Low Wave Speed Research Articles

Predicting the seismic ground-motion parameters: 3D physics-based numerical simulations combined with artificial neural networks

Typically, it is challenging to incorporate near-surface soils into 3D physics-based numerical simulations (PBSs) for ground-motion prediction. The low shear wave speed of near-surface soils, coupled with the complexity of the soil seismic response, poses significant difficulties. To overcome these limitations, a hybrid approach was proposed in this study, combining PBSs with artificial neural networks (ANNs). The essence of the hybrid method can be summarized as follows: (1) development of ANN models, establishing a strong-motion database, training the ANNs on it to predict the ground-motion parameters for East–West (EW), North–South (NS), and Vertical (UD) directions afterward; (2) establishment of 3D PBS model, obtaining the ground-motion parameters of the bedrock face corresponding to a certain shear wave speed; (3) application of the trained ANNs to predict the ground-motion parameters on the ground surface, taking the simulated results and related site parameters as inputs, and the outputs are peak ground acceleration (PGA) and 5% damped spectral accelerations (Sa) at different periods on the ground surface. In this study, ANN models were trained on a strong-motion database based on Kiban–Kyoshin Network (KiK-net). After several verifications of the ANN predictions, a case study of the 21 October 2016 Mw6.2 Central Tottori earthquake was conducted. In addition to the comparison with observations, the broadband (0.1–10 Hz) results of the hybrid method were also compared with the results that obtained by transfer function based on recorded data and Next Generation Attenuation (NGA)-West2 ground-motion prediction equations (GMPEs) to demonstrate the effectiveness and applicability of the proposed method. In addition, the distribution of Sa for four periods in simulated area was presented. The performance of the hybrid method for predicting broadband ground-motion characteristics was generally satisfactory.

Low voltage and low wave speed are rarely present outside the left atrium-pulmonary veins junction in paroxysmal atrial fibrillation but frequently present in persistent forms

Abstract Background and aim Multielectrode high-density catheters have enabled acquisition of comprehensive and dense maps of electrogram’s amplitude and timing. However, automated activation and voltage mapping are flawed by catheter orientation in relation to the wavefront activation. Omnipolar mapping technology (OT) uses both unipolar and bipolar signals to obtain OT signals and increases the accuracy of automatic point acquisition allowing for a high- density map in a quick and efficient way. The aim of this study was to automatically assess with OT, the left atrium (LA) voltage and wave speed propagation map in sinus rhythm (SR) according to the type of atrial fibrillation (AF). Methods We studied 14 consecutive patients referred for ablation of AF, either paroxysmal (PAF) (n=6) or persistent (Pers-AF) (n=8) using catheters with OT. Patients with Pers-AF were cardioverted, and a SR map was obtained before ablation in all patients and voltage and propagation wave speed maps were obtained. The cut-off for low voltage areas (LVAs) was less than 0.1 mV and for low wave speed (LWS) was less than 0.4 mm/ms. Results were compared according to the type of AF. Results The results are depicted in the Table. The median duration of AF in Pers-AF was 18 (5-24) months. Patients with Pers-AF had larger LA volumes and displayed presence of LVAs and LWS areas in 100% of cases in comparison with 17% and 33% of cases respectively for PAF patients (p=0.003 and p=0.015). Also, the total LVAs and LWS areas in cm2 were significantly higher, (p=0.001 and p=0.005)). The LVAs and the LWS areas were more frequently located outside the PV-LA junction in Pers-AF than in PAF, respectively 75% vs 0% and 88% vs 0%, p=0.010 and p=0.005 (Figure). Conclusions Patients with PAF never presented LVAs or LWS outside the PV-LA junction. On the contrary, patients with Pers-AF had frequently anatomical and electrophysiological abnormalities in the LA body. These findings may suggest the need for a wider ablation strategy in Pers-AF.Normal voltage in purple ; LWS in white

Design Study of an HTS Linear Power Generator Suitable for Wave Energy Converter

In this paper, we show a conceptual structure of a direct drive linear power generator with REBaCuO high-temperature superconducting (HTS) bulk field poles for wave energy conversion. The proposed linear power generator structure is comprised of a moving armature coil and HTS bulks concept, which we call it a dual translator power generation system. Its performance has been analyzed using a finite-element electromagnetic field analysis method. A determination of the main machine parameters for the dual translator HTS linear power generator was conducted. Simulation results were compared between our dual translator HTS linear generator structure and the use of permanent magnets (PMs) for the field poles of that structure. It was found that the electromotive forces, electromagnetic forces and output power of the dual translator HTS linear power generator structure are 1.67, 2.63 and 2.60 times superior to that of dual translator PM linear generator structure, respectively. A combination of the dual translator power generation system and HTS bulks has the potential to help us to achieve higher torque and higher output power despite low ocean wave speed, which is important to achieve practical application of ocean power generation systems.

Wave speed guided point-by-point cavo-tricuspid isthmus ablation to reduce ablation and fluoroscopy in typical atrial flutter

Abstract Funding Acknowledgements Type of funding sources: None. Background Ablation procedures for cavo-tricuspid isthmus (CTI) dependent typical atrial flutter (AFL) have traditionally been performed with conventional electrophysiology mapping guided by fluoroscopy. Three-dimensional electroanatomical mapping (EAM) systems have routinely been used to guide ablation of more complex arrhythmias, but in recent years, have been used for simple ablations with the aim of reducing irradiation. High voltage regions in the CTI have been described as a target in order to minimize procedure times, but there is a lack of knowledge regarding the utility of slow conduction zones for this purpose. Methods In a prospective cohort of patients undergoing 3D EAM assisted AFL ablation, we evaluated a novel strategy (NS) based on omnipolar vectors, using both wave speed and voltage information to focus radiofrequency (RF) lesions (see figure), and compared with a recent retrospective cohort of patients undergoing voltage-guided ablation (VG). Results A group of 26 consecutive patients with mean age 68±12 years and 88% male, underwent NS ablation, and were compared to 23 patients undergoing VG ablation, with no differences among patient characteristics. Of the 26 patients in group NS, 16 (53.9%) were successfully ablated targeting only 2 sites, representing the 2 most confluent areas of low wave speed and high voltage. A total of 24 (92.5%) were ablated successfully with targeting all sites of low wave speed. Only a single patient in the NS group required the completion of a classic CTI line. Average fluoroscopy time and radiation dose were lower in the NS group compared to VG group (1.1±2.4 min 19.5±41.7 mGray vs 8.2±10.2 min and 64.4±76.5 mGray, p=0.0014 and p=0.013, respectively). Furthermore, RF time trended towards reduction in the NS group. Conclusions A strategy based on targeting only areas of low wave speed is effective, limiting the need to complete a traditional CTI line, and reducing fluoroscopy in patients undergoin CTI ablation.

Super-resolution imaging with modulation of point spread function

The spatial resolution of an imaging system using waves is limited by the spatial bandwidth of the point spread function (PSF) of the system, which is related to the wavelength. However, when the PSF is modulated either in amplitude or phase or in both, the resulting spatial bandwidth of the PSF is increased. In this study, the PSF-modulation method is used to obtain super-resolution imaging of objects and to distinguish wave sources that are closely located in space and are not normally separable due to diffraction limit. In imaging using waves, such as ultrasound, acoustics, optics, electromagnetics, radar, and sonar, the PSF can be modulated in their respective fields. For example, in ultrasound, shear wave in biological soft tissues has a low wave speed and thus has a small wavelength. A ring-shaped shear wave can be generated locally (remotely) deep in the tissue by the radiation force of a focused Bessel beam, X wave, or other limited-diffraction beam at their focuses [Lu, 2021 IEEE IUS, 2021 ASA POMA] to produce a sharp peak at the center of the ring (due to shear wave focusing with a small wavelength). This sharp peak of the shear wave modulates the center of a conventional focused beam transmitted after the shear wave ring is produced. The modulated focused beam is then used to scan through an object to obtain a super-resolution image after removing the contribution of the original beam. In this talk, the theory, computer simulation, and experiment results of the super-resolution method will be presented.

Ambient Noise Tomography of the Lipari Volcanic Island (Southern Italy) From a Dense Nodal Array

AbstractWe applied ambient noise tomography to continuous data recorded by a dense seismic array deployed on the volcanic island of Lipari in the southern Tyrrhenian Sea. Since most of Lipari's seismicity occurs offshore and is not evenly distributed, this technique allowed us to obtain the first high‐resolution images beneath the island down to ∼2.5 km depth. Results show a complex seismic structure related to the various ages and compositions of the volcanic products characteristic of the different regions of the island. High shear wave velocities are found in western Lipari, where active hydrothermal vents and N‐S faults are mapped. Low wave speeds are revealed beneath southern and northeastern Lipari, where more recent volcanic activity developed along N‐S dike‐like structures that are aligned with rhyolitic vents. We suggest these dikes likely represent the probable pathways of future volcanic eruptions.

Variably Premixed Rotating Detonation Engine for Evaluation of Detonation Cycle Dynamics

A variably premixed rotating detonation engine using gaseous hydrogen and air reactants is introduced to enable investigation of key cycle processes while varying the homogeneity of the reactant inlet conditions. Two chamber configurations are investigated, the first with reactants filling the entire span of a straight annular channel and the second with a slightly larger channel and a backward-facing step. The first configuration permits both premixed and non-premixed fuel injection, enabling mixing quality modulation. The second configuration is operated only at fully premixed conditions. Operating modes and detonation wave speeds are characterized using exhaust-plume imaging, while the chamber heat release field is captured by transverse imaging through a transparent outer body. Tests using the first configuration were characterized by unstable counterpropagating modes with low detonation wave speeds regardless of the state of premixing, while the second configuration rendered single-wave behavior with wave speeds up to 86% of the Chapman–Jouguet velocity. Comparisons with a simple computational fluid dynamics model of the second configuration indicate that reactant preheating significantly influences the detonation wave topology, highlighting the potential utility of the test platform for isolating key physics associated with the effects of reactant premixing, preheating, and chamber geometry on rotating detonation engine operation.

Influence Analysis of Traveling Wave Effect on Rail Interaction of Long-Span Suspension Bridge

At present, there are few research results on the seismic response of the track system on railway suspension bridges, and relevant research has not yet formed a certain standard. In order to provide a certain reference for the development of industry standards and the design of seamless lines on railway suspension bridges, based on the large-mass method based on multi-point excitation, taking China’s longest high-speed railway suspension bridge-Wufengshan Yangtze River Bridge as the engineering background, a beam–rail integrated dynamic calculation space model was established with ANSYS, and the influence of traveling wave effect on the beam-rail interaction of large span suspension bridges was studied. The study shows that: the traveling wave effect will increase the relative displacement of the beam and rail, and then increase the stress of the rail; the traveling wave effect will cause the stress of the rail far from the source measurement to lag behind the stress of the rail near the source side, and the lag phenomenon gradually disappears with the increase of the apparent wave speed; the traveling wave effect has a greater effect on the displacement of the main bridge end than that of the approach bridge end; the longitudinal displacement of the main bridge end on the north and south side keeps changing synchronously under high apparent wave speed and consistent excitation, and no longer keeps changing synchronously under low apparent wave speed. The longitudinal displacement of the north-south side of the main bridge end keeps changing synchronously at high apparent wave speed and consistent excitation, but no longer at low apparent wave speed.

Anomalous intraslab structure revealed by the analysis of aftershocks of the Mw 6.7 Coquimbo-La Serena earthquake of 20 January 2019

Seismograms of about 12,000 aftershocks of the Mw6.7 Coquimbo-La Serena earthquake of 20 January 2019 that were recorded during six months by an array of 33 short-period seismic stations are used to generate wavespeed images of the subduction wedge beneath the Chilean forearc between 28.0°-30.6°S. This part of the margin is near the northern terminus of the 2015 Mw8.4 Illapel earthquake and the southern terminus of the 1922 Mw8.5 Copiapo earthquake. The distribution of hypocenters and wavespeeds at the slab interface and within the subduction wedge, are like those determined by Comte et al. (2019) in the Illapel aftershock region, and we infer that the same processes of subduction erosion and re-accretion inferred by them are active here. Moreover, anomalous wavespeeds are found in the immediate vicinity of a dense cluster of aftershocks located in a well-defined double seismic zone, within the subducting Nazca plate. Vp/Vs ratios in the upper seismic zone are uniformly high (∼1.9), those in most of the lower seismic zone are average to low, and those in the intraslab region of the aftershock cluster are exceptionally low (∼1.6). These values suggest dehydration of the slab interior and hydration of the mantle near the slab interface. While the origin of this feature is enigmatic, its limited spatial extent suggests that it most likely is due to an inherited structure, such a seamount or petit-spot volcano, in the Nazca plate, rather than a feature that developed in situ. The high rate of activity coupled with the low wavespeeds suggests a region of local weakness that could eventually evolve into a tear or hole in the slab, and its location at the bounds of two megathrust earthquakes suggests that it plays a significant role in delimiting the rupture of such events.

Hydroacoustic interaction between draft tube and penstock eigenmodes under Francis turbine full load instability

At high load, Francis Turbines may experience self-sustained pressure surge leading to significant power swing and pressure fluctuations along the waterway. The physical mechanism initiating this instability phenomenon has been the subject of much research. The development of the axisymmetric cavitating vortex rope at the runner outlet modifies the hydroacoustic properties of the draft tube waterway. Very low wave speed due to high cavitation volume combined with a high swirling number initiates the unstable axial pulsations of the cavitating vortex rope which frequency corresponds to a penstock’s eigenfrequency. The 15 MW power plant of Monceaux-la-Virole in France, composed of two units fed by a single penstock, experiences such full-load surge. On-site tests have been carried out to analyze the envelope of pressure fluctuations along the penstock once instability occurs. Combined with a 1D SIMSEN model of the power plant, these measurements have allowed to enhance the understanding of this instability phenomenon. To achieve this, an advanced draft tube modelling taking into account distributed wave speed, convective terms and divergent geometry is used and frequency analysis is carried out. Unstable draft tube eigenmodes and stable penstock eigenmodes are predicted. The key draft tube model parameters such as wave speed and second viscosity are calibrated to set the draft tube eigenmode frequency to the unstable measured frequency for different operating points. This frequency analysis concludes that high load instability occurs when a matching between the draft tube and the penstock eigenfrequencies is experienced. Moreover, it is shown that the unstable draft tube eigenmode is able to interact with different order penstock eigenmodes as function of the operating point of the unit.

New Constraints for the On‐Shore Makran Subduction Zone Crustal Structure

AbstractThe Makran Subduction Zone is the primary seismic/tsunami hazard of the northwestern Indian Ocean, but little is known of its on‐shore seismic structure. We derived a shear wave velocity model extending to 100 km depth beneath a ∼400 km‐long seismic profile oriented parallel to the convergence vector of the Arabian Sea Plate. Receiver function/surface wave analysis shows that the average structure in the coastal region comprises a ∼22–28 km‐thick low wavespeed sedimentary cover and a 6–8 km‐thick gradient zone overlying 100 km‐thick high wavespeed upper mantle. The ocean‐basement interface dips gently northward, remaining a positive impedance contrast to ∼50 km depth at ∼250 km north of the coast where it disappears as the basaltic/gabbroic oceanic crust has probably transformed to eclogite. Further north, a weak arrival at ∼5 s in the receiver functions appears, grading northward into the Moho arrival of the continental Iranian Plateau. This disruption in the seismic signature of the Moho occurs in the forearc region where the dip of the subducting oceanic plate steepens. The southern Iranian Plateau's continental crust has an average Vs of 3.55 ± 0.05 km s−1, an almost flat Moho 40–45 km deep, and a sub‐Moho mantle Vs of 3.75 ± 0.05 km s−1 in the 50–80 km depth range. Weak Moho conversions probably result from ∼20% serpentinization of peridotite in the mantle wedge. Receiver functions indicate a flat continental Moho – no crustal root beneath the high topography region of the volcanic belt, which therefore must be compensated by low upper mantle densities. The high Vp/Vs ratio observed for the mantle wedge suggests ∼1%–2% partial melt.

Pore and Mineral Fabrics Control the Elastic Wave Velocities of Metapelite With Implications for Subduction Zone Tomography

AbstractSeismic tomography is used to infer compositional variations and environmental conditions in active subduction zones. In some regions, low P‐wave and S‐wave velocities and high are documented near and along the plate boundary, and are interpreted to reflect high pore fluid pressures in metasedimentary rocks. However, there are limited experimental data with which to constrain the in‐situ wave speeds of metasedimentary rocks, particularly under elevated pore pressure. To determine whether the velocities of metasedimentary rocks are consistent with low wave speeds and high , we measured the ultrasonic velocities of the Orocopia schist, a greenschist facies metapelite, in the laboratory. Velocities were measured under dry and saturated conditions, at effective pressures between 1 and 136 MPa, parallel and normal to foliation, and at room temperature. We find that, under fluid‐saturated conditions, the velocities of the Orocopia schist and are consistent with anomalous values in subduction zones ( km/s, km/s, and ) at effective pressures less than 10 MPa and normal to foliation. We used differential effective medium models and microscopy to quantify the contributions of mineralogy, mineral fabric, porosity, and pore shape to , , and their anisotropies. We find that, although mineralogy and mineral anisotropy impart some control on the wave velocities, aligned, small aspect ratio pores (0.005–0.03) are required to explain the measured velocities and high . The low and and high in subduction zones can be explained by phyllosilicate‐rich metapelitic rocks under near‐lithostatic pore pressure.

Body‐Wave Tomographic Imaging of the Turkana Depression: Implications for Rift Development and Plume‐Lithosphere Interactions

AbstractThe Turkana Depression, a topographically subdued, broadly rifted zone between the elevated East African and Ethiopian plateaus, disrupts the N–S, fault‐bounded rift basin morphology that characterizes most of the East African Rift. The unusual breadth of the Turkana Depression leaves unanswered questions about the initiation and evolution of rifting between the Main Ethiopian Rift (MER) and Eastern Rift. Hypotheses explaining the unusually broad, low‐lying area include superposed Mesozoic and Cenozoic rifting and a lack of mantle lithospheric thinning and dynamic support. To address these issues, we have carried out the first body‐wave tomographic study of the Depression's upper mantle. Seismically derived temperatures at 100 km depth exceed petrological estimates, suggesting the presence of mantle melt, although not as voluminous as the MER, contributes to velocity anomalies. A NW–SE‐trending high wavespeed band in southern Ethiopia at 200 km depth is interpreted as refractory Proterozoic lithosphere which has likely influenced the localization of both Mesozoic and Cenozoic rifting. At 100 km depth below the central Depression, a single localized low wavespeed zone is lacking. Only in the northernmost Eastern Rift and southern Lake Turkana is there evidence for focused low wavespeeds resembling the MER, that bifurcate below the Depression and broaden approaching southern Ethiopia further north. These low wavespeeds may be attributed to melt‐intruded mantle lithosphere or ponded asthenospheric material below lithospheric thin‐spots induced by the region's multiple rifting phases. Low wavespeeds persist to the mantle transition zone suggesting the Depression may not lack mantle dynamic support in comparison to the two plateaus.

Fluid pumping of peristaltic vessel fitted with elastic valves.

Using numerical simulations, we probe the fluid flow in an axisymmetric peristaltic vessel fitted with elastic bi-leaflet valves. In this biomimetic system that mimics the flow generated in lymphatic vessels, we investigate the effects of the valve and vessel properties on pumping performance of the valved peristaltic vessel. The results indicate that valves significantly increase pumping by reducing backflow. The presence of valves, however, increases the viscous resistance therefore requiring greater work compared to valveless vessels. The benefit of the valves is the most significant when the fluid is pumped against an adverse pressure gradient and for low vessel contraction wave speeds. We identify the optimum vessel and valve parameters leading to the maximum pumping efficiency. We show that the optimum valve elasticity maximizes the pumping flow rate by allowing the valve to block more effectively the backflow while maintaining low resistance during the forward flow. We also examine the pumping in vessels where the vessel contraction amplitude is a function of the adverse pressure gradient as found in lymphatic vessels. We find that in this case the flow is limited by the work generated by the contracting vessel, suggesting that the pumping in lymphatic vessels is constrained by the performance of lymphatic muscle. Given the regional heterogeneity of valve morphology observed throughout the lymphatic vasculature, these results provide insight into how these variations might facilitate efficient lymphatic transport in the vessel's local physiologic context.

Shear Wave Speed of Shallow Seafloor Sediments in the Northern South China Sea and Their Correlations With Physical Parameters

AbstractIn this study, shear wave speed and related physical properties were measured for 21 seafloor sediment cores collected in the northern South China Sea. Results reveal that the shear wave speed in this study area ranges between 15.57 and 75.55 m/s and demonstrates a pattern of regional distribution. The sediment in the continental shelf area shows characteristics of high shear wave speed, high wet bulk density, low porosity, and large grain size. Contrarily, the sediment in the continental slope area exhibits characteristics of low shear wave speed, low wet bulk density, high porosity, and fine particles. Accordingly, the correlation between shear wave speed and physical properties such as porosity, wet bulk density, water content, and average grain size was studied. Prediction formulas of shear wave speed of sediments in this study area containing single and double physical parameters were established via regression analysis. The comparison between single‐parameter and double‐parameter prediction formulas indicates that the determined coefficient of the double‐parameter prediction formulas based on porosity and average grain size is higher than that of single‐parameter prediction formulas. It indicates that the double‐parameter prediction formulas based on porosity and average grain size have better prediction performance. Moreover, the prediction formulas established in this study were further compared with those of other study areas, and their differences were analyzed accordingly.

Upper mantle and mantle transition zone thermal and water content anomalies beneath NE Asia: Constraints from receiver function imaging of the 410 and 660 km discontinuities

The 410 and 660 km discontinuities (d410 and d660) bordering the mantle transition zone (MTZ) beneath NE Asia, including NE China, Eastern Mongolia, and southern Siberia, are imaged in successive circular bins with a radius of 1 degree by stacking a total of 274,413 P-to-s radial receiver functions recorded by 799 broadband seismic stations. After moveout correction based on the 1-D IASP91 Earth model, the resulting apparent depths of the discontinuities exhibit significant and spatially systematic variations. Three approximately N-S elongated narrow zones with significantly thickened MTZ are observed, which may be associated with the thermal effect and dehydration of subducted slabs. The major volcanoes in NE China are underlain by a d660 that is apparently depressed by ∼19 km, which can be interpreted by the presence of an anomalously high water concentration in the lower MTZ released from the stagnated slabs. Low wavespeed anomalies above the d410 west of the Datong volcanic fields are underlain by an MTZ with normal thickness, and are attributable by dehydration of the leading portion of the stagnant Pacific slab in the MTZ that is revealed in an N-S oriented narrow zone east of this area. The lateral shift of the upper mantle low wavespeed zone and the area with thickened MTZ may suggest a westward drift of the upper mantle relative to the subducted slab. An abnormally thin MTZ is observed beneath the Hangay Dome in central Mongolia, suggesting the possible existence of thermal upwelling from the lower mantle through the MTZ.

Lithospheric foundering and underthrusting imaged beneath Tibet

Long-standing debates exist over the timing and mechanism of uplift of the Tibetan Plateau and, more specifically, over the connection between lithospheric evolution and surface expressions of plateau uplift and volcanism. Here we show a T-shaped high wave speed structure in our new tomographic model beneath South-Central Tibet, interpreted as an upper-mantle remnant from earlier lithospheric foundering. Its spatial correlation with ultrapotassic and adakitic magmatism supports the hypothesis of convective removal of thickened Tibetan lithosphere causing major uplift of Southern Tibet during the Oligocene. Lithospheric foundering induces an asthenospheric drag force, which drives continued underthrusting of the Indian continental lithosphere and shortening and thickening of the Northern Tibetan lithosphere. Surface uplift of Northern Tibet is subject to more recent asthenospheric upwelling and thermal erosion of thickened lithosphere, which is spatially consistent with recent potassic volcanism and an imaged narrow low wave speed zone in the uppermost mantle.

Shear Stiffness and Energy Absorption of Auxetic Open Cell Foams as Sandwich Cores

This work describes the identification of the shear modulus of open cell polyurethane thermoformed auxetic foams from 3‐ and 4‐point bending tests. The foams are incorporated in sandwich beams with carbon fibre/epoxy face skins, and benchmarked against similar sandwich structures made with the conventional counterpart open cell foam. Three types of beams are tested: one with auxetic foams, another type related to a conventional foam core with the same thickness of the auxetic porous materials, and a third type of beam consisting in conventional foam with a thickness corresponding to an iso‐weight configuration to the auxetic specimen. The auxetic foam has a shear modulus 7% lower than the one of the bulk conventional specimens, but higher shear stresses at large deformations and a smoother strain stiffening response compared to the beams with the conventional thinner core. The paper also highlights the low shear wave speed of these auxetic foams compared to other porous polymers used in helmet and head protection applications, as well as potential uses of the quasi‐zero‐stiffness behavior here observed for the auxetic foam sandwich beam.

The structure of ion-acoustic waves in a low-frequency three-component electron–ion space plasma with two-electron populations

Low-frequency ion-acoustic waves are analysed on the ion time-scale, in a three-component electron–ion space plasma. The solitary waves propagate in the positive x direction relative to an ambient magnetic field $$\vec {B}_0$$ which forms static background for a configuration consisting of cool fluid ions and both warm and hot Boltzmann-distributed electrons with temperatures $$T_{ic},$$ $$T_{ew}$$ and $$T_{eh},$$ respectively. We derive linear dispersion relation for the waves by introducing first-order density, pressure and velocity perturbations into the ion fluid equations. Additionally, the variation in the nonlinear structure of the waves are investigated by carrying out a full parametric analysis utilising our numerical code. Our results reveal that ion-acoustic waves exhibit well-defined nonlinear spikes at speeds of $$M\ge 2.25$$ and an electric field amplitude of $$E_0=0.85.$$ It is also shown that low wave speeds $$(M\le 2),$$ higher densities of the hot electrons, antiparallel drifting of the cool fluid ions, and increased ion temperatures all lead to significant dispersive effects. The ion-acoustic plasma waves featured in this paper have forms that are consistent with those classified as the type-A and type-B broadband electrostatic noise (BEN) observed in the data obtained from earlier satellite missions.

Lithospheric layering beneath the contiguous United States constrained by S-to-P receiver functions

To image upper mantle seismic discontinuities beneath the contiguous United States, a total of 284,121 S-to-P receiver functions (SRFs) recorded by 3,594 broadband seismic stations in the EarthScope Transportable Array and other permanent and temporary deployments are stacked in circular bins of 2° in radius. A robust negative arrival, representing a sharp discontinuity of wave speed reduction with depth, is visible in virtually all the stacked traces in the depth range of 30–110 km. Beneath the western U.S., the mean depth of this discontinuity is 69±17 km, and beneath the eastern U.S., it is 76±5 km, both are comparable to the depth of the tomographically-determined lithosphere–asthenosphere boundary (LAB). In contrast, the depth of the discontinuity beneath the stable cratonic region of the central U.S. is 87±6 km, which is significantly shallower than the ∼250 km LAB depth determined by seismic tomography. Based on the amplitude of the corresponding arrival in the SRFs and findings from previous seismic tomography and mantle xenolith studies, this discontinuity beneath the central U.S. is interpreted as the top of an intra-lithospheric low wave speed, probably phlogopite-rich layer. The observations provide new constraints on a number of regional scale tectonic processes, such as lithospheric stretching in the Texas–Louisiana Gulf Coastal Plain and the Basin and Range Province, and possible lithospheric basal erosion beneath the northeastern U.S.