping to the SE down to about 400 km suggests a slab of European origin limited to the east by the Giudicarie fault in the upper 200 km but extending beyond this fault at greater depths. In its eastern part it is detached from overlying lithosphere. Further to the east, well-separated in the upper 200 km from the slab beneath the central Alps but merging with it below, another deep-reaching, nearly vertically dipping high-velocity anomaly suggests the existence of a slab beneath the eastern Alps of presumably the same origin which is completely detached from the orogenic root. Our image of this slab does not require a polarity switch because of its nearly vertical dip and full detachment from the overlying lithosphere. Fast anomalies beneath the Dinarides are weak and concentrated to the northernmost part and shallow depths. Low-velocity regions surrounding the fast anomalies beneath the Alps to the west and northwest follow the same dipping trend as the overlying fast ones, indicating a kinematically coherent thick subducting lithosphere in this region. Alternatively, these regions may signify the presence of seismic anisotropy with a horizontal fast axis parallel to the Alpine belt due to asthenospheric flow around the Alpine slabs. In contrast, low-velocity anomalies to the east suggest asthenospheric upwelling presumably driven by retreat of the Carpathian slab and extrusion of eastern Alpine lithosphere towards the east while low velocities to the south are presumably evidence of asthenospheric upwelling and mantle hydration due to their position above the European slab.

Microscopic-characterization of photo-induced birefringence of azo-polymer thin film by focused surface plasmon

In this paper, we develop a technique to quantitatively evaluate the orientational dependence of local anisotropic properties (of azo-polymers fabricated in the form of thin films ∼ 100 nm) by means of focused surface plasmon. The magnitude of photo-induced birefringence and orientation of fast axis at microscopic sites in thin films are extracted by the developed method on evaluating the propagation constants of surface plasmon obtained from the elliptical absorption pattern in the reflected spatial frequency distribution. Low birefringence (∼0.01) has been successfully measured with ultra-low probe volume.

Upper mantle seismic anisotropy beneath the Deccan Volcanic Province and the adjacent Eastern Dharwar Craton in south Indian shield from shear wave splitting analysis

We investigate the upper mantle seismic anisotropy beneath the Deccan Volcanic Province (DVP) and the adjacent Eastern Dharwar Craton (EDC) in the south Indian shield to provide evidence of their differentiated evolution. For this objective, we use the shear wave splitting analysis of the teleseismic SKS, SKKS, PKS and PKKS phases data recorded across a transect, with a linear array of fifteen broadband seismograph stations that spans these two predominant tectonic domains. The resulting splitting parameters (i.e. fast polarization orientation, Φ, and the delay time, δt) show an interesting pattern that is not easily explained by a simple source of anisotropy for the entire transect study region, such as ‘fossil’ (or frozen) anisotropy in the lithosphere or asthenospheric (or geologically recent) anisotropy. However, we observe distinct splitting patterns highlighting deeper geological structures, with which we are able to segregate the transect study region into two zones. In the first zone consisting of DVP, Φ is nearly NE-SW for most stations, along with an average (at each station) δt varying between ~0.5 s and ~1.2 s. In the second zone consisting of predominantly EDC and partly EGMB (Eastern Ghat Mobile Belt), our results show Φ predominantly in the NW-SE orientations for EDC and NE-SW orientations for EGMB, with δt varying between 1.0 and 1.4 s. Based on the splitting and predominant non-APM (Absolute Plate Motion) trending in the EDC, we infer that the observed splitting is related to ‘frozen’ anisotropy in a thick lithosphere, which is preserved between late Archean to Proterozoic, or the last major episode of tectonic and magmatic activity affecting the region ~2.6 Ga ca. In the DVP, the deformation seems to represent a predominant APM-trending asthenospheric anisotropy beneath a thinned lithosphere. This study suggests that the upper mantle fabric is possibly influenced by shear interactions from the geologically recent ~65 Ma Deccan plume event and the fast-moving Indian plate. At the edge of the EDC, encompassing the EGMB, our results show APM-trending anisotropy. This signature is possible from the ancient rifted margins and other lithospheric-scale heterogeneities. Our study uses a single-layer anisotropy model with a horizontal fast axis to approximate the first-order anisotropy beneath each station. Although multiple layers of anisotropy are not ruled out, it needs to be resolved with further studies having richer back-azimuthal coverage.

Dynamic Quarter‐Wave Metasurface for Efficient Helicity Inversion of Polarization Beyond the Single‐Layer Conversion Limit

AbstractTerahertz chiral sensing and polarization‐multiplexing communication demand subwavelength devices that dynamically invert polarization helicity. Metasurfaces can enhance anisotropy and fine tunability at subwavelength scales for this purpose. Although metasurfaces enabling deep modulation between orthogonal polarizations have been designed, they suffer from low conversion efficiencies. In this study, it is shown that the efficiency of conversion from linear to circular polarization by conventional single‐layer transmissive metasurfaces cannot exceed a fundamental limit. A dynamic reflective metasurface free from this limitation is then proposed. The device includes multilayer structures working as a terahertz quarter‐wave plate with switchable slow and fast axes. A phase transition of vanadium dioxide induces the necessary structural transformation of the metallic patterns. A practical fabrication method, based on the room‐temperature bonding technique of sapphires, is presented. Dynamic helicity inversion is demonstrated at 0.90 THz, with a conversion efficiency of over 80% that is beyond the fundamental limit of single‐layer transmissive metasurfaces (65%) and more than four times greater than that of previously reported devices.

Comparative Study of Modified Mueller Matrix Transformation and Polar Decomposition Parameters for Transmission and Backscattering Tissue Polarimetries

Mueller matrix polarimetry is widely used in biomedical studies and applications, for it can provide abundant microstructural information about tissues. Recently, several methods have been proposed to decompose the Mueller matrix into groups of parameters related to specific optical properties which can be used to reveal the microstructural information of tissue samples more clearly and quantitatively. In this study, we thoroughly compare the differences among the parameters derived from the Mueller matrix polar decomposition (MMPD) and Mueller matrix transformation (MMT), which are two popular methods in tissue polarimetry studies and applications, while applying them on different tissue samples for both backscattering and transmission imaging. Based on the Mueller matrix data obtained using the setups, we carry out a comparative analysis of the parameters derived from both methods representing the same polarization properties, namely depolarization, linear retardance, fast axis orientation and diattenuation. IN particular, we propose several modified MMT parameters, whose abilities are also analyzed for revealing the information about the specific type of tissue samples. The results presented in this study evaluate the applicability of the original and modified MMT parameters, then give the suggestions for appropriate parameter selection in tissue polarimetry, which can be helpful for future biomedical and clinical applications.

Tectonic Inheritance During Plate Boundary Evolution in Southern California Constrained From Seismic Anisotropy

AbstractThe style of convective force transmission to plates and strain‐localization within and underneath plate boundaries remain debated. To address some of the related issues, we analyze a range of deformation indicators in southern California from the surface to the asthenosphere. Present‐day surface strain rates can be inferred from geodesy. At seismogenic crustal depths, stress can be inferred from focal mechanisms and splitting of shear waves from local earthquakes via crack‐dependent seismic velocities. At greater depths, constraints on rock fabrics are obtained from receiver function anisotropy, Pn and P tomography, surface wave tomography, and splitting of SKS and other teleseismic core phases. We construct a synthesis of deformation‐related observations focusing on quantitative comparisons of deformation style. We find consistency with roughly N‐S compression and E‐W extension near the surface and in the asthenospheric mantle. However, all lithospheric anisotropy indicators show deviations from this pattern. Pn fast axes and dipping foliations from receiver functions are fault‐parallel with no localization to fault traces and match post‐Farallon block rotations in the Western Transverse Ranges. Local shear wave splitting orientations deviate from the stress orientations inferred from focal mechanisms in significant portions of the area. We interpret these observations as an indication that lithospheric fabric, developed during Farallon subduction and subsequent extension, has not been completely reset by present‐day transform motion and may influence the current deformation behavior. This provides a new perspective on the timescales of deformation memory and lithosphere‐asthenosphere interactions.

Micromachined two-axis water-immersible scanning mirror using torsional and bending hinges

We report a two-axis water-immersible microscanning mirror using torsional and bending biaxially oriented polyethylene terephthalate hinges. Two different designs based on a four- or single-coil electromagnetic actuator are investigated. A micromachining-based fabrication process is developed to enable high patterning resolution and alignment accuracy and to reduce the amount of manual assembly. With a torsional hinge, the fast axis has a resonance frequency of 300 to 500 Hz in air and 200 to 400 Hz in water. With a bending hinge, the slow axis has a resonance frequency of 60 to 70 Hz in air and 20 to 40 Hz in water. 2D B-scan and 3D volumetric ultrasound microscopy are demonstrated using the hybrid-hinge scanning mirror. The ability of scanning the slow axis at DC or very low frequencies allows a dense raster scanning pattern to be formed for improving both the imaging resolution and field of view.

Analysis of non-idealities in rhomb compensators.

In-line rotatable rhombs that are only weakly chromatic are desired as compensators for a wide variety of applications in spectroscopic polarimetry and Mueller matrix spectroscopic ellipsometry. These devices employ multiple total internal reflections to generate differences in the phase shifts upon reflection for orthogonal fast and slow axis optical electric field components. A framework has been developed for characterization of non-idealities in the performance of rhombs due to dissipation and associated dichroism upon each reflection as well as stress-induced birefringence along each beam path. External oblique reflection measurements by spectroscopic ellipsometry for the internally reflecting interface structures has enabled characterization of the dichroic effects and retardance generated by the reflections. The framework for analysis of the effects of stress relies on simulations demonstrating that the contributions to polarization modification from each beam path depend only on the accumulated stress-induced retardance and average azimuthal angle of the fast principal stress axis along the given path. The overall approach has been applied to straight-through Mueller matrix measurements of a three-reflection rhomb in its operational configuration to establish the set of stress parameters for each of the four beam paths needed to fit the measurements. Thus, device geometry and optical structure, including layer thicknesses and component media optical properties, as well as stress-induced retardances and average stress azimuthal angles, which are all deduced in the analysis, enable a complete description of the polarization modifying properties of the rhomb when serving as a compensator.

Dichroic Scanning Nonlinear Optical Microscope With Photodetecting Polymer Scanner and Objective Lens With External Back Focal Plane

In this article authors present a prototype of a dichroic scanning nonlinear optical microscope with a photodetecting polymer Scanner and a customized objective lens with an external back focal plane. Two types of two-axis electromagnetic actuated polymer scanners are manufactured by a standard printed circuit board (PCB) fabrication process. Both scanners share the same overall dimensions of 45 mm by 45 mm. The maximum optical scan angle of a type I scanner is 13 degrees at 420 Hz by the fast axis in resonance and 4 degrees by the slow axis in the linear mode. The type II scanner shows similar performance. A scan lens as well as a tube lens in a conventional nonlinear optical microscope are replaced by placing such a polymer scanner on the external back focal plane of a customized water immersion objective lens. The objective lens has the back working distance of 6 mm, the numerical aperture of 0.7, the front working distance of 1.16 mm, a field of view of 1.67 mm in the diameter and the angle of view of +/−15 degrees. Preliminary imaging results are also presented.

On the Implementation and Usability of Crystal Preferred Orientation Evolution in Geodynamic Modeling

AbstractFlow in the Earth's mantle causes the preferred orientation of crystals called lattice/crystal preferred orientation (LPO or CPO). This preferred orientation is one of the main reasons why seismic anisotropy is observed. Seismic anisotropy observations could therefore be used to constrain the mantle flow in geodynamic models through tracking CPO evolution, and computing the resulting elastic tensor and the anisotropy predicted at the surface. Even though there are many types of CPO models, only a few studies include CPO calculations due to the complexity and computational cost. Here, we implemented an extended version of the CPO model D‐Rex into the open‐source community geodynamics code ASPECT. We show that the implementation is correct, how to use it, and that it is feasible and important to use in large 3D models. We also show that it is important to calculate CPO, especially for models focusing on plate boundary of smaller scale flow because the resulting fast axis directions can greatly deviate from the flow direction. The added infrastructure will also allow for future enhancement, testing, and even replacement of the CPO model.

Spectrally combined three-diode-pumped compact femtosecond Ti:sapphire laser exceeding 1 W mode-locked power.

The achievement of a high average power exceeding 1 W remains a major challenge for direct-diode pumped and mode-locked femtosecond Ti:sapphire lasers. Herein, we demonstrate high-power soliton-like pulses from a direct spectrally combined three-diode-pumped and semiconductor saturable absorber mirror (SESAM)-based mode-locked Ti:sapphire laser. Its mode-locked output power of up to 1 W was obtained in correspondence with a 68.8 MHz repetition rate and 55 fs pulse duration; thus, the pulse energy and peak power are 14.5 nJ and 264 kW, respectively. To the best of our knowledge, this is the highest reported output power and pulse energy from a Ti:sapphire laser with three spectrally combined pump diodes (471 nm, 491 nm, and 525 nm) and a simple beam expander. For efficient pumping, the combined pump beam, directed into the lens (f = 60 mm), which comprised three aspheric lenses along the fast axis and a shared cylindrical beam telescope (8× magnification) along the slow axis, resulting in a circular-focused beam in the Ti:sapphire crystal. The beam waist was measured to be 39 μm ×38 μm along the slow and fast axes.

Spectral beam combining of discrete quantum cascade lasers

In this paper, we report a spectral beam combining technique based on discrete quantum cascade lasers at λ ~ 4.8 μm. Good beam qualities of M2 < 1.3 for both fast and slow axes are obtained. The entire spectrum span is approximately 29.1 cm−1, which is consistent with the theoretical results of grating equation. Maximum beam combining efficiency of 58.9% with output power exceeding 1 W is demonstrated under continuous wave operation at room temperature. The limit of beam combining efficiency is theoretically investigated. The independent temperature control for the discrete lasers circumvented the issue of thermal crosstalk between the lasers on an array and pave the way to high power and high efficiency laser spectral beam combining.

A new method for membrane manufacturing from polyamide with semiconductor diode laser

In this study, a semiconductor diode laser assisted Laser Drilling Process (LDP) for the polyamide (PA, nylon 66) membranes is presented. Along with an experimental work, based on a theoretical investigation a mathematical modelling was constructed as a computer program to simulate the whole experimental procedure. The simulation includes two parts; the material laser interaction for the drilling process and the coordinate of the interaction region on the material with respect to the targeted burst point and the interaction timing.In the theoretical side of the laser-material interaction (in our case PA), the main parameters such as thermal analysis, porous surface analysis, melting front yield widening, and Heat-Affected-Zone (HAZ) radius were investigated in terms of laser pulse length and laser beam energy density (i.e. focused laser beam diameter, spot size). The porous surface analysis was performed and pore sizes on the polyamide surface were observed that they changed from 0.5 μm to 5 μm. The melting front yield widening was calculated as 88.0%. The theoretical and experimental data harmony percentages of widening under 0.005 s and 0.01 s pulse durations for the fast axis beam waist (FABW) and the slow axis beam waist (SABW) were calculated as 91%–86% and 98%–99%, respectively. The HAZ radius was evaluated under 0.0005 s, 0.005 s, and 0.01 s pulse durations and the theoretical and experimental data harmony percentages was calculated as 99.6%, 99.1%, and 99.6%, respectively.Due to the elliptical shape of the diode laser beam the simulation based on the mathematics was constructed regarding to the FABW and the SABW radii. The theoretical and experimental data harmony percentages related with the motorized system mathematics of the LDP under 0.0005 s, 0.005 s, and 0.01 s pulse durations were determined as 97.0%, 99.4% and 95.0%, respectively. The theoretical and experimental data harmony percentages of thermal analysis under the pulse durations were obtained as 88.0%, 91.0% and 93.0%, respectively.

Resolving Seismic Anisotropy of the Lithosphere–Asthenosphere in the Central/Eastern Alps Beneath the SWATH-D Network

The Alpine orogeny is characterized by tectonic sequences of subduction and collision accompanied by break-off events and possibly preceded by a flip of subduction polarity. The tectonic evolution of the transition to the Eastern Alps has thus been under debate. The dense SWATH-D seismic network as a complementary experiment to the AlpArray seismic network provides unprecedented lateral resolution to address this ongoing discussion. We analyze the shear-wave splitting of this data set including stations of the AlpArray backbone in the region to obtain new insights into the deformation at depth from seismic anisotropy. Previous studies indicate two-layer anisotropy in the Eastern Alps. This is supported by the azimuthal pattern of the measured fast axis direction across all analyzed stations. However, the temporary character of the deployment requires a joint analysis of multiple stations to increase the number of events adding complementary information of the anisotropic properties of the mantle. We, therefore, perform a cluster analysis based on a correlation of energy tensors between all stations. The energy tensors are assembled from the remaining transverse energy after the trial correction of the splitting effect from two consecutive anisotropic layers. This leads to two main groups of different two-layer properties, separated approximately at 13°E. We identify a layer with a constant fast axis direction (measured clockwise with respect to north) of about 60° over the whole area, with a possible dip from west to east. The lower layer in the west shows N–S fast direction and the upper layer in the east shows a fast axis of about 115°. We propose two likely scenarios, both accompanied by a slab break-off in the eastern part. The continuous layer can either be interpreted as frozen-in anisotropy with a lithospheric origin or as an asthenospheric flow evading the retreat of the European slab that would precede the break-off event. In both scenarios, the upper layer in the east is a result of a flow through the gap formed in the slab break-off. The N–S direction can be interpreted as an asthenospheric flow driven by the retreating European slab but might also result from a deep-reaching fault-related anisotropy.

Pulsed Diode Laser Minibars for Pumping Space-Borne Solid-State Lasers

Laser diode benches (LDBs) with lasing wavelength of 808 nm were developed, manufactured and endurance-tested for utilization as pump lasers for the Nd:YAG oscillator in the climate satellite MERLIN. The LDBs combine customized TE-polarized GaAs-based 5-mm minibars with fast axis collimation lenses to produce a reliable, space qualified component at optical power of 63 W per minibar. Accelerated life tests of 20 LDBs are presented. Except for a small increase in threshold current, no wear out in operating current or sudden failures were observed, and reliable operation over the full mission time of >4 gigashots was confirmed. The electro-optical characteristics remain in the specified ranges after exposure to the total operational load of the mission.

Mantle deformation beneath the Western Ghats, India: Insights from core-refracted shear wave splitting analysis

The Western margin of the Indian sub-continent that hosts one of the world’s largest escarpments, called the Western Ghats (WG), is a collage of diverse geological formations with different structural and physical properties. Characterizing upper mantle seismic anisotropy offers insights into the mantle deformation and its role in the evolution of such regions. In this study, we measured the upper mantle anisotropy using shear wave splitting analysis of SK(K)S and PK(K)S waveforms recorded at 17 broadband seismological stations located along and in the vicinity of the WG. Results indicate that the fast axis polarization directions are primarily in the NE-SW direction, with delay times varying from 0.3 s to 1.8 s. This direction is parallel to the Absolute Plate Motion of the Indian sub-continent, suggesting that shear at the base of the lithosphere is the dominant mechanism for anisotropy along the WG. E-W oriented anisotropy at stations close to the west coast, especially in the northern part of the WG can be associated with lithospheric stretching along the west coast, associated with the rifting process. Further, the coast-parallel fast axis polarization directions (N-S, NNW-SSE), with delay times varying from 0.6 s to 1.2 s, at stations away from the coast, could be due to the edge flow associated with transition from a thinner to a thicker lithosphere. In addition, we model the variation in splitting parameters at station PCH in the Southern Granulite Terran in terms of two-layer anisotropy, with an E-W orientation in the upper layer and a NE-SW one in the lower layer. The variable fast axis polarization directions from coast perpendicular to coast parallel at stations close and away from the west coast of the Indian sub-continent imply different rifting episodes.

High-speed wide-angle interleaved scanning technique for a 3D imaging lidar

In this paper, we describe a novel interleaved scanner for an eye-safe 3D scanning lidar system to measure aerodynamic phenomena in a wind tunnel using elastic backscatter from seeding particles. The scanner assembly consists of a rotating polygon scanner for line scanning along the fast axis, a galvanometer (galvo) scanner for scanning along the slow axis, angular position sensors, and motor controllers. The polygon scanner sweeps the lidar beam at up to 10,000 lines/s across a 27 deg angular field of regard in the fast axis, while the galvo scanner covers an angular range of 20 deg in the slow axis. Using this scanner, the lidar can perform nonintrusive flow visualization, velocimetry, and hard target mapping at millimeter-scale spatial resolution to a standoff range of 5 m at an update rate of 50 Hz for the full field of regard. An interleaved scanning methodology for the acquisition of two snapshots of 3D lidar intensity data with a 100 µs time offset is discussed. The design of the control and data acquisition electronics is described. The effects of rapid scanning on the response of a narrow field-of-view lidar are addressed. The design of the scanner is scalable and can be tailored to meet the requirements of other applications by increasing or decreasing the standoff range, angular field of regard, and scan rate.

Crustal structure and upper mantle anisotropy of the Afar triple junction

The Afar region is a tectonically distinct area useful for studying continental break-up and rifting. Various conflicting models have been suggested to explain the lateral variations of the anisotropy in this region. To address this issue, we investigated the tectonics of the Afar region using receiver function and shear-wave splitting measurements based on broadband seismic data from 227 stations in the region. Further, the receiver function results were inverted to obtain the crustal thickness and Vp/Vs ratio of the region. Our results reveal a thick African crust (thicker than 40 km) with typical Vp/Vs values for the continental crust, elongated down to 21 km along the rift system with very high Vp/Vs values near the fractured zones, suggesting crustal thinning near the fractured zones. Our shear-wave splitting measurements indicate a general fast axis orientation of N030E. However, substantial disparities in the fast anisotropy direction exist in the triple junction region, with some stations displaying a direction of N120E, which is perpendicular to the fast directions measured at the surrounding stations. In addition, many stations located close to the rifts and within the Arabian Plate provide mostly null measurements, indicating the presence of fluids or isotropic media. This study uses several methodologies to unravel the structure and evolution of the Afar region, providing valuable insight into the Afar, a tectonically distinct region, which will be useful for elucidating the mechanisms and characteristics of a continental break-up and the rifting process.

Velocity and azimuthal anisotropy structure underneath the Reelfoot Rift region from Rayleigh wave phase velocity dispersion curves

SUMMARYPhase velocity and azimuthal anisotropy maps for fundamental mode Rayleigh waves are determined for a portion of the central United States including the seismically active Reelfoot Rift (RFR) and the enigmatic Illinois Basin. Dense seismic array installations of the Northern Embayment Lithosphere Experiment, the EarthScope transportable array and the Ozarks Illinois Indiana Kentucky array allow a detailed investigation of phase velocity and anisotropy in a broad period range (20–100s).We obtain more than 12 000 well-constrained, unique two-station paths from teleseismic events. The two-station method is used to determine dispersion curves and these are inverted for isotropic phase velocity maps and azimuthal anisotropy maps for each period. The presence of fast phase velocities at lower crustal and uppermost mantle depths is found below the RFR, and Ste. Genevieve and Wabash Valley fault zones. At periods of 30s and higher, the RFR is underlain by slow phase velocities and is flanked to the NW and SE by regions of fast velocity. Fast phase velocities are present below the centre of the Illinois Basin in the period range 75–100s. Anisotropy fast axis orientations display complex patterns for each period and do not trend parallel to the direction of absolute plate motion. Anisotropy fast directions are consistently parallel to the trend of the RFR from 50s to higher periods, suggesting the presence of either frozen-in anisotropic fabric or fabric related to material transport from a recently discovered, pronounced low velocity zone below the Mississippi Embayment.