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SUMMARY: We define double (S1S2) and triple (PS1S2) singularity points and their degeneracy classes in triclinic anisotropic media. We derive equations for the group velocity image for all these cases. The degeneracy classes are defined by factorization of quadratic (double singularity point) and cubic (triple singularity point) forms with three variables. [ABSTRACT FROM AUTHOR]

On 11 May 2022, an earthquake of Mw 5.2 occurred in the Dharchula region of Uttarakhand Himalayas, India. The Uttarakhand State Earthquake Early Warning System (UEEWS) detected and transmitted the warning within 11.61 s from the origin time, taking only 4.26 s for processing, location, and magnitude estimation and warning dissemination. The complete analysis was performed using three seconds of waveforms. Using the initial earthquake parameters provided by the UEEWS, moment tensor inversion was performed using the broadband seismometers network installed in northern India. The moment tensor (MT) inversion was performed for the event using both the body waves and the surface waves. The first motion polarity was used along with waveform data to enhance the solution's stability. This paper discusses the importance of real-time event detection and efforts towards real-time MT source inversion of earthquakes in the Uttarakhand Himalayas. Relocation of two past earthquakes near Dharchula is also undertaken in this study. The outcome of this study provides insights into mitigating seismic hazards, understanding earthquake source mechanisms, and enhancing knowledge of local fault structures. [ABSTRACT FROM AUTHOR]

This paper analyzes shear horizontal (SH) waves propagating in thin metal strips deposited periodically onto the surface of a magneto-electro-elastic (MEE) substrate. The MEE substrate is assumed to be a transversely isotropic crystal with 6 mm symmetry or a hexagonal (6 mm) crystal. A field analysis method, which is based on the Bloch–Floquet theory and the coupled equations of wave motion, is employed to derive the dispersion equations and wave mode shapes. Numerical examples are presented for an MEE composite (PZT4/CoFe2O4) and an aluminum (Al) strip. For comparison, different constituent ratios of PZT4/CoFe2O4 are considered. The effect of the ratios of strip height and width to grating periodicity on frequencies, phase velocities, as well as wave mode shapes is discussed in detail. The results show that there is a bandgap at the resonance condition when the grating periodicity matches the wavelength of the SH surface wave. This metal grating surface can trap SH surface waves by slowing the SH waves. The wave mode shapes indicate that the SH surface waves are trapped mainly on these metal strips. As the ratio of strip height to grating periodicity increases, the trapping effect is more pronounced and the gap becomes wider. The ratio of strip width to grating periodicity significantly affects the behavior of SH surface waves. The present results are relevant in the application of MEE surface acoustic wave resonators. [ABSTRACT FROM AUTHOR]

Seismic interferometry is often proposed as a cost-efficient technique for reservoir monitoring including CO2 sequestration due to its low cost and environmental advantages over active source imaging. Although many studies have demonstrated the ability of seismic interferometry to retrieve surface waves, body wave imaging remains challenging due to their generally lower amplitudes of body waves in seismic interferometry data. An optimum data acquisition strategy can help retrieve low amplitude body waves better, however, rare attempts have been made to evaluate various data acquisition strategies. In this study, we use numerical modeling to examine three different acquisition schemes to evaluate the retrievability of P waves from seismic interferometry data. From our numerical results, we observe that (1) positing receivers beneath the attenuated weathered layer improves the data quality and signal to noise ratio, but additional processing steps including predictive deconvolution and Radom transform filter are necessary to remove the downgoing surface multiples, artifacts that are generated from this data acquisition; (2) vertical seismic profiling (VSP) alongside with the conventional surface seismic acquisition improve the target zone detection; and (3) crosswell acquisition of seismic interferometry is an ineffective means to obtain reflection events due to the non-similarity of ray paths from the noise sources meaning that the required stationary phase theory is not fulfilled. [ABSTRACT FROM AUTHOR]

Strong waveform complexities, including multipathing of the S diffracted phase and rapid changes in differential ScS‐S times, are observed for multiple deep Fiji earthquakes recorded at the USArray. The complexities occur at the northeastern edge of the Pacific Large Low Shear Velocity Province (LLSVP), about 12 degrees southeast of present‐day Hawaiʻi. Waveform modeling of the multipathing provides good constraints on an ultra‐low velocity zone (ULVZ) with a width of 5 degree located near the inner edge of the LLSVP. Based on the mineralogical‐modeling of the ULVZ as a solid iron‐rich magnesiowüstite‐bearing assemblage with compatible morphology predicted from geodynamical simulations, a ULVZ model with a thickness of 30 km and a shear wave velocity reduction of 18% is preferred. The rapid change in differential ScS‐S travel time is best explained by having both the aforementioned ULVZ and an adjacent high velocity structure near the LLSVP. Furthermore, a low‐velocity plume‐like structure could potentially explain the observed S travel time delay independent of ScS. These seismic features are proposed to be a ULVZ driven toward the edge of the LLSVP while potentially pushed by a subducted slab. This configuration may trigger plume generation due to strong thermal instabilities and is in the same vicinity where mantle flow models place the present‐day Hawaiian plume source. Multiple ScS can potentially be used to verify vertical plume structure in tomographic models but the accuracy of upper mantle structure, which is a key reflection point, needs to be considered. Plain Language Summary: Seismic waves from earthquakes in Fiji recorded by seismometers in the United States travel close to the core‐mantle boundary (CMB) and can be used to image fine‐scale structures along the edge of a previously known large province near the CMB with low seismic wave velocity, namely the Pacific LLSVP. The edges of the Pacific LLSVP are of interest because they contain many structural anomalies, thought to be correlated with hotspots on Earth's surface, including Hawaiʻi, and can host the plume sources for these hotspots. In this study, we observed two phenomena: (a) seismic waves with an additional unexpected pulse due to the presence of a very low velocity structure and (b) a rapid change in the travel time behavior of two seismic phases which can be explained by the same low velocity structure adjacent to a high velocity structure. We constrained these two structures to be at the edge of the LLSVP, a configuration favorable for generating long‐lasting plume, that is, the source for the Hawaiian hotspot. The location of these structures is in agreement with the hypothesized source location from recent geodynamical studies. We also showed that this low velocity structure could be composed of a solid iron‐rich material. Key Points: Seismic waves sampling the northeastern edge of the Pacific Large Low Shear Velocity Province (LLSVP) show strong waveform complexity and rapid change in differential timeWaveform and mineralogical modeling suggest a magnesiowüstite‐bearing ultra‐low velocity zone adjacent to slab at edge of LLSVP, conducive to plume formationThe structural anomalies and proposed plume are located ∼12° southeast of present‐day Hawaiʻi, in agreement with recent mantle flow models [ABSTRACT FROM AUTHOR]

As the junction of two important tectonic units, North China Craton and South China Block, East‐Central China has undergone multi‐period tectonic events, including the Triassic continental collision and the Mesozoic and Cenozoic subduction of the western Pacific plate, which forms unique crustal and upper mantle structures in East‐Central China. In this study, by jointly inverting body and surface waves, we construct a high‐resolution 3D Vs model of East‐Central China at the depths of 0–800 km. Our results show that along the eastern Qinling‐Dabie orogen formed by past continental collision, there are significant changes in velocity patterns in the upper mantle. From the eastern Dabie to the western Dabie, velocity features mainly vary from low velocities to high velocities, implying a westward weakening of the influence of the Paleo‐Pacific tectonic domain. The high‐velocity anomaly is partially missing beneath eastern Qinling, which may be attributed to the lower crustal and lithospheric mantle delamination. In addition, we find a significant low‐velocity anomaly in the upper mantle beneath the Lower Yangtze Craton, which may represent an upwelling of thermal fluids caused by the dehydration of stagnant slabs in the mantle transition zone. This low‐velocity anomaly extends upward into the shallow lithosphere and is consistent with the area of crustal and lithospheric thinning as well as the concentrated exposure of Cenozoic continental basalts. These connections reveal the reactivation of the lithospheric mantle by the dehydration of the stagnant slab and the asthenosphere upwelling during the Cenozoic. Plain Language Summary: East‐Central China has developed complex geological structures over a long history of tectonic evolution. Traces of past tectonic movements are preserved in the deep Earth. Imaging the present‐day subsurface structures is essential for understanding past tectonic evolution processes that occurred millions of years ago. In this study, by jointly inverting body and surface waves, we construct a high‐resolution 3D model of East‐Central China at the depths of 0–800 km. Our model shows that there is a clear change of the upper mantle structures along the eastern Qinling‐Dabie orogen, which is related to the westward subduction of Paleo‐Pacific plate. In addition, we find a low‐velocity anomaly in the upper mantle beneath the Lower Yangtze Craton. The low‐velocity anomaly may be caused by the dehydration of the stagnant Pacific slab. Altogether, our results provide new insights into the dynamic processes in East‐Central China. Key Points: A 3D velocity model of East‐Central China is obtained by a joint inversion of surface and body wavesChanges in Vs structure along the Qinling‐Dabie orogen suggest a westward weakening of the impact of Paleo‐Pacific plate subductionAsthenosphere upwelling induced by the dehydration of the stagnant Pacific slab during the Cenozoic reactivated the lithospheric mantle [ABSTRACT FROM AUTHOR]

Improving our understanding of the relation between the water content and the seismic signatures of unconsolidated superficial soils is an important objective in the overall field of hydrogeophysics. Current approaches to constrain the water content in the vadose zone from seismic data are based on computing the ratio between compressional and shear wave velocities Vp/Vs. While this allows for the detection of pronounced changes in saturation, such as the groundwater table, it is essentially insensitive to variations in the saturation‐depth profile. Conversely, evidence shows that surface waves are sensitive to both the location of the water table and the saturation‐depth profile. Classic rock physics models are unable to explain the corresponding observations. We propose to estimate surface‐wave signatures accounting for capillary suction effects. We extend the Hertz‐Mindlin model using Bishop's effective stress definition, thus accounting for stiffness changes associated with capillary stresses acting on the soil's frame. We then compute the elastic properties of the partially saturated medium using the Biot‐Gassmann‐Wood model. Considering a 1D unconsolidated porous medium under steady‐state saturation conditions, as given by Richards' equation, we simulate body‐wave travel times and surface‐wave dispersion characteristics for different water table depths and overlying soil textures. Our results illustrate that surface‐wave phase velocity dispersion curves are remarkably sensitive to capillary effects in partially saturated soils, exhibiting velocity changes of up to 20% in the 10–100 Hz frequency range. These effects, which are particularly important in medium‐to fine‐grained soils, are virtually nonexistent in the corresponding Vp/Vs profiles. Plain Language Summary: Seismic waves are usually employed to study the water content in the shallow subsurface for environmental purposes. Most studies estimate the water content of the soil at different depths relating compressional and shear wave velocities, which are estimated by measuring body‐wave travel times. Even though this method permits to locate the depth at which the soil becomes fully water saturated, it is rather insensitive to changes in the water content of the overlaying portion of the soil, where partial air‐water saturation prevails. Surface waves, however, appear to be significantly sensitive to changes in saturation within this partially saturated region. Classic rock physics models cannot explain this difference in sensitivity between surface‐wave velocity and body‐wave travel times. In this work, we propose to include the effects of capillary forces, which arise when the soil is partially saturated, in the rock physics models. By doing so, we show that capillary action, which acts bringing soil particles further together, can explain why surface‐wave dispersion curves are sensitive to saturation variations in the partially saturated zone while body‐wave travel times remain virtually unperturbed. These results may help to better interpret seismic data for environmental studies. Key Points: Water content variations in the partially saturated zone hardly alter Vp/Vs ratios, but have large effects on surface‐wave signaturesThe effective soil stress is strongly affected by capillary forces, which stiffen the medium at relatively low saturationsCapillary effects permit to explain observed changes in surface‐wave dispersion due to small water content variations in the vadose zone [ABSTRACT FROM AUTHOR]

Accuracy of hypocenter location, in particular focal depth, is a precondition for high‐resolution seismotectonic analysis of natural and induced seismicity. For instance, linking seismicity with mapped fault segments requires hypocenter accuracy at the sub‐kilometer scale. In this study, we demonstrate that inaccurate velocity models and improper phase selection can bias absolute hypocenter locations and location uncertainties, resulting in errors larger than the targeted accuracy. To avoid such bias in densely instrumented seismic networks, we propose a coupled hypocenter‐velocity inversion procedure restricted to direct, first‐arriving, mainly upper‐crustal Pg and Sg phases. On the basis of synthetic tests and selected ground‐truth events we demonstrate that a sub‐kilometer hypocenter accuracy can be achieved by regional‐scale, three‐dimensional Pg and Sg velocity models combined with dynamic phase selection and a non‐linear location algorithm. The tomographic inversion uses about 60,000 Pg and 30,000 Sg quality‐checked phases of local earthquakes in the Central Alps region. The derived models image the VP and VS structure of the Central Alps upper crust at unprecedented resolution, including small‐scale anomalies such as those caused by Subalpine Molasse units below the Alpine front. The relocation procedure is applied to more than 18,000 earthquakes and the relocated hypocenters reveal previously unrecognized seismogenic structures, for instance in the Swiss Molasse basin south of Bern. The ML 4.6 Urnerboden earthquake of 2017 is used as an example to demonstrate how the derived 3D velocity structure and relocated hypocenters can be jointly interpreted to constrain the lithology hosting upper‐crustal seismicity in the Central Alps. Plain Language Summary: To better understand how mountain belts like the European Alps presently deform and what are the plate‐tectonic forces driving this deformation requires accurate knowledge of the location of earthquakes within these continental collision zones. In this study, we achieve an accuracy of less than a kilometer for earthquake locations in Switzerland, based on detailed knowledge of the subsurface structure of the Earth's crust. We use three‐dimensional tomographic imaging methods to improve subsurface structural models of the Central Alps. These models also provide new insights into the geological structure of this mountain range and in combination with the improved earthquake locations allow for detailed studies of present‐day tectonic processes. Key Points: Sub‐kilometer hypocenter accuracy is achieved with dynamically selected Pg and Sg phases, in combination with 3D crustal velocity modelsNew 3D VP and VS models image the upper crust of the Central Alps region at unprecedented resolutionJoint interpretation of relocated hypocenters and seismic velocities can constrain lithologies hosting seismicity in the Central Alps [ABSTRACT FROM AUTHOR]

Ocean swells or storms can generate persistent localized microseismic sources in deep oceans, which emanate body wave energy traveling through the Earth's interior, carrying abundant information about subsurface structures as earthquake waves do. However, body waves from localized microseismic sources have not been fully exploited to map the internal structures of the Earth. Here, we report that the travel times of body wave reflections from the Moho can be obtained from body waves generated by localized microseismic sources. We further demonstrate that body waves from these sources can be utilized to constrain the Moho morphology through synthetic experiments and comparisons with earthquake receiver function results. Our work suggests that localized microseismic sources can be exploited to map the internal structure of the Earth without relying on earthquake data. Plain Language Summary: Movements of ocean waves can trigger the vibration of the solid earth, generating seismic waves which can propagate downward into the deep interior of the Earth and be recorded by a distant seismograph. Such seismic waves are called body waves, which carry information about the interior structures of the Earth. In the past, body waves generated by oceanic swells or storms have not been exploited to map the internal structures of the Earth. In this study, we report that body waves generated by sources related to oceanic swells or storms can be utilized to map the boundary between the crust and the upper mantle of the Earth. Our method may be also applicable to ground motion data triggered by large storms in Mars to image Mars's interior. Key Points: Travel times of PmP phases from Moho can be obtained from persistent microseismic sourcesThese travel times of PmP phases can be utilized to constrain the Moho morphologyIndividual localized microseismic sources can be exploited to map the internal structure of the Earth without relying on earthquake data [ABSTRACT FROM AUTHOR]

Recent developments in sensor technology have allowed for the low-cost deployment of dense seismic arrays which continuously record the seismic ambient noise field. Generally, the extraction of body waves from the seismic ambient noise field is more challenging than for surface waves, due to the dominant amplitude of the latter. In this study, we work with data from a dense seismic array deployment in the Dehdasht area, southwestern Iran. We show that by using the polarization properties of seismic waves, we can simultaneously retrieve both high frequency body waves and surface waves from the cross-correlated noise field. As a by-product of this study, we also observe high energy spurious artefacts, particularly those associated with the direct P -phases. Numerical simulations show that these artefacts are a consequence of an uneven distribution of noise sources, and are difficult to suppress during the simultaneous retrieval of body and surface waves. [ABSTRACT FROM AUTHOR]

The sensitivity of seismic compressional and shear waves and their velocity ratios to rock lithology, pore fluids, and high‐temperature materials makes these parameters very useful for constraining the physical state of the crust. In this study, we develop a joint inversion approach utilizing both radial and vertical components' autocorrelations of teleseismic P‐wave coda for imaging the crust by simultaneously characterizing the crustal Vp, Vs, and Vp/Vs ratio. Autocorrelations of the radial and vertical components contain P and S waves that are reflected from the subsurface. Therefore, joint inversion of them can account for the variations of both Vp and Vs, and consequently, the Vp/Vs ratio. Synthetic inversions show significant improvement in the estimation of these parameters compared to those from the inversion of either, P receiver functions or the autocorrelation of the vertical component. The velocity models inferred from the application of the approach to teleseismic data recorded along a north‐south passive seismic profile (BILBY experiment) in central Australia reveal a distinct pattern of the Moho and the Vp/Vs variations across the crustal blocks/domains. The general trend of the Moho structure corresponds well with the change of the reflectivity that can normally be seen at the base of the crust and also with the Moho estimated from previous studies including the deep seismic reflection profiling method. The Vp/Vs structure at depths greater than 10 km shows dominant high values beneath locations where the crustal domains interact (e.g., at transition from one domain to another). Plain Language Summary: Understanding variations of the crustal P and S waves (Vp and Vs) can indicate rock lithology. Here, we introduce a new approach to estimate these wavespeeds simultaneously using vibrations from earthquakes recorded at seismic stations. Autocorrelations of the distant P‐wave coda recorded on the vertical and radial components of a seismogram contain P‐ and S‐wave reflections. Thus, they are sensitive to both the crustal wavespeeds and the crustal discontinuities. We test the feasibility of the new method with synthetic data, which reveals a substantial improvement in recovering the input Vp/Vs models. We then apply the approach to teleseismic waveforms recorded along a passive seismic experiment in central Australia to infer the crustal structure. The results reveal a distinct variation of the thickness of the crust and the Vp/Vs across geological units along the passive seismic line. The overall pattern of the crustal thickness variations also matches the change of reflectivity at the base of the crust. The inferred Vp/Vs model exhibits high Vp/Vs values at depths greater than 10 km beneath locations where the crustal domains interact. Key Points: We introduce a new joint inversion approach to estimate Vp, Vs simultaneously, and Vp/Vs ratio structures below a seismic stationWe jointly invert both the vertical and radial component autocorrelations of the teleseismic P‐wave coda to image and characterize the crustThe joint inversion provides better constraints than the inversion of either P receiver functions or the vertical component autocorrelation [ABSTRACT FROM AUTHOR]

The core‐mantle boundary (CMB) is the most extreme boundary within the Earth where the liquid, iron‐rich outer core interacts with the rocky, silicate mantle. The nature of the lowermost mantle atop the CMB, and its role in mantle dynamics, is not completely understood. Various regional studies have documented significant heterogeneities at different spatial scales. While there is a consensus on the long scale length structure of the inferred S‐wave speed tomograms, there are also notable differences stemming from different imaging methods and datasets. Here we aim to overcome over‐smoothing and avoid over‐fitting data for the case where the spatial coverage is sparse and the inverse problem ill‐posed. We present an S‐wave tomography model at a global scale for the Lowermost Mantle (LM) using the Hierarchical Trans‐Dimensional Bayesian Inversion (HTDBI) framework, LM‐HTDBI. Our LM‐HTDBI analysis of ScS‐S travel times includes uncertainty, and the complexity of the model is deduced from the data itself through an implicit parameterization of the model space. Our comprehensive resolution estimates indicate that short‐scale anomalies are significant and resolvable features of the lowermost mantle regardless of the chosen mantle‐model reference to correct the travel times above the D" layer. The recovered morphology of the Large‐Low‐Shear‐wave Velocity Provinces (LLSVPs) is complex, featuring small high‐velocity patches among low‐velocity domains. Instead of two large, unified, and smooth LLSVPs, the newly obtained images suggest that their margins are not uniformly flat. Plain Language Summary: The lowermost mantle sits atop the core‐mantle boundary, the most dramatic boundary within our planet, with contrasts in physical properties that exceed those that exist at the surface. Despite significant progress in recent years, this part of the Earth is not well understood, and various tomographic studies on a global scale, along with regional studies that focus on seismic waveform modeling, pave the path towards higher resolution and new understanding. Important questions to answer are on the distribution, shape, size and composition of inhomogeneities in the lowermost mantle, and their critical role in the mantle and core dynamics. While there is a general consensus on the long‐scale length structures inferred from long‐period shear waves, there are notable differences in details of the tomograms of the lowermost mantle, stemming from the use of different imaging methods and datasets. Here, we utilize a large travel time data set of ScS and S waves with a significant addition of new measurements sensitive to the lowermost mantle to perform a probabilistic shear‐wave tomography, and we retrieve a high‐resolution image of the lowermost mantle. The new shear‐wave speed tomogram and comprehensive resolution‐estimations indicate that short and medium scale inhomogeneities are omnipresent features of the lowermost mantle. Key Points: S‐wave tomography of the lowermost mantle using state‐of‐the‐art Bayesian approach with 2D spherical Voronoi cellsThe inversion technique treats the model complexity and the data noise as free parameters and avoids damping and smoothingThis study provides an important bridge between long‐scale features at a global scale and short‐scale features of regional models [ABSTRACT FROM AUTHOR]

This study presents a forward model to quantify the P‐wave velocity (VP), S‐wave velocity (VS), and electrical conductivity (σ) of the solid‐liquid mixtures for a given set of pressure, temperature, lithology, liquid phase (aqueous fluid or melt), liquid fraction, and geometrical parameters in relation to the aspect ratio and connectivity of the liquid phase. This is based on previous experimental and theoretical studies on seismic velocity and electrical conductivity of solid rocks and liquid phases. A total of 78 lithologies, an aqueous fluid with NaCl (∼0–10 wt.%), and mafic to felsic melt appropriate for the crust and the uppermost mantle conditions were described in terms of VP, VS, and σ, as per previous experimental measurements and molecular dynamics simulation. This forward model is provided as a Windows executable program, and generates synthetic VP, VS, and σ, referring to the seismic velocities and electrical conductivity observed in the northeast Japan arc. After generation of the synthetic VP, VS, and σ, the original lithology and liquid parameters (phase, fraction, aspect ratio, and connectivity) were searched by implementing the grid search algorithm to map the misfit over the broad parameter space. The mapping shows the presence of a global misfit minimum around the optimized solution and the possibility of resolving the lithology and the liquid phase parameters based on the observed VP, VS, and σ by using the forward model presented in this study. Plain Language Summary: Liquid phases such as aqueous fluid and magma present within the solid Earth play crucial roles in various geodynamic processes including earthquake and volcanic eruption, as well as in the evolution of our planet. This study aims to improve the subsurface imaging of the potentially variable types of liquid phase and solid rock, based on the near‐surface observations concerning seismic velocity and electrical conductivity of the Earth's interior. We constructed a quantitative model to predict the physical properties of liquids and rocks. Then we used the model to perform "inversion," which estimates the physical properties of liquid and rock based on the observed seismic velocity and electrical conductivity. The inversion test using synthetic data shows the utility of our model. Key Points: A forward model is presented to quantify seismic velocity and electrical conductivity of solid‐liquid mixtures in crust‐uppermost mantleThe input parameters include pressure, temperature, lithology, liquid phase composition and fraction, and solid‐liquid geometryFor a given set of seismic velocity and electrical conductivity, the optimal parameter values can be found by inversion [ABSTRACT FROM AUTHOR]

Tomographic imaging based on long‐term ambient seismic noise measurements, mainly the phase information from surface waves, has been shown to be a powerful tool for geothermal reservoir imaging and monitoring. In this study, we utilize seismic noise data from a dense nodal array (192 3C nodes within 20 km2) over a ultra‐short observation period (4.7 days) to reconstruct surface waves and determine the high‐resolution (0.2 km) three‐dimensional (3‐D) S wave velocity structure beneath a rural town in Zhejiang, China. We report the advantage of cross‐coherence over cross‐correlation in suppressing pseudo‐arrivals caused by persistent sources. We use ambient noise interferometry to retrieve high quality Rayleigh waves and Love waves. Body waves are also observed on the R‐R component interferograms. We apply phase velocity dispersion measurements on both Rayleigh waves and Love waves and automatically pick more than 23,000 dispersion curves by using a Machine Learning technique. 3‐D surface wave tomographic results after depth inversion indicate low‐velocity anomalies (between −1% and −4%) from the surface to 2 km depth in the central area. Combined with the conductive characteristics observed on resistivity profile, the low‐velocity anomalies are inferred to be a fluid saturated zone of highly fractured rock. Joint interpretation based on horizontal‐to‐vertical spectral ratio (HVSR) measurements, and existing temperature and fluid resistivity records observed in a nearby well, suggests the existence of the high‐temperature geothermal field through the fracture channel. Strong correlation between HVSR measurements and the S wave velocity model highlights the potential of extraction of both amplitude and phase information from ambient noise. Plain Language Summary: The geothermal energy potential beneath our feet is vast and clean. Increased development of geothermal energy provides people a promising solution for the nation and the world as we become ever more concerned about global warming, as well as air pollution. However, the development of national geothermal resources in China is far behind the target. We utilize seismic ambient noise to produce a high‐resolution three‐dimensional (3‐D) tomographic image of the geothermal system beneath a rural town in Zhejiang, China. This technique allows us to detect velocity variations/contrasts due to subsurface inhomogeneity over an ultra‐short time window which offers the opportunity for high temporal‐resolution (several days scale) geothermal reservoir imaging and monitoring/tracking. We identify low‐velocity variations outlining a highly fractured zone from the surface to 2 km depth in the central area. We interpret the low‐velocity/high‐conductivity/high‐temperature fracture zone as a heat transfer channel of the local geothermal system. Key Points: Ambient noise data have been recorded using a dense nodal array (192 3C nodes within 20 km2) over ultra‐short observation period (4.7 days)Both surface waves (Rayleigh and Love waves) and P waves are identified in the cross‐coherence functionsS wave velocity model is consistent with existing geophysical data and suggests the existence of high‐temperature geothermal resources [ABSTRACT FROM AUTHOR]

The Gulf of Lions—Ligurian basin, in the Western Mediterranean, opened from the end of Oligocene to the Miocene as a back‐arc basin driven by rollback of the Ionian slab. Geophysical surveys across the margin have explored the crustal structure and the continent‐ocean transition (COT), but its location and petrological nature is still a matter of debate. Here, we apply joint refraction and reflection travel‐time tomography that combines travel‐times from multichannel seismic and wide‐angle seismic data to provide improved constraints on the P‐wave velocity structure of the sediment cover and the geometry of the top of the basement with respect to previous studies. Similar to earlier works, the velocity model shows three crustal domains, but their geometry and internal velocity differ. We identify a ∼100 km‐wide domain of oceanic crust with an anomalously high upper crustal velocity possibly caused by porosity‐canceling processes triggered by 7–8 km thick overlying sediment cover. The vertical velocity structure and the thickness of the oceanic crust are similar to back‐arcs in the Pacific and the Western Mediterranean. In contrast to previous interpretations, we propose that this oceanic domain is confined between two domains of ultra‐thin (4–5 km‐thick) continental crust, and that the COT is <10 km wide on either side of the oceanic crust. Plain Language Summary: The Gulf of Lions and Ligurian Basin formed during the opening of the Mediterranean Sea 30–20 Ma ago. Understanding the geology of the crust along these basins is key to infer the formation processes that governed the opening of the Western Mediterranean, and more generally, the evolution of Earth's crust. Many studies have explored this region with geophysical data but the crustal structure along these basins is yet debated. For the first time, here, we integrate two seismic data sets to explore the crustal structure along the Gulf of Lions and Ligurian Basin. The integration of these two data sets allows us to decrease the uncertainty on the final interpretation regarding the extent and petrological nature of crustal domains. Our results reveal that the oceanic crust is present offshore the Gulf of Lions, but in contrast to earlier studies, it is confined between continental crust. These results redefine the configuration of crustal domains in this region of the Western Mediterranean, showing that the continent‐ocean transition is spatially sharper than previously thought, implying that the process of oceanization was brief in space and time during the opening of this portion of the Mediterranean basins. Key Points: We interpret a confined lens‐shaped oceanic domain in between highly stretched continental crust challenging previous interpretationsThe oceanic crust shows similarities in vertical velocity structure and thickness with oceanic back‐arcs in the Pacific and MediterraneanOur results imply that the continent‐ocean transition occurs abruptly in less than 10 km at each side of the oceanic domain [ABSTRACT FROM AUTHOR]

For 2 years, the InSight lander has been recording seismic data on Mars that are vital to constrain the structure and thermochemical state of the planet. We used observations of direct (P and S) and surface-reflected (PP, PPP, SS, and SSS) body-wave phases from eight low-frequency marsquakes to constrain the interior structure to a depth of 800 kilometers. We found a structure compatible with a low-velocity zone associated with a thermal lithosphere much thicker than on Earth that is possibly related to a weak S-wave shadow zone at teleseismic distances. By combining the seismic constraints with geodynamic models, we predict that, relative to the primitive mantle, the crust is more enriched in heat-producing elements by a factor of 13 to 20. This enrichment is greater than suggested by gamma-ray surface mapping and has a moderate-to-elevated surface heat flow. [ABSTRACT FROM AUTHOR]

Seismic deployments in the Alaska subduction zone provide dense sampling of the seismic wavefield that constrains thermal structure and subduction geometry. We measure P and S attenuation from pairwise amplitude and phase spectral ratios for teleseismic body waves at 206 stations from regional and short‐term arrays. Parallel teleseismic travel‐time measurements provide information on seismic velocities at the same scale. These data show consistently low attenuation over the forearc of subduction systems and high attenuation over the arc and backarc, similar to local‐earthquake attenuation studies but at 10× lower frequencies. The pattern is seen both across the area of normal Pacific subduction in Cook Inlet, and across the Wrangell Volcanic Field where subduction has been debated. These observations confirm subduction‐dominated thermal regime beneath the latter. Travel times show evidence for subducting lithosphere much deeper than seismicity, while attenuation measurements appear mostly reflective of mantle temperature less than 150 km deep, depths where the mantle is closest to its solidus and where subduction‐related melting may take place. Travel times show strong delays over thick sedimentary basins. Attenuation signals show no evidence of absorption by basins, although some basins show signals anomalously rich in high‐frequency energy, with consequent negative apparent attenuation. Outside of basins, these data are consistent with mantle attenuation in the upper 220 km that is quantitatively similar to observations from surface waves and local‐earthquake body waves. Differences between P and S attenuation suggest primarily shear‐modulus relaxation. Overall the attenuation measurements show consistent, coherent subduction‐related structure, complementary to travel times. Plain Language Summary: Seismic waves lose more energy passing through hot and partly molten volumes than cold regions. As a result, measurements of variation in their amplitudes, or attenuation, provides a tool for mapping out the upper mantle, complementing more traditional measurements of their variation in travel time. New high‐quality arrays across southern Alaska, along with recent methodological developments, now allow this measurement to be made systematically across the entire region. They show consistently low attenuation where subducting plates are near the surface or along paths that follow the cold subducting plates. These regions are in southernmost Alaska. By contrast, signals traveling beneath volcanic regions or north of them, where hot mantle flows toward subduction zones, show high attenuation. The attenuation patterns resemble those from travel time, but seem to show more sensitivity to the upper 150 km of the Earth while travel time delays more uniformly sample deeper. Sedimentary basins show confusing signals, with travel time delays as expected for low‐wavespeed sediments, but high amplitudes that are difficult to explain. These signals allow quantitative mapping of temperature and melt variations in the upper mantle, even in regions as complex as subduction zones where both properties vary rapidly over short distances. Key Points: Body‐wave attenuation measurements reveal upper mantle structure, including subducting plates and the Alaskan sub‐/back‐arcThe Wrangell Volcanic Field is underlain by Yakutat subduction, but geometry is complexSedimentary basins unexpectedly produce negative apparent attenuation in body‐wave spectra [ABSTRACT FROM AUTHOR]

Seismic attenuation is an important parameter for characterizing subsurface morphology and thermal structure. In this study, we use P-wave amplitude spectra from 588 teleseismic events recorded by 477 broadband seismic stations in the southeastern United States to examine the spatial variations of seismic attenuation in the crust and upper mantle. The resulting seismic attenuation parameter (Δt*) measurements obtained using the spectral ratio technique reveal a zone of relatively low attenuation in the Gulf of Mexico Coastal Plain and the southwestern terminus of the Piedmont province. Spatial coherency analysis of the Δt* observations suggests that the center of the low attenuation layer is located within the uppermost mantle at about 70 km depth. This low attenuation anomaly lies along the suture zone between Laurentia and Gondwana and approximately coincides with the east-west trending Brunswick magnetic anomaly. The origin of this low attenuation anomaly can be attributed to low attenuation bodies in the form of remnant lithospheric fragments in the deep crust and the uppermost mantle. The contribution of scattering to the observed Δt* is estimated by calculating the ratio of amplitude on the transverse and vertical components in the P-wave window. Relative to the rest of the study area, the Gulf of Mexico Coastal Plain demonstrates weaker scattering which is suggestive of a more homogenous crustal and uppermost mantle structure. [ABSTRACT FROM AUTHOR]

Metasurfaces are devices employing the notion of resonance to transform seismic surface waves into body waves, thereby protecting the subwavelength structures. Using metasurfaces to control surface waves is a rapidly expanding field with intrinsic theoretical value and potential applications everywhere. In this paper, we have combined the concepts of nonlinearity, double mass system, and nonlocal elasticity to unveil the dispersive properties of hybrid Rayleigh waves. To enable a more compact construction for multi-frequency attenuation, two spring–mass resonators coupled with a nonlinear spring are used instead of single-mass resonators. A novel metasurface consisting of an array of nonlinear two-degree-of-freedom (two-mass) spring–mass systems is explored. This metasurface is connected to a nonlocal elastic substrate through a linear elastic spring. The paper provides a simple but effective analytical approach to study the dispersive properties of seismic metasurfaces. The complete explicit solutions are derived for the displacement of spring–mass systems and the Rayleigh waves in the nonlocal host substrate. Two frequency bandgaps are formed due to the presence of a dual spring–mass system in the metasurface. An attempt is made to explain the existence of these multi-frequency bandgaps as a result of the presence of multi-resonators. The effects of nonlinearities (softening and hardening) of the springs, nonlocal elasticity and relative amplitude inputs of the spring–mass system on the frequency (spectral) bandgaps are analyzed in detail via numerous plots. • The notion of resonance is utilized to convert seismic surface waves into body waves. • The aim is to provide multi-frequency attenuation by a more compact construction. • The concepts of nonlinearity, nonlocal elasticity are combined for dispersion analysis. • The effects of these variations on the produced frequency bandgaps are investigated. [ABSTRACT FROM AUTHOR]

Since explosive and impulsive seismic sources such as dynamite, air guns, gas guns or even vibroseis can have a big impact on the environment, some companies have decided to record ambient seismic noise and use it to estimate the physical properties of the subsurface. Big challenges arise when the aim is extracting body waves from recorded passive signals, especially in the presence of strong surface waves. In passive seismic signals, such body waves are usually weak in comparison to surface waves that are much more prominent. To understand the characteristics of passive signals and the effect of natural source locations, three simple synthetic models were created. To extract body waves from simulated passive signals we propose and test a Radon-correlation method. This is a time-spatial correlation of amplitudes with a train of time-shifted Dirac delta functions through different hyperbolic paths. It is tested on a two-layer horizontal model, a three-layer model that includes a dipping layer (with and without lateral heterogeneity) and also on synthetic Marmousi model data sets. Synthetic tests show that the introduced method is able to reconstruct reflection events at the correct time-offset positions that are hidden in results obtained by the general cross-correlation method. Also, a depth migrated section shows a good match between imaged horizons and the true model. It is possible to generate off-end virtual gathers by applying the method to a linear array of receivers and to construct a velocity model by semblance velocity analysis of individually extracted gathers. [ABSTRACT FROM AUTHOR]

Injection methods are a very efficient means to compute synthetic seismograms of short-period teleseismic body waves in 3-D regional models. The principle is to inject an incident teleseismic wavefield inside a regional 3-D Cartesian spectral-element grid. We have developed an opern-source package that allows us to inject either an incident plane wave, computed with a frequency–wavenumber method, or the complete wavefield, computed in a spherically symmetric reference earth model with AxiSEM. The computations inside the regional spectral-element grid are performed with SPECFEM3D_Cartesian. We compare the efficiency and reliability of the two injection methods for teleseismic P waves, considering a wide range of epicentral distance and hypocentral depths. Our simulations demonstrate that in practice the effects of wave front and Earth curvature are negligible for moderate size regional domains (several hundreds of kilometres) and for periods larger than 2 s. The main differences observed in synthetic seismograms are related to secondary phases that have a different slowness from the one of the reference P phase. [ABSTRACT FROM AUTHOR]

The eastern, passive margin of the North American continent is more heterogeneous than originally envisioned. Imaged by teleseismic body wave arrivals, both compressional Vp and shear Vsh, the lithospheric mantle of the stable, eastern North America contains rapid and large wave speed variations from the fast, older interior toward the slower, margin structures along the continental edge. At multiple regions at the continental edge, wave speeds indicate the presence of a small amount of melt, <1%–2%. Melt assessments were computed using a suite of mineralogical/petrological models to convert Vp and Vsh into absolute temperatures and melt fraction. Moreover, along the Atlantic margin, wave speeds are lower than expected for a previously rifted margin. Within the continent interior, wave speeds in the mantle lithosphere vary spatially at wavelengths, suggesting strong compositional and mineralogical influence likely a result of later stage alteration from volatile rich fluids. Key Points: Small amounts of mantle melt is present in isolated regions along the eastern coast of North AmericaNorth American continental lithosphere was likely modified by later‐stage, volatile rich melts from the underlying mantleCompressional and Shear wave‐speed, finite‐frequency model of eastern North American show a stable, rich, but complex tectonic plate [ABSTRACT FROM AUTHOR]

Three‐dimensional, high‐resolution crustal and upper mantle P and S wave velocity (Vp and Vs) models are presented for the central United States using body wave tomography. The study utilizes local and teleseismic data recorded by the Northern Embayment Lithospheric Experiment stations, the CERI New Madrid Seismic Network, the Earthscope Transportable Array, and the Ozark Illinois INdiana Kentucky Flexible Array. Vp and Vs solutions are very similar and are well resolved in the depth range 40 to 400 km. Two anomalously slow regions are present below the Illinois Basin, forming a northwest dipping low‐velocity zone (LVZ) extending from ~200 to 400 km. Maximum anomaly magnitudes in the LVZ reach about −4% and −5% for Vp and Vs, respectively. The LVZ appears to connect to a well‐documented LVZ located below the northern Mississippi Embayment. The Illinois Basin velocity anomalies cannot be explained by elevated temperature alone and require compositional heterogeneity involving elevated orthopyroxene content in addition to an increase in iron and water content. The need for additional orthopyroxene suggests that the LVZ is being produced by metasomatism of mantle rocks by hydrous, silica‐rich fluids ascending from a slab fragment trapped in or near the transition zone. This supports previous interpretations for the existence of the LVZ below the Embayment. We suggest that the LVZs below the Mississippi Embayment and the Illinois Basin are linked to the presence of a large igneous province, which convection models suggest is currently located below the central United States. Plain Language Summary: We present three‐dimensional, high‐resolution velocity models from the Earth's surface to 410 km depth under the central United States. Our study utilizes seismic tomography—a technique that images the velocity structure of the Earth using seismic waves that traveled from earthquake sources to seismic stations. Travel‐time data recorded by several seismic arrays (dense deployments of seismic stations) were acquired to increase the resolution of our model. In the resulting model, compressional wave and shear wave velocity anomalies are similar and are well‐resolved in the depth range 40 to 400 km. An anomalously slow region is present below the Illinois Basin that dips to the northwest and extends to 400 km. The low velocity cannot be explained by elevated temperature alone and requires an increase in iron and water content as well as high amounts of orthopyroxene. Elevated orthopyroxene suggests that the mantle rocks have been chemically altered due to fluids coming from a trapped slab fragment. We suggest that the low‐velocity zone below the Illinois Basin is associated with the presence of a slab fragment containing a thick oceanic plateau. According to several studies, the plateau was subducted millions of years ago and is presently located underneath the central United States. Key Points: We present detailed 3‐D Vp and Vs models for the upper mantle beneath the central United States using data recorded by two FlexArraysTwo prominent low‐velocity anomalies are imaged at 240 and 370 km depth beneath the Illinois BasinThe origin of these anomalies is attributed in part to the metasomatism of mantle rocks by silica‐rich fluids [ABSTRACT FROM AUTHOR]

The northwestern part of the Gulf of Mexico has undergone two episodes of continental rifting and collision and produced structural artifacts that are now buried under many kilometers of sediments, complicating investigations of the region. The deep sedimentary package precludes outcrops and points to a need for the application of seismic techniques, but low rates of seismicity in the region and sparse seismic monitoring limit the utility of traditional seismic methods. We therefore use diverse data to perform two‐dimensional seismic tomography across the dry land portion of the margin. Data are gleaned from teleseismic and regional earthquakes, postcritical SsPmp arrivals, and direct P wave energy identified with seismic interferometry that were recorded by a broadband, three‐component array and partially overlapping short‐period, vertical‐component array. The Pn and postcritical SsPmp phase help constrain the Moho discontinuity, which a previous receiver function study suggested was absent beneath the seaward portion of this transect. A high‐velocity body is observed in the crust at the same location as the Houston Magnetic Anomaly, possibly marking rocks from the Alleghenian continental assembly. The crust thins from NW to SE, indicating that extension occurred mostly to the south of the Ouachita orogeny. Our model indicates that the margin's sediment package reaches a maximum thickness of ~15 km at the coast and becomes unresolvably thin near the Llano Uplift. Plain Language Summary: Structural features buried under the thick sediments in the onshore portion of the Gulf coastal plain may be keys to understanding the process that led to the formation of the present‐day Gulf of Mexico. However, obtaining a model of these structures is challenging for several reasons. First, the Gulf of Mexico has very low rates of seismicity; second, the sediments rapidly reduce seismic energy; third, recording instruments have historically been quite sparse. Together, these limit the use of traditional seismic methods. We therefore use complementary data sets to model the structures, including records of distant earthquakes and local earthquakes plus P arrivals identified with seismic interferometry. We observe a high‐velocity body in the crust that lies beneath the Houston Magnetic anomaly, indicating a possible origin associated with the continental assembly of Africa and North America that occurred approximately 325 to 260 million years ago. Key Points: A high seismic velocity body is observed at the approximate location of the Houston Magnetic Anomaly possibly marking rocks from the Alleghenian orogenyCrustal extension occurred mostly to the south of the Ouachita orogenySediment thickness varies from a maximum thickness of approximately 15 km at the coast to unresolvably thin near the Llano uplift [ABSTRACT FROM AUTHOR]

Biological cells and many living organisms are mostly made of liquids and therefore, by analogy with liquid drops, they should exhibit a range of fundamental nonlinear phenomena such as the onset of standing surface waves. Here, we test four common species of earthworm to demonstrate that vertical vibration of living worms lying horizontally on a flat solid surface results in the onset of subharmonic Faraday-like body waves, which is possible because earthworms have a hydrostatic skeleton with a flexible skin and a liquid-filled body cavity. Our findings are supported by theoretical analysis based on a model of parametrically excited vibrations in liquid-filled elastic cylinders using material parameters of the worm's body reported in the literature. The ability to excite nonlinear subharmonic body waves in a living organism could be used to probe, and potentially to control, important biophysical processes such as the propagation of nerve impulses, thereby opening up avenues for addressing biological questions of fundamental impact. [ABSTRACT FROM AUTHOR]

The rupture process of the 2018 Mw 7.9 offshore Kodiak (Alaska) earthquake is still in hot dispute because of a lack of offshore observations, thus causing difficulties for understanding seismogenic tectonics and tsunami hazards. In this study, teleseismic body waves, high‐rate GPS, seismic waves preceding tsunami waves, static GPS, and tsunami data are jointly used to resolve the faulting geometry and coseismic slip distribution of the offshore Kodiak earthquake. Tests of a series of finite fault rupture models illustrate that an optimal five‐fault model can reconcile all available data sets well. The results reveal that the asperity on each fault segment is located near the hypocenter, with peak slip of ~7.8 m. The aftershock loci appear to be complementary to the mainshock slips, demonstrating the velocity‐strengthening regions that predominantly slip aseismically. Based on a tectonic stress field analysis, we propose that the 2018 Kodiak earthquake is attributed to a combination of enhanced tensional stress following the 1964 Mw 9.2 Alaska earthquake and compressional stress produced by the collision of the Yakutat terrane with North America. Key Points: A complex quasi‐orthogonal strike‐slip fault system is revealed by seismic, geodetic, and DART dataNear‐field high‐rate GPS and DART data are involved in the joint inversion to improve the rupture modelShear stress between the 1964 Mw 9.2 Alaska earthquake and collision of the Yakutat terrane with North America favors strike‐slip events [ABSTRACT FROM AUTHOR]

The reconstruction of body waves from the cross‐correlation of random wavefields has recently emerged as a promising approach to probe the fine‐scale structure of the Earth. However, because of the nature of the ambient noise field, the retrieval of body waves from seismic noise recordings is highly challenging and has only been successful in a few cases. Here, we use seismic noise data from a 5,200‐node oil‐company survey to reconstruct body waves and determine the velocity structure beneath Long Beach, California. To isolate the body wave energy from the ambient noise field, we divide the entire survey into small‐aperture subarrays and apply a modified double‐beamforming scheme to enhance coherent arrivals within the cross‐correlated waveforms. The resulting beamed traces allow us to identify clear refracted P waves traveling between different subarray pairs, which we then use to construct a high‐resolution 3D velocity model of the region. The inverted velocity model reveals velocity variations of the order of 3% and strong lateral discontinuities caused by the presence of sharp geologic structures such as the Newport‐Inglewood fault (NIF). Additionally, we show that the resolution that is achieved through the use of high‐frequency body waves allows us to illuminate small geometric variations of the NIF that were previously unresolved with traditional passive imaging methods. Key Points: Body waves are identified within the ambient noise recorded by a dense seismic array in Long Beach, CaliforniaArray processing tools are used to retrieve refracted P waves propagating through different sections of the arrayTraveltime measurements are made to estimate 3D P wave velocities beneath the array [ABSTRACT FROM AUTHOR]

EarthScope's USArray Transportable Array has shortcomings for the purpose of interpreting geologic features of wavelengths less than the Transportable Array station spacing, but these can be overcome by using higher spatial resolution gravity data. In this study, we exploit USArray receiver functions to reduce nonuniqueness in the interpretation of gravity anomalies. We model gravity anomalies from previously derived density variations of sedimentary basins, crustal Vp/Vs variation, Moho variation, and upper mantle density variation derived from body wave imaging informed by surface wave tomography to estimate Vp/Vs. Although average densities and density contrasts for these seismic variations can be derived, the gravity anomalies modeled from them do not explain the entire observed gravity anomaly field in the United States. We use the unmodeled gravity anomalies (residuals) to reconstruct local variations in densities of the crust associated with geologic sources. The approach uses velocity‐density relationships and differs from density computations that assume isostatic compensation. These intracrustal densities identify geologic sources not sampled by and, in some cases, aliased by the USArray station spacing. We show an example of this improvement in the vicinity of the Bloomfield Pluton, north of the bootheel of Missouri, in the central United States. Key Points: Central and eastern U.S. Moho density contrast and velocity‐density relationships for crustal and upper mantle Vp/Vs ratios are estimated using gravity dataDenser sampling of gravity data interpolates seismic models and reduces aliasing of geologic features caused by the USArray station spacingHigh‐resolution crustal density variation constrained by seismic information in the central and eastern United States provides detailed information for interpretation [ABSTRACT FROM AUTHOR]

The Northern Canadian Cordillera (NCC) is an actively deforming orogenic belt in northwestern Canada. Geochemical and geophysical data show that the NCC is underlain by a thin and hot lithosphere, in contrast with the adjacent cold and thick cratonic lithosphere to the east. This juxtaposition of cold/hot and thick/thin lithosphere across a narrow transition zone has important implications for regional geodynamics. The recent deployment of USArray Transportable Array and other seismic stations across Alaska, USA, and northwestern Canada allows us to image lithosphere and upper mantle three-dimensional seismic velocity structure at significantly improved resolution. Our model reveals a broad high-velocity anomaly across northern Yukon and Northwest Territories, which is interpreted as buried cratonic lithosphere and which we refer to as the Mackenzie craton. Another prominent high-velocity anomaly is imaged beneath northeastern British Columbia and is interpreted to indicate cratonic lithosphere beneath the Northern Rocky Mountains. These two mechanically strong lithospheric blocks, also suggested by regional magnetic data, are interpreted to buttress the ends of the Mackenzie Mountains fold and thrust belt, guiding intervening cordilleran mantle flow toward the Canadian Shield and controlling the arcuate geometry of the Mackenzie Mountains fold and thrust belt. Both P and S wave models also reveal the signature of a northward dipping, subductingWrangell slab across the southern region of the Alaska/Yukon border. Strong P and S wave velocity contrasts across the Tintina Fault suggest that it is a lithosphere-scale shear zone that extends into the upper mantle beneath the NCC and demarcates distinct regions of lithospheric mantle. [ABSTRACT FROM AUTHOR]

A new azimuthal anisotropy model for the North American and Caribbean Plates, namely, US32, is constructed based on full waveform inversion and records from the USArray and other temporary/permanent networks deployed in the study region. A total of 180 earthquakes and 4,516 seismographic stations are employed in the inversion to simultaneously constrain radially and azimuthally anisotropic model parameters: L, N, Gc, and Gs, within the crust and mantle. Thirty-two preconditioned conjugate gradient iterations have been utilized to minimize frequency-dependent phase discrepancies between observed and predicted seismograms for three-component short-period (15-40 s) body waves and long-period (25-100 s) surface waves. Model US32 exhibits complicated variations in anisotropic fabrics underneath the western and eastern United States, especially at depths shallower than 100 km. For instance, the fast axis orientations in model US32 suggest the presence of trench-perpendicular mantle flows underneath the Cascadia Subduction Zone and also follow the strikes of the Snake River Plain, the Ouachita Orogenic Front, and the Grenville and Appalachian Orogenic Belts. The amplitudes of azimuthal anisotropy reduce to around 1% at depths greater than 200 km, and the orientations are subparallel to the global plate motion directions to the east of the Rocky Mountain, except for large discrepancies in central and eastern Canada. At a depth of 700 km, the fast axes change along the trajectory of the Farallon slab underneath the Great Lakes region and Gulf of Mexico, which might indicate the development of 2-D poloidal-mode mantle flows perpendicular to the strike of the sinking slab within the uppermost lower mantle. Comparisons between model US32 with a western U.S. model from ambient noise tomography and SKS splitting measurements demonstrate a relatively good agreement for the fast axis orientations, considering the usage of different data sets and imaging techniques. However, the absolute magnitude of azimuthal anisotropy in model US32 might be underestimated, especially at greater depths, given the poor agreement on the amplitudes of predicted and observed SKS splitting times. At the current stage, the agreement among different azimuthal anisotropy models at global and continental scales is still poor even for the United States with a dense station coverage. [ABSTRACT FROM AUTHOR]

Ambient noise seismic interferometry performed by cross-correlation has been proven to be a potential cost-effective technique for geological studies. To improve the resolution of images created by interferometry, additional techniques using deconvolution and cross-coherence have been introduced. While all three methods have previously been evaluated using surface wave data for shear-wave imaging of the near surface, comparatively little study has been devoted to assess the three methods for the retrieval of body waves in reflection surveys for time-lapse application. Moreover, although the application of seismic interferometry to CO2 sequestration by cross-correlation has been investigated by many researchers, to our knowledge, similar time-lapse studies have not been conducted using deconvolution and cross-coherence methods. We evaluate the three methods of cross-correlation, deconvolution and cross-coherence for the retrieval of phase information contained in virtual seismic records by applying seismic interferometry to synthetic data, using a model reservoir before and after CO2 injection. By examining two approaches of regularization and smoothing factors to suppress spurious reflection events observed on the deconvolution and cross-coherence results, we note that both approaches provide similar results. We investigate noise effects by adding random noise independently at each geophone. Finally, we apply these techniques to field data recorded near the CO2 storage site in Ketzin, Germany. For both our numerical and field data studies, we find that the cross-coherence technique retrieves the phase information of body-wave data more effectively than the cross-correlation and deconvolution techniques, and is less sensitive to uncorrelated noise from shallow sources. [ABSTRACT FROM AUTHOR]

Slip during large crustal earthquakes continues for extended periods of time on spatially extensive fault planes. High‐frequency body waves from other parts of the fault system thus impinge on patches of the fault that are still actively sliding. Intuitively, the material within sliding fault planes does not rheologically distinguish between the low‐frequency stresses driving the gross earthquake and high‐frequency dynamic stresses from impinging body waves. Nonlinear interaction occurs, interrogating the rheology of the fault plane. High‐frequency S waves nonlinearly produce additional inelastic slip ΔS on the sliding fault. The ratio of this slip to the elastic displacement (ΔS/ΔSE), where ΔSE is the elastic displacement, depends on the slip‐reflection number, (∂V/∂τ)ρβ, where ∂V/∂τ is the derivative of sliding velocity with respect to shear traction, ρ is density, and β is S‐wave velocity. The sliding fault transmits and weakly reflects S waves for small values ≪2 of the parameter; it weakly transmits and strongly reflects for high values ≫2. It is relevant to evaluate ∂V/∂τ at the long‐period slip velocity V0 and shear traction τ0 of the gross earthquake. The ratio is then ΔS/ΔSE = ρβV0Φ/τ0, where Φ ≡ (∂V/∂τ)(τ0/V0) is measure of the nonlinearity the fault rheology. For example, the parameter Φ at high‐frequency impinging S waves is the ratio μ0/a of the coefficient of friction to the rate parameter a ≈ 0.01 for rate and state friction. Weak sliding faults (low τ0 and high V0) strongly reflect impinging S waves. During waning slip, dynamic stresses from S waves antithetical to the gross slip direction and compressional P waves may heterogeneously lock the fault. Key Points: High‐frequency S waves should reflect from sliding faults and be partially transmittedThe compressional cycle of P waves transiently strengths the fault surface slowly the slip rate, which generates secondary body wavesHigh‐frequency body waves cause the fault to lock heterogeneously during waning slip [ABSTRACT FROM AUTHOR]

The mantle transition zone (MTZ) of Earth is demarcated by solid‐to‐solid phase changes of the mineral olivine that produce seismic discontinuities at 410 and 660‐km depths. Mineral physics experiments predict that wadsleyite can have strong single‐crystal anisotropy at the pressure and temperature conditions of the MTZ. Thus, significant seismic anisotropy is possible in the upper MTZ where lattice‐preferred orientation of wadsleyite is produced by mantle flow. Here, we use a body wave method, SS precursors, to study the topography change and seismic anisotropy near the MTZ discontinuities. We stack the data to explore the azimuthal dependence of travel‐times and amplitudes of SS precursors and constrain the azimuthal anisotropy in the MTZ. Beneath the central Pacific, we find evidence for ~4% anisotropy with a SE fast direction in the upper mantle and no significant anisotropy in the MTZ. In subduction zones, we observe ~4% anisotropy with a trench‐parallel fast direction in the upper mantle and ~3% anisotropy with a trench‐perpendicular fast direction in the MTZ. The transition of fast directions indicates that the lattice‐preferred orientation of wadsleyite induced by MTZ flow is organized separately from the flow in the upper mantle. Global azimuthal stacking reveals ~1% azimuthal anisotropy in the upper mantle but negligible anisotropy (<1%) in the MTZ. Finally, we correct for the upper mantle and MTZ anisotropy structures to obtain a new MTZ topography model. The anisotropy correction produces ±3 km difference and therefore has minor overall effects on global MTZ topography. Key Points: The MTZ anisotropy is investigated from the SS precursors at both regional and global scalesWe observe ~3% anisotropy in the MTZ beneath subduction zones but negligible MTZ anisotropy (<1%) at a global scaleWe propose a new MTZ topography model corrected for the upper mantle and MTZ anisotropy structures [ABSTRACT FROM AUTHOR]

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Abstract An alternative manner is provided for solving the classical linearised problem of the radiation and diffraction of regular water waves caused by oscillation of a floating body in deep water. It is shown that the singular wave integrals of the three-dimensional free surface Green function G and its gradient ∇ G can be regarded as regular wave integrals and are integrated directly. The method is validated by comparing with benchmark data for a floating or submerged body undergoing oscillatory wave motions. The comparison shows that the evaluation is sufficiently accurate for practical purposes. As the significance of the method, the numerical approximation stability for the gradient ∇ G is shown to be the same with that for G. [ABSTRACT FROM AUTHOR]

We retrieve the local P wave empirical Green's functions between the elements of five different regional arrays across the globe by cross‐correlating and bin stacking the teleseismic earthquake coda waves recorded at each array. The stack is made using the coda of P and S wave phases for events in the distance range from 40° to 50° from the center of the array. With a sequence of time windows along the coda the various body wave arrivals can be tracked, using record sections constructed by binning the stacked interstation correlograms in less than 1‐km distance increments. The correlation of the coda part of each principal seismic phase produces highly coherent interstation arrivals for different analysis windows. Such arrivals can be reproduced by just stacking 100 arrivals from a pool of more than a thousand events, showing the stability of the observed Green's functions. Modeling for the structure beneath the Warramunga array in the Northern Territory, Australia, demonstrates that these arrivals correspond to multiply reflected arrivals from layers at different depths. The recovery of high‐frequency interstation body waves from the teleseismic earthquake coda opens the prospect of conducting local high‐resolution seismic imaging with teleseismic energy. Key Points: We observe emergence of high‐frequency interstation body wave reflections from the correlation of teleseismic coda arrivals across regional arrays The retrieved body waves at regional distances are useful to conduct local seismic imaging beneath seismic arrays Modeling with a simplified three‐layer medium shows some level of agreement with the observed arrivals for Warramunga Array in Australia [ABSTRACT FROM AUTHOR]

As new techniques exploiting the Earth's ambient seismic noise field are developed and applied, such as for the observation of temporal changes in seismic velocity structure, it is crucial to quantify the precision with which wave‐type measurements can be made. This work uses array data at the Homestake mine in Lead, South Dakota, and an array at Sweetwater, Texas, to consider two aspects that control this precision: the types of seismic wave contributing to the ambient noise field at microseism frequencies and the effect of array geometry. Both are quantified using measurements of wavefield coherence between stations in combination with Wiener filters. We find a strong seasonal change between body‐wave and surface‐wave content. Regarding the inclusion of underground stations, we quantify the lower limit to which the ambient noise field can be characterized and reproduced; the applications of the Wiener filters are about 4 times more successful in reproducing ambient noise waveforms when underground stations are included in the array, resulting in predictions of seismic time series with less than a 1% residual, and are ultimately limited by the geometry and aperture of the array, as well as by temporal variations in the seismic field. We discuss the implications of these results for the geophysics community performing ambient seismic noise studies, as well as for the cancellation of seismic Newtonian gravity noise in ground‐based, sub‐Hertz, gravitational‐wave detectors. Key Points: Correlation measurements indicate that body waves often prevail in ambient seismic noise at 0.2 HzPrecise prediction of seismic signals using data from an array including underground seismometers is possible [ABSTRACT FROM AUTHOR]

Full‐waveform inversion (FWI) optimizes the subsurface properties of geophysical Earth models in such a way that the modeled data, based on these subsurface properties, match the observed data. The anisotropic properties, whether monoclinic, orthorhombic, triclinic, or vertical transversally isotropic (VTI), of the subsurface, be it a fractured reservoir or the core‐mantle boundary, are necessary to describe the observed wave phenomena. There are no principal limitations on the complexity of the anisotropy that can be inverted using FWI. However, the question remains—what kind of anisotropic descriptions of the elastic properties of the Earth can or cannot be inverted reliably from seismic waveforms? We reveal the resolution that can be achieved through reconstructions of each elastic parameter by building vertical resolution patterns from the scattering radiation patterns of body waves. A visual analysis of these patterns indicates "trade‐offs," that is, perturbations of parameters that have the same reflection‐based scattering patterns as other perturbations. Each trade‐off leads to an apparent ambiguity in the inversion, which must be addressed by additional assumptions, constraints, or regularizations. For orthorhombic media, we find the exact trade‐offs that exist between parameters. Our parameterization isolates the VTI parameters, and therefore, we also obtain trade‐offs for every scattering mode for VTI media. We discover that only monoclinic parameters are recoverable from the first‐order scattering of monotypic waves. We summarize the trade‐offs in tables for easy reference. This paper is intended to be useful for researchers setting up anisotropic FWI problems, and interpreting or controlling the quality of such inversions. Key Points: We share the resolution limits for each anisotropic elastic parameter in waveform inversionFor orthorhombic and VTI media we map the null space of the inversionConverted waves are necessary to move the inversion beyond monoclinic anisotropy type [ABSTRACT FROM AUTHOR]

We determine m B, the original body wave magnitude developed by Gutenberg and Richter over the period 1942–1956, for about 3300  M w ≥ 6 earthquakes for the period 1988–present using modern broad-band seismograms. The main objective is to extend the database of energy-related parameters by combining m B databases for recent and old events. The radiated energy E R_B (in erg) computed from m B using the Gutenberg & Richter relation |$\log {E_{\mathrm{ R}\_\mathrm{ B}}} = 2.4{m_\mathrm{ B}} + 5.8$| agrees very well with E R estimated with modern techniques, especially for large deep earthquakes. Thus, E R_B is useful as a proxy for E R to investigate the global diversity of earthquake characteristics and physics over an extended period of time. [ABSTRACT FROM AUTHOR]

The article offers information on earthquakes. Topics include patterns exhibited by earthquakes at all depths in the oceanic regimes around the Pacific margin; and discusses the Earth's crust which must be in a natural state, if it is going to be susceptible to Reservoir-Induced Seismicity (RIS). Article also presents that once RIS begun beneath or close to the reservoir, it tends to migrate around, affecting a whole series of discontinuities in the rock mass, over time.

The article offers information on the series of earthquakes named as the New Madrid, Missouri, earthquakes. Topics include that body-wave magnitude values was estimated from isoseismal maps; and discusses about the first earthquake which caused slight damage to man-made structures, mainly because of the sparse population in the epicentral area. Article also presents strange happenings during earthquake as it was biggest earthquakes in American history.

This study, analyses the seismic response for a rigid massless square foundation resting on a viscoelastic soil layer limited by rigid bedrock. The foundation is subjected either to externally applied forces or to obliquely incident seismic body or surface harmonic seismic waves P, SV and SH. A 3-D frequency domain BEM formulation in conjunction with the thin layer method (TLM) is adapted here for the solution of elastodynamic problems and used for obtained the seismic response. The mathematical approach is based on the method of integral equations in the frequency domain using the formalism of Green's functions (Kausel and Peck 1982) for layered soil, the impedance functions are calculated by the compatibility condition. In this study, The key step is the characterization of the soil-foundation interaction with the input motion matrix. For each frequency the impedance matrix connects the applied forces to the resulting displacement, and the input motion matrix connects the displacement vector of the foundation to amplitudes of the free field motion. This approach has been applied to analyze the effect of soil-structure interaction on the seismic response of the foundation resting on a viscoelastic soil layer limited by rigid bedrock. [ABSTRACT FROM AUTHOR]

Abstract The Green's function between two receivers can be retrieved using seismic interferometry (SI) by cross-correlation, as if one of the receivers were a virtual seismic source. When the wavefields experience intrinsic losses during propagation, non-physical arrivals (ghosts) would appear in the retrieved result. These ghosts are a result of internal reflections inside the different layers lying between the subsurface sources and the receivers. Recent studies have introduced a stable method to monitor the layer-specific changes in quality factor (Q) using the ghosts retrieved by SI applied to a horizontal-well data. However, drilling a horizontal well is much more complicated and expensive than drilling a conventional vertical well. Because of this, we show here how the Q-estimation method introduced for the horizontal well can be adapted to monitor layer-specific changes of Q using a vertical well. In order to improve the accuracy of the Q-estimation, we propose a grid-searching method to detect the optimal effective Q. We illustrate our method using numerically modelling data from a horizontal and a vertical well. Highlights • Method for quality-factor estimation using body-wave reflections from a vertical-well data • The method uses ghost body-wave reflections retrieved by seismic interferometry by autocorrelation • Method validated with data from numerical modelling [ABSTRACT FROM AUTHOR]

Robust and automatic seismic phase picking always remain a challenging task for accurate locating of hypocentre and understanding of focal mechanism. Several algorithms have been previously proposed to meet the requirement to some extent. However, most of them depend on manually presetting and adjusting parameters during the picking process, which readily cause large uncertainties for the recognition of wave arrivals. We here develop a new refining picker which adopts the frequency spectrum variation analysis (FSVA) to perform the adaptive phase onset identification for P and S wave. Its characteristics of high sensitivity to seismic signals in time and frequency domains allow the accurate recognition of P and S phase arrivals. For the regional data, our proposed method can improve the picking efficiency of P -wave phase together with STA/LTA. For the preliminary picking of S -wave recorded at regional distance, we first use the polarization filter to remove P -wave energy from seismograms, then combine STA/LTA and proposed method to refine the identification. For the teleseismic records, the wavelet transformation is first utilized to enhance the signal-to-noise ratio of waveform data. We then utilize the AIC (Akaike Information Criterion) for the preliminary identification, and the proposed method for refining the pick. The results from synthetic seismic data show that the developed method decreases significantly the poor picks by 20.2 per cent for P -phase and 28.2 per cent for S -phase, respectively. We select a catalogue of nearly 20 earthquakes larger than M w4.0 which occurred around the plate boundaries in the eastern Pacific coast during 2014–2017, and utilize the developed method to refine the phase onset identification. The proportion of quality picks with absolute error less than 1.0 s improves from 64.84 to 83.52 per cent for P -phase, and 39.19–56.04 per cent for S -phase, which shows good robustness and automation capability of the developed picker for the data sets recorded at both regional and teleseismic stations. [ABSTRACT FROM AUTHOR]

We investigate the usability of the converted phase SP and its precursors, which reflect off the underside of upper-mantle seismic discontinuities. In contrast to PP and SS waves, the SP phases do not reflect midway between source and receiver but about three quarters on the great circle path. This leads to extended data coverage, especially in oceanic regions, where usually few receivers are deployed. Due to similar traveltimes and incidence angles, SP is difficult to distinguish from the PS phase. One feature that makes it possible to separate these two waves is their polarization. Therefore, we developed a polarization filter, which allows detecting precursor signals of SP in vespagrams. For this feasibility study, we analysed events from all azimuthal directions with M w≥ 5.8 and ranging between 80° and 140° epicentral distance recorded at the western part of the Transportable Array in the United States. Even though this method has several restrictions like limited distance and depth ranges for which the precursor signals are clearly identifiable, this study resulted in 52 events showing signals reflected off the underside of the 410 and/or 660 km discontinuity. Our averaged results show a deepened 410 km discontinuity beneath the Gulf of Alaska, central Alaska and western Canada and a shallower 410 km discontinuity beneath the northern east Pacific Ocean and the coast of Mexico. For the 660 km discontinuity we find fewer reflections. This discontinuity seems to be slightly elevated in central Alaska and beneath the Pacific shore of Mexico. The southern coast of Alaska and parts of Canada show a deepened 660 km discontinuity. These observations agree with previous results of PP -precursors, SS -precursors and receiver function studies. We show that SP precursors are a successful new approach to map upper-mantle seismic discontinuities. [ABSTRACT FROM AUTHOR]

Time-lapse seismic tomography aims at obtaining temporal velocity variations of the subsurface medium in different time periods, which could be very useful for volcanic monitoring, earthquake prediction and fault zone damage evaluation. The standard time-lapse velocity tomograms are generally obtained by subtracting velocity models resulting from separate seismic tomography for different time periods. However, this could introduce some artefacts in temporal velocity changes because of different data distribution and data quality at different time periods. In this study, we propose a new time-lapse seismic velocity tomography method that is based on the concept of double-difference (DD) seismic tomography. We redefine the DD equation by considering two events from two different time periods in order to evaluate the temporal velocity changes. The new method inverts data from two adjacent epochs simultaneously for the temporal velocity model changes to minimize differential arrival time residuals. As a result, it is less affected by different data distribution and quality in different time periods. We have applied the new method to a surface microseismic monitoring dataset for underground longwall coal mining. It is known that risks associated with unstable rock formation due to stress redistribution caused by coal mining could be devastating. By evaluating temporal velocity changes, we could derive stress redistribution due to mining process and then better understand potential risks. The resolution tests show that compared to standard time-lapse tomography method, our proposed method has higher resolutions in recovering temporal velocity changes. The application to the real microseismic dataset clearly shows that velocity increases by ∼0.15 km s–1in the front of mining face where the stress increases and velocity decreases by ∼0.23 km s–1in the gob area where the materials are loose, which is expected from numerical modelling of stress distribution and is more consistent than the standard time-lapse seismic tomography. [ABSTRACT FROM AUTHOR]

We apply receiver function (RF) methodology to map the geometry of a sedimentary basin along a ∼10-km-long profile of broadband seismometers that recorded continuously for approximately 3 months. For a subset of the stations, we apply the Neighbourhood Algorithm inversion scheme, to quantify the geometry of basin bounding fault directly beneath the stations. We compare our results with active reflection seismic data and with the lithostratigraphy from a well located along the profile. We find that the P -to-s conversions from the sediments–basement interface (SBI), recorded in RF data sets together with information on intrabasin structures, are useful for obtaining high resolution images of the basin. The depth of the SBI derived from RF inversion is consistent (within ∼0.4 km) with the estimates from active reflection seismic and the well data. This study highlights that analysis of teleseismic waveforms can retrieve relevant information on the structure of a sedimentary basin. [ABSTRACT FROM AUTHOR]

Different from the stacked seismic data, pre-stack data includes abundant information about shear wave and density. Through inversing the shear wave and density information from the pre-stack data, we can determine oil-bearing properties from different incident angles. The state-of-the-art inversion methods obtain either low vertical resolution or lateral discontinuities. However, the practical reservoir generally has sharp discontinuities between different layers in vertically direction and is horizontally smooth. Towards obtaining the practical model, we present an inversion method based on the regularized amplitude-versus-incidence angle (AVA) data to estimate the piecewise-smooth model from pre-stack seismic data. This method considers subsurface stratum as a combination of two parts: a piecewise smooth part and a constant part. To fix the ill-posedness in the inversion, we adopt four terms to define the AVA inversion misfit function: the data misfit itself, a total variation regularization term acting as a sparsing operator for the piecewise constant part, a Tikhonov regularization term acting as a smoothing operator for the smooth part, and the last term to smoothly incorporate a priori information for constraining the magnitude of the estimated model. The proposed method not only can incorporate structure information and a priori model constraint, but also is able to derive into a convex objective function that can be easily minimized using iterative approach. Compared with inversion results of TV and Tikhonov regularization methods, the inverted P-wave velocity, S-wave velocity and density of the proposed method can better delineate the piecewise-smooth characteristic of strata. [ABSTRACT FROM AUTHOR]