Slow Wave Speeds Research Articles

Gondwanan flood basalts linked seismically to plume-induced lithosphere instability

Delamination of the continental lithospheric mantle is well recorded beneath several continents. However, the fate of the removed continental lithosphere has been rarely noted, unlike subducted slabs reasonably well imaged in the upper and mid mantle. Beneath former Gondwana, recent seismic tomographic models indicate the presence of at least 5 horizontal fast-wavespeed anomalies at ~600 km depths that do not appear to be related to slab subduction, including fast structures in locations consistent with delamination associated with the Paraná Flood Basalt event at ~134 Ma and the Deccan Traps event at ~66 Ma. These fast-wavespeed anomalies often lie above broad slow seismic wavespeed trunks at 500 to 700 km depths beneath former Gondwana, with slow wavespeed anomalies branching around them. Numerical experiments indicate that delaminated lithosphere tends to stagnate in the transition zone and mid-mantle above a mantle plume where it shapes subsequent plume upwelling. For hot plumes, the melt volume generated during plume-influenced delamination can easily reach ~2 to 4 × 106 km3, consistent with the basalt eruption volume at the Deccan Traps. This seismic and numerical evidence suggests that observed high-wavespeed mid-mantle anomalies beneath the locations of former flood basalts are delaminated fragments of former continental lithosphere, and that lithospheric delamination events in the presence of subcontinental plumes induced several of the continental flood basalts associated with the multiple breakup stages of Gondwanaland. Continued upwelling in these plumes can also have entrained subcontinental lithosphere in the mid-mantle to bring its distinctive geochemical signal to the modern mid-ocean spreading centers that surround southern and western Africa.

Poroelastic model of the lungs at low frequencies predicted by Biot's theory

Biot's theory of poroelastic wave propagation inherently lends itself to elucidate the characteristics of a biphasic medium comprising solid and fluid components, such as biological tissues. One of the intricately complex biological tissues that remains poorly understood is the lungs since their properties diversify significantly through their pore geometries affected by inspiratory positive airway pressure (IPAP) and applied frequency range. One hypothesizes that the vibroacoustic behaviour of the lungs can be predicted by Biot's theory, as the nature of the lungs aligns with the principles of the theory at low frequencies. This study aims to analytically investigate the vibroacoustic behaviour of the lungs, considering 10 and 20-cm H;2O IPAP. Utilizing a fractional derivative formulation, one predicts the complex-valued shear wave speed, as well as the fast and slow compression wave speeds, for frequencies ranging from 5 to 100 Hz. A 3D digital thorax twin study using these predicted wave speeds, particularly at 28 Hz and 20 cm H;2O IPAP, is validated against experimental data from the literature. Consequently, applying Biot's theory provides a valuable framework for understanding the dynamic vibroacoustic behaviour of the lung tissues in response to varying IPAP and low frequencies.

Mantle Wavespeed and Discontinuity Structure Below East Africa: Implications for Cenozoic Hotspot Tectonism and the Development of the Turkana Depression

AbstractEthiopia's Cenozoic flood basalt magmatism, uplift, and rifting have been attributed to one or more mantle plumes. The Nubian plate, however, has drifted 500–1,000 km north since initial magmatism at ∼45 Ma, having developed above mantle that now underlies the northern Tanzania craton and the low‐lying Turkana Depression. Unfortunately, our knowledge of mantle wavespeed structure and mantle transition zone (MTZ) topography below these regions is poorest, due to a historical lack of seismograph stations. The same data gap means we lack constraints on lithospheric structure in and around the NW–SE trending Mesozoic Anza rift. We exploit data from new seismograph networks in the Turkana Depression and neighboring northern Uganda to develop AFRP22, a new African absolute P‐wavespeed tomographic model that resolves whole mantle structure along the entire East African rift system. We also map MTZ thickness using Ps receiver functions. East Africa's thinnest MTZ (∼25 km thinning) underlies the northwest Turkana Depression. AFRP22 reveals a co‐located, previously unrecognized, slow wavespeed plume tail, extending from the MTZ, deep into the lower mantle. This plume may thus have contributed, along with the African Superplume, to the development of the 45–30 Ma flood basalt province that preceded extension. Pervasive sub‐lithospheric slow wavespeeds imply that Turkana's present‐day low elevation is explained best by Mesozoic and Cenozoic‐age crustal thinning. At ∼100 km depth, AFRP22 illuminates a fast wavespeed SE Ethiopian plateau. In addition to governing the northernmost limit of Mesozoic Anza rifting, the refractory nature of this lithospheric block likely minimized Cenozoic flood basalt magmatism there.

A lightweight waterborne acoustic meta-absorber with low characteristic impedance rods

We investigate a meta-absorber composed of impedance-matched rods for the broadband acoustic absorption. However, with a slow longitudinal wave speed, the absorber’s density is high to keep the characteristic impedance matching with that of water. To obtain a lightweight high efficiency absorber, we analyze the influence of characteristic impedance and loss factor on absorption behaviors and find that the characteristic impedance matching is not necessary and the low characteristic impedance structure can also attain the efficient absorption performance, and the high dissipation can reduce the density required for an efficient absorption. Based on this, we propose a meta-absorber composed of low characteristic impedance rubber rods in parallel coated by a rubber cover layer and backed with a stepped resin base. By the combination of the transfer matrix method and transmission line impedance transfer equation, the acoustic performance of meta-absorber is analyzed. Compared with the characteristic impedance matching rods, the low characteristic impedance rods can release restrictions on design to attain better acoustic behaviors with a lower density. Further, the integration of rods with designed lengths can motivate coherently coupled weak resonances to obtain a broadband absorption. Finally, an optimized meta-absorber is tested in a water-filled impedance tube and the experimental results agree well with the theoretical results which can achieve a broadband low frequency quasi-perfect absorption (α>0.9) in a deep subwavelength region (λ/158 at 950 Hz) with the density close to that of water. Our work finds a new mechanism that low characteristic impedance absorbing materials can achieve the high efficiency absorption with a slow longitudinal wave speed and low density and promise broad applications in the sound stealth of marine vehicles.

A New P‐Wave Tomographic Model (CAP22) for North America: Implications for the Subduction and Cratonic Metasomatic Modification History of Western Canada and Alaska

AbstractOur understanding of the present‐day state and evolution of the Canadian and Alaskan mantle is hindered by a lack of absolute P‐wavespeed constraints that provide complementary sensitivity to composition in conjunction with existing S‐wavespeed models. Consequently, cratonic modification, orogenic history of western North America and complexities within the Alaskan Proto‐Pacific subduction system remain enigmatic. One challenge concerns the difficulties in extracting absolute arrival‐time measurements from often‐noisy data recorded by temporary seismograph networks required to fill gaps in continental and global databases. Using the Absolute Arrival‐time Recovery Method (AARM), we extract >180,000 new absolute arrival‐time residuals from seismograph stations across Canada and Alaska and combine these data with USArray and global arrival‐time data from the contiguous US and Alaska. We develop a new absolute P‐wavespeed tomographic model, CAP22, spanning North America that significantly improves resolution in Canada and Alaska over previous models. Slow wavespeeds below the Canadian Cordillera sharply abut fast wavespeeds of the continental interior at the Rocky Mountain Trench in southwest Canada. Slow wavespeeds below the Mackenzie Mountains continue farther inland in northwest Canada, indicating Proterozoic‐Archean metasomatism of the Slave craton. Inherited tectonic lineaments colocated with this north‐south wavespeed boundary suggest that both the crust and mantle may control Cordilleran orogenic processes. In Alaska, fast upper mantle wavespeeds below the Wrangell Volcanic Field favor a conventional subduction related mechanism for volcanism. Finally, seismic evidence for the subducted Kula and Yukon slabs indicate tectonic reconstructions of western North America may require revision.

Coronal seismology by slow waves in non-adiabatic conditions

Slow magnetoacoustic waves represent an important tool for probing the solar coronal plasma. The majority of seismological methods with slow waves are based on a weakly non-adiabatic approach, which assumes the coronal energy transport has only weak effects on the wave dynamics. Despite it significantly simplifies the application of coronal seismology by slow waves, this assumption omits a number of important and confidently observed effects and thus puts strong limitations on the reliability of seismological estimations. We quantitatively assess the applicability of the weak thermal conduction theory to coronal seismology by slow waves. We numerically model the linear standing slow wave in a 1D coronal loop, with field-aligned thermal conduction κ‖ as a free parameter and no restrictions on its efficiency. The time variations of the perturbed plasma parameters, obtained numerically with full conductivity, are treated as potential observables and analysed with the standard data processing techniques. The slow wave oscillation period is found to increase with κ‖ by about 30%, indicating the corresponding modification in the effective wave speed, which is missing from the weak conduction theory. Phase shifts between plasma temperature and density perturbations are found to be well consistent with the approximate weakly conductive solution for all considered values of κ‖. In contrast, the comparison of the numerically obtained ratio of temperature and density perturbation amplitudes with the weak theory revealed relative errors up to 30–40%. We use these parameters to measure the effective adiabatic index of the coronal plasma directly as the ratio of the effective slow wave speed to the standard sound speed and in the polytropic assumption, which is found to be justified in a weakly conductive regime only, with relative errors up to 14% otherwise. The damping of the initial perturbation is found to be of a non-exponential form during the first cycle of oscillation, which could be considered as an indirect signature of entropy waves in the corona, also not described by weak conduction theory. The performed analysis and obtained results offer a more robust scheme of coronal seismology by slow waves, with reasonable simplifications and without the loss of accuracy.

Control of diffusion-driven pattern formation behind a wave of competency

In certain biological contexts, such as the plumage patterns of birds and stripes on certain species of fishes, pattern formation takes place behind a so-called “wave of competency”. Currently, the effects of a wave of competency on the patterning outcome are not well-understood. In this study, we use Turing’s diffusion-driven instability model to study pattern formation behind a wave of competency, under a range of wave speeds. Numerical simulations show that in one spatial dimension a slower wave speed drives a sequence of peak splittings in the pattern, whereas a higher wave speed leads to peak insertions. In two spatial dimensions, we observe stripes that are either perpendicular or parallel to the moving boundary under slow or fast wave speeds, respectively. We argue that there is a correspondence between the one- and two-dimensional phenomena, and hypothesize that (as others have) pattern formation behind a wave of competency can account for the pattern organization observed in many biological systems.

Quantification of Gastric Slow Wave Velocity Using Bipolar High-Resolution Recordings.

Gastric bio-electrical slow waves are, in part, responsible for coordinating motility. High-resolution (HR) in vivo recordings can be used to capture the wavefront velocity of the propagating slow waves. A standard "marking-and-grouping" approach is typically employed along with manual review. Here, a bipolar velocity estimation (BVE) method was developed, which utilized local directional information to estimate the wavefront velocity in an efficient manner. Unipolar in vivo HR recordings were used to construct bipolar recordings in different directions. Then, the local directionality of the slow wave was extracted by calculating time delay information. The accuracy of the method was verified using synthetic data and then validated with in vivo HR pig experimental recordings. Against ventilator noise amplitude of 0% - 70% of the average slow wave amplitude, the direction and speed error increased from 4.4° and 0.9 mm/s to 8.6° and 1.4 mm/s. For signals added with high-frequency noise with SNR of 60 dB - 12 dB, the error increased from 8.0° and 1.0 mm/s to 9.8° and 1.2 mm/s. With experimental data, the BVE algorithm resulted in 19.2 ±1.7° of direction error and 2.0 ± 0.2 mm/s of speed error, when compared to the standard "marking-and-grouping" method. Gastric slow wave wavefront velocities were estimated rapidly using the BVE algorithm with minimal errors. The BVE algorithmenables the ability to estimate wavefront velocities in HR recordings in an efficient manner.

The collective motion of thermophoretic-type active particle suspension under spatial–temporal modulation

The mesoscopic collective behavior of a thermophoretic-type active particle suspension under a spatial–temporal modulated excitation field is experimentally studied. By using a digital light processing chip, a sinusoidal-like spatial–temporal varying laser pattern (intensity wave) is projected on the particle suspension to modulate the particle propulsion strength through the thermophoretic effect and to exert spatial confinement through the optical trapping effect. For static and slow (wave speed ≪ particle propulsion speed) modulations, in contrast to the passive particles trapped around the wave crest, the self-propulsion strength splits the particle distribution into two branches adjacent to the edges of confinement similar to the bacteria system, and the profile of particle distribution moves coherently with the modulation wave. Once the wave speed increases to the order of the particle propulsion speed, the splitting of particle distribution disappears. Furthermore, it is remarkable that the forward propagating modulation-induced certain fraction of backward streaming particles are observed. This is a generic behavior of a self-propelling active particle suspension, which is not found in the bacteria system. This finding should be useful for particle manipulation at the mesoscopic scale.

AFRP20: New P‐Wavespeed Model for the African Mantle Reveals Two Whole‐Mantle Plumes Below East Africa and Neoproterozoic Modification of the Tanzania Craton

AbstractAfrica's Cenozoic tectonism is often attributed to mantle plumes, particularly below East Africa, but their morphology, number, location, and impact on the African lithosphere are debated. The broad slow wavespeed African Superplume, ubiquitous in large‐scale tomographic models, originates below South Africa, reaching the surface somewhere below East Africa. However, whether the diverse East African mantle geochemistry is best reconciled with one heterogeneous upwelling, or current tomographic models lack the resolution to image multiple distinct plumes, remains enigmatic. S‐wavespeed tomographic images of Africa are legion, but higher frequency P‐wavespeed whole‐mantle models possessing complementary diagnostic capabilities are comparatively lacking. This hinders attempts to disentangle the effects of Cenozoic hotspot tectonism and Pan African (and older) tectonic events on the East African lithosphere. Here we develop a continental‐scale P‐wave tomographic model capable of resolving structure from upper‐to‐lower mantle depths using a recently developed technique to extract absolute arrival‐times from noisy, temporary African seismograph deployments. Shallow‐mantle wavespeeds are δVP ≈ −4% below Ethiopia, but less anomalous (δVP ≥–2%) below other volcanic provinces. The heterogeneous African Superplume reaches the upper mantle below the Kenyan plateau. Below Ethiopia/Afar we image a second sub‐vertical slow wavespeed anomaly rooted near the core‐mantle boundary outside the African LLVP, meaning multiple disparately sourced whole‐mantle plumes may influence East African magmatism. In contrast to other African cratons, wavespeeds below Tanzania are only fast to 90–135 km depth. When interpreted alongside Lower Eocene on‐craton kimberlites, our results support pervasive metasomatic lithospheric modification caused by subduction during the Neoproterozoic Pan‐African orogeny.

Insights Into Deep Mantle Thermochemical Contributions to African Magmatism From Converted Seismic Phases

AbstractThe contribution of mantle upwellings of varying spatial extent to Cenozoic magmatism across Africa is debated because geochemical and seismological tools used to interrogate them are primarily sensitive to either composition or temperature. Thermochemical conditions control the depth at which mantle materials undergo phase changes, which cause seismic discontinuities. Mapping seismic discontinuities across the mantle transition zone (MTZ) and below provides insight into the variable thermochemical nature of upwellings. We present observations of seismic discontinuities beneath Africa obtained from a compilation of P‐to‐s receiver functions (RFs; using Pds, PPds, and PKPds phases), recorded at seismograph networks across Africa between 1990–2019. We exploit a recent high‐resolution African continental P‐wavespeed model to migrate our RFs to depth in a common conversion point stack. Cenozoic magmatism along the East African Rift is largely underlain by a thin MTZ implying a contribution to rift magmatism from sources at or below MTZ depths. The Ethiopian rift is underlain by a depressed d410 and uplifted d660 indicating a moderate positive thermal anomaly at MTZ depths (∼100–150 K). The southern East African Rift displays a greater d410 depression and a regional d660 depression, suggesting a stronger thermochemical anomaly at MTZ depths. Here, seismic conversions at ∼1,025 km depth are collocated with slow wavespeeds within the African Superplume, corroborating evidence for a compositional anomaly. We suggest that the contribution of a purely thermal plume directly below Ethiopia augments conditions for mantle melting and rifting. Distinct upwellings may also affect the MTZ below Cenozoic magmatism in Cameroon and Madagascar.

Seismic Structure of the Antarctic Upper Mantle Imaged with Adjoint Tomography

AbstractThe upper mantle and transition zone beneath Antarctica and the surrounding oceans are among the poorest‐imaged regions of the Earth's interior. Over the last 15 years, several large broadband regional seismic arrays have been deployed, as have new permanent seismic stations. Using data from 297 Antarctic and 26 additional seismic stations south of ~40°S, we image the seismic structure of the upper mantle and transition zone using adjoint tomography. Over the course of 20 iterations, we utilize phase observations from three‐component seismograms containing P, S, Rayleigh, and Love waves, including reflections and overtones, generated by 270 earthquakes that occurred from 2001–2003 and 2007–2016. The new continental‐scale seismic model (ANT‐20) possesses regional‐scale resolution south of 60°S. In East Antarctica, thinner continental lithosphere is found beneath areas of Dronning Maud Land and Enderby‐Kemp Land. A continuous slow wave speed anomaly extends from the Balleny Islands through the western Ross Embayment and delineates areas of Cenozoic extension and volcanism that span both oceanic and continental regions. Slow wave speed anomalies are also imaged beneath Marie Byrd Land and along the Amundsen Sea Coast, extending to the Antarctic Peninsula. These anomalies are confined to the upper 200–250 km of the mantle, except in the vicinity of Marie Byrd Land where they extend into the transition zone and possibly deeper. Finally, slow wave speeds along the Amundsen Sea Coast link to deeper anomalies offshore, suggesting a possible connection with deeper mantle processes.

490 High-Resolution Electrogastrogram Identifies Disturbed Gastric Slow Waves and Dysautonomia in Subjects With Dumping Syndrome

INTRODUCTION: Rapid gastric emptying is the sine qua non of Dumping Syndrome (DS), which can arise from multiple etiologies including gastric surgery and diabetes mellitus. The relationship between DS and gastric myoelectric activity is unknown, but dysautonomia may play a role. We sought to characterize features of gastric slow waves and autonomic indices among subjects with DS using the ambulatory high-resolution electrogastrogram (HR-EGG). METHODS: Eleven subjects completed 24-hour ambulatory HR-EGG. Fasting abdominal computed tomography scan was utilized for stomach localization and placement of the ambulatory HR-EGG array, consisting of 10 skin mounted electrodes. Subjects were divided into two groups: DS (4F, age 53.2 ± 9.3, BMI 31.7 ± 11, 79-93% gastric emptying at 1 hour on scintigraphy, two diabetics, one prior gastric surgery, one gastrointestinal amyloidosis) and healthy age-matched controls (4M/3F, age 47.6 ± 28, BMI 24.5 ± 3.7). All subjects completed the PAGI-SYM foregut symptom questionnaire and a diary capturing all meals and sleep/wake times. RESULTS: Subjects with DS had significantly higher mean PAGI-SYM foregut symptom severity scores (1.9 ± 1.0 vs 0.1 ± 0.1; P = 0.0007). HR-EGG showed that DS subjects had a significantly lower slow wave speed (4.6 vs 5.9 ± 0.4; P = 0.0003) and higher post-prandial power change (11 vs 3.5 ± 3.6; P = 0.002) compared to healthy controls. The global percent stomach activity throughout the 24-hour ambulatory recordings was not different between groups. DS subjects trended towards higher post-prandial area under the curve EGG activity at both the 1-hour (261.9 ± 360.7 vs 172.9 ± 137.5) and 4-hour marks (1108.8 ± 1589.4 vs 656.3 ± 442.2). In addition, subjects with DS had significantly lower heart rate variability than controls: RMSDD (39.9 ± 9.7 vs 58.1 ± 7.8) and SDNN (12.6 ± 4.2 vs 23.7 ± 3.2); no differences were noted in mean heart rate (Table 1). CONCLUSION: Ambulatory HR-EGG clearly identifies that there are spatial abnormalities of the gastric slow wave present among subjects with DS. Moreover, reduced heart rate variability - a marker of autonomic dysfunction - was observed among subjects with DS. These findings suggest that gastric dysrhythmias and autonomic disturbances should be given important consideration in the pathogenesis of Dumping Syndrome.

Acquisition of Logging-While-Drilling (LWD) Multipole Acoustic log Data during the India National Gas Hydrate Program (NGHP) Expedition 02

Abstract Gas hydrates are ice-like crystalline minerals with high energy concentrations, which generally occur in deepwater environments, at low temperature and high pressure, usually close to the seafloor. Gas hydrate research wells are normally drilled riser-less, with multiple wells being drilled in rapid succession. Logging-while-drilling (LWD) is the preferred logging method as the borehole condition in such shallow formations deteriorates very rapidly after drilling. LWD helps minimize the time between drilling and logging. The LWD sonic measurement is key in such projects and is used typically for geomechanics and formation evaluation purposes. The water-bearing sedimentary section within the shallow gas hydrate stability zone is often shale-rich, unconsolidated and the measured acoustic wave slowness is “extremely slow”, with shear slowness above 1400 μs/ft and bulk density of around 1.7 gm/cc. Quadrupole measurements are commonly used in the industry to log shear slowness with LWD tools in slow formations, where the shear slowness is slower than the borehole fluid slowness. However, it is very challenging to log shear slownesses with LWD sonic tool in these extremely slow formations, even with a quadrupole measurement. This paper details the challenges related to borehole acoustics logging in gas hydrate research wells, outlines the key technological enablers and gives an overview of the performance of LWD sonic tools during the National Gas Hydrate Program Expedition 02 (NGHP-02) in the offshore of India. It will spotlight the innovative approach followed to modify the LWD sonic acquisition and processing to adapt to the extremely slow acoustic wave speeds, which facilitated recording LWD shear slowness up to approximately 1400 μs/ft.

Compression waves in porous media containing gas hydrate

Wave processes in a porous medium containing water, gas and gas hydrate are numerically investigated. The two-velocity model of porous medium is used. The nonlinearity of the oscillations of the bubbles is taken into account in the equation of gas state and the equation for bubble radius. The transmission of compression of step waves from the water into hydrate-containing porous medium is investigated. Beyond the line of phase equilibrium the association of gas hydrate is possible. The influence of the medium parameters and the intensity of the incident wave on the evolution of waves in this porous medium is studied. When entering the porous medium, the initial pulse is divided into fast (deformation) and slow (filtration) waves. In a slow wave damped oscillations associated with pulsations of bubbles are observed. The increase in the initial equilibrium pressure in the medium leads to an increase in the velocities of the deformation and filtration waves. With the initial volume content of the gas in the pore space unchanged, an increase in the initial hydrate content leads to a small increase in the velocities and amplitudes of fast and slow waves. At constant initial volume content of hydrate, the increase of gas content leads to a noticeable decrease of slow wave speed and decrease of amplitudes of both fast and slow waves. It was found that the accounting of the processes of formation of gas hydrate in a porous medium at the considered time intervals does not have a noticeable effect on the character of wave propagation.

Across-night dynamics in traveling sleep slow waves throughout childhood.

Sleep slow waves behave like traveling waves and are thus a marker for brain connectivity. Across a night of sleep in adults, wave propagation is scaled down, becoming more local. Yet, it is unknown whether slow wave propagation undergoes similar across-night dynamics in childhood-a period of extensive cortical rewiring. High-density electroencephalography (EEG; 128 channels) was recorded during sleep in three groups of healthy children: 2.0-4.9 years (n = 11), 5.0-8.9 years (n = 9) and 9.0-16.9 years (n = 9). Slow wave propagation speed, distance, and cortical involvement were quantified. To characterize across-night dynamics, the 20% most pronounced (highest amplitude) slow waves were subdivided into five time-based quintiles. We found indications that slow wave propagation distance decreased across a night of sleep. We observed an interesting interaction of across-night slow wave propagation dynamics with age (p < 0.05). When comparing the first and last quintiles, there was a trend level difference between age groups: 2- to 4.9-year-old children showed an 11.9% across-night decrease in slow wave propagation distance, which was not observed in the older two age groups. Regardless of age, cortical involvement decreased by 10.4%-23.7% across a night of sleep. No across-night changes were observed in slow wave speed. Findings provide evidence that signatures of brain connectivity undergo across-night dynamics specific to maturational periods. These results suggest that across-night dynamics in slow wave propagation distance reflect heightened plasticity in underlying cerebral networks specific to developmental periods.

Precambrian Plate Tectonics in Northern Hudson Bay: Evidence From P and S Wave Seismic Tomography and Analysis of Source Side Effects in Relative Arrival‐Time Data Sets

AbstractThe geology of northern Hudson Bay, Canada, documents more than 2 billion years of history including the assembly of Precambrian and Archean terranes during several Paleoproterozoic orogenies, culminating in the Trans‐Hudson Orogen (THO) ∼1.8 Ga. The THO has been hypothesized to be similar in scale and nature to the ongoing Himalaya‐Karakoram‐Tibetan orogen, but the nature of lithospheric terrane boundaries, including potential plate‐scale underthrusting, is poorly understood. To address this problem, we present new P and S wave tomographic models of the mantle seismic structure using data from recent seismograph networks stretching from northern Ontario to Nunavut (60–100∘W and 50–80∘N). The large size of our network requires careful mitigation of the influence of source side structure that contaminates our relative arrival time residuals. Our tomographic models reveal a complicated internal structure in the Archean Churchill plate. However, no seismic wave speed distinction is observed across the Snowbird Tectonic Zone, which bisects the Churchill. The mantle lithosphere in the central region of Hudson Bay is distinct from the THO, indicating potential boundaries of microcontinents and lithospheric blocks between the principal colliders. Slow wave speeds underlie southern Baffin Island, the leading edge of the generally high wave speed Churchill plate. This is interpreted to be Paleoproterozoic material underthrust beneath Baffin Island in a modern‐style subduction zone setting.

High strain-rate soft material characterization via inertial cavitation

Mechanical characterization of soft materials at high strain-rates is challenging due to their high compliance, slow wave speeds, and non-linear viscoelasticity. Yet, knowledge of their material behavior is paramount across a spectrum of biological and engineering applications from minimizing tissue damage in ultrasound and laser surgeries to diagnosing and mitigating impact injuries. To address this significant experimental hurdle and the need to accurately measure the viscoelastic properties of soft materials at high strain-rates (103–108 s−1), we present a minimally invasive, local 3D microrheology technique based on inertial microcavitation. By combining high-speed time-lapse imaging with an appropriate theoretical cavitation framework, we demonstrate that this technique has the capability to accurately determine the general viscoelastic material properties of soft matter as compliant as a few kilopascals. Similar to commercial characterization algorithms, we provide the user with significant flexibility in evaluating several constitutive laws to determine the most appropriate physical model for the material under investigation. Given its straightforward implementation into most current microscopy setups, we anticipate that this technique can be easily adopted by anyone interested in characterizing soft material properties at high loading rates including hydrogels, tissues and various polymeric specimens.

Observations of a Fast-mode Magnetosonic Wave Propagating along a Curving Coronal Loop on 2011 November 11

Abstract The detailed analysis of an interesting quasi-periodic fast-propagating (QFP) magnetosonic wave is presented using high-resolution observations taken by the Solar Dynamic Observatory. The QFP wave occurred over the west solar limb during the fast eruption phase of a nearby prominence. It propagated along a group of curving coronal loop and manifested two types of wave trains that showed different morphologies and propagation characteristics. The wavefronts of the first type wave trains are relatively broad, and they changed their propagation direction when they pass through the turning part of the guiding loop. On the contrary, the wavefronts of the other type wave trains are narrow, and their propagation did not affected by geometric changes of the guiding loop. Measurements indicate that the average speeds of the broad (narrow) wave trains is , and the period of the wave trains ranges from 54 to 458 s. We propose that the narrow wave trains may manifest the leakage of the wave trains from the guiding coronal loop, or were guided by another group of invisible coronal loop. In addition, the projection effect and weak magnetic field strength of the guiding coronal loop are proposed to explain the slow wave speed.

Geophysics: The buoyancy of Earth's deep mantle.

The physical nature of two regions called large low-shear-velocity provinces at the base of Earth's mantle is uncertain. A measurement of their density has implications for our understanding of mantle dynamics. See Article p.321 The interior composition of Earth can be estimated by imaging seismic waves changing speed as they travel through different materials, but some anomalies in the deep mantle remain challenging to explain. Fast wave speed anomalies appear in areas with a history of subduction, indicating relatively cold and dense mantle material driving downward flow. However, slow wave speed anomalies in the form of large domes above the core–mantle boundary remain contentious—in particular their net buoyancy. Harriet Lau and co-authors estimate Earth's deep mantle buoyancy using GPS-based measurements of the daily deformation of Earth in response to the gravitational pull of the Sun and the Moon. They show that the mean excess density across the bottom two-thirds of these lower-mantle domes is about 0.5 per cent. The authors conclude that these structures are enriched with high-density chemical components, probably originating from subducted oceanic plates or primordial material associated with Earth's formation.