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Abstract We present a novel three-dimensional model of compressional wave attenuation (1/Q P ) for the Eastern and eastern Southern Alps in Europe that includes the eastern part of the Adriatic indenter, termed here the Dolomites Sub-Indenter. Our approach employed waveform data from the SWATH-D network, a dense temporary network operational between 2017 and 2019, as well as selected stations of the larger AlpArray Seismic Network. A spectral inversion method using frequency-independent quality factor Q P , was applied to derive 3578 path-averaged attenuation values (t*) from 126 local earthquakes. These were then inverted using the damped least square inversion (local earthquake tomography) for the attenuation structure. The resulting Q P model, which builds on and complements a previously calculated 3-D velocity model (V P and V P /V S ), exhibits good resolution down to ~ 20 km depth. Several anomalies can be correlated with the distribution of other physical parameters (V P and V P /V S ) and regional tectonic features. Notably, the Friuli-Venetian region exhibits the highest attenuation (lowest Q P ) anomaly, coinciding with low V P values and increased V P /V S . This anomaly is likely associated with a high density of faults and fractures, as well as the presence of fluid-filled sediments along the active thrust front in the eastern segment of the Southern Alps. Another intriguing observation is the low attenuation (high Q P ) anomaly along the northwestern edge of the Dolomites Sub-Indenter (NWDI), located south of the Periadriatic fault and east of the Giudicarie fault, where seismicity is notably absent. This anomaly coincides with Permian magmatic rocks at the surface and may be a measure of their strength at depth. Graphical Abstract

Abstract Body wave extraction from oceanic secondary microseismic sources with seismic interferometry provides alternative information to better constrain the Earth's structure. However, sources' spatiotemporal variations raise concerns about travel time measurement robustness. Therefore, we study the cross‐correlations’ stability during a single oceanic event. This study focuses on 3 days of data and three seismic arrays' combinations between 8 and 11 December 2014 during storm Alexandra, a “weather bomb” event in southern Greenland. We use the WAVEWATCH III hindcast to model P‐wave noise sources and assess the impact of short‐term source variations on cross‐correlations. Model‐based cross‐correlations compared to data show coherent delays to reference 3D Earth models (∼0–3 s) confirming the robustness of the source model which could explain minor travel time variations (≤1 s).

Abstract The 2016 Kaikoura earthquake, New Zealand, ruptured more than a dozen faults, making it difficult to prescribe a model fault for analyzing the event by inversion. To model this earthquake from teleseismic records, we used a potency density tensor inversion, which projects multiple fault slips onto a single model fault plane, reducing the non-uniqueness due to the uncertainty in selecting the faults’ orientations. The resulting distribution of potency-rate density tensors is consistent with observed surface ruptures. In its initial stage, the rupture propagated northeastward primarily at shallow depths. Later, the rupture propagated northeastward at greater depths beneath a gap in reported surface ruptures. The main rupture phase started in the northeastern part of the Kekerengu fault after 50 s and propagated bilaterally to the northeast and southwest. The non-double-couple component grew to a large fraction of the source elements as the rupture went through the junction of the Jordan Thrust and the Papatea fault, which suggests that the rupture branched into both faults as it back-propagated toward the southwest. The potency density tensor inversion sheds new light on the irregular evolution of this earthquake, which produced a fault rupture pattern of unprecedented complexity. Our source model of the 2016 Kaikoura earthquake (e.g., back-rupture propagation) could prompt research to determine a more realistic model with segmented faults using near-field data.

Global seismological observations are sensitive to oceanic forcings, namely microseisms. In addition to dominant surface waves, these sources generate body waves that travel through the deep structures of our planet. Despite these sources’ inherent complexity, interferometric methods allow isolating coherent waves for imaging applications. For a given station pair, only specific microseism events contribute to the illumination of a specific target. We propose an opportune workflow based on ocean sea state models to extract robust interferences. This approach is illustrated with a strong microseism source in the North Atlantic Ocean, occurring around December 9, 2014.

Abstract We measure group velocity dispersion of surface waves generated by two meteoroid impacts on Mars close to the lander of the InSight mission. This allows us to probe the crustal structure in the first few kilometers beneath the InSight lander. In combination with body wave arrival times from five impact events, we obtain direct seismic constraints on the seismic velocity of the crust in the vicinity of the InSight landing site. We confirm the existence of a uppermost low‐velocity layer with a mean thickness of ∼1.2 km, interpreted as layered volcanic materials, possibly interstratified with sedimentary and altered materials. Our joint inversion of surface and body waves shows a four‐layer model for the Martian crust, compatible with high‐ and low‐frequency P‐to‐S receiver functions estimated in previous studies.

Abstract Subducting the buoyant crustal material of an aseismic oceanic ridge has been regarded as a dominant contributor to flat slab subduction. However, normal‐dip subduction is also observed in some cases where ridges are subducting. In this study, we compare the subduction of two ridges on the Nazca Plate: Nazca Ridge (flat slab) and Iquique Ridge (normal‐dip slab). Anisotropy determined by shear wave splitting observation suggests that the low‐velocity anomalies found beneath the ridges are mapping anisotropic structure into isotropic velocities. After a tomographic inversion incorporating anisotropy models for both ridges, we find that the low‐velocity anomalies found beneath the Nazca Ridge are not anisotropic and therefore likely represent warm mantle, and those beneath the Iquique Ridge are caused by anisotropy. We conclude that subslab mantle buoyancy has a larger impact on the subduction angle than the crustal material of the ridge.

The Kathmandu region becomes situated in very close to the major thrust faults such as Main Boundary Thrust (MBT) and Main Central Thrust (MCT) has been significantly affected by a series of several major earthquakes in the past. The current study analyses the attenuation of body waves in the region using the coda normalization method based on the data of two stations (NA430 and NA060) essentially from a temporary seismic network deployed after the occurrence of the 25th April 2015 earthquake. The events considered for the study have a local magnitude of 4–5.5 ​at a depth range of 10–27 ​km. The estimated quality factor of primary and secondary waves are QP = (44 ​± ​12.896)f (0.82 ± 0.173), QS = (62 ​± ​19.265)f (0.94 ± 0.185) for NA430 station and QP = (35 ​± ​9.467)f (0.93 ± 0.155), QS = (68 ​± ​22.510)f (0.91 ± 0.198) for NA060 station. The QS/QP ratio more than one observed for the entire frequency range specifies the highly heterogeneous crust medium beneath the study region. The obtained quality factors could be used in the generation of target ground motion for engineering purposes and in the calculation of source parameters for seismic hazard assessment as risk reduction strategies in the study region.

Abstract NASA's InSight has detected a large magnitude seismic event, labeled S1222a. The event has a moment magnitude of MWMa4.7, with five times more seismic moment compared to the second largest event. The event is so large that features are clearly observed that were not seen in any previously detected events. In addition to body phases and Rayleigh waves, we also see Love waves, minor arc surface wave overtones, and multi‐orbit surface waves. At long periods, the coda event exceeds 10 hr. The event locates close to the North‐South dichotomy and outside the tectonically active Cerberus Fossae region. S1222a does not show any evident geological or tectonic features. The event is extremely rich in frequency content, extending from below 1/30 Hz up to 35 Hz. The event was classified as a broadband type event; we also observe coda decay and polarization similar to that of very high frequency type events.

Abstract This study investigates lateral variations in density contrast across the 660‐km discontinuity, which is critical for understanding mantle convection and composition. We produce a global map of density jump, with values ranging from 3.8% to 9.3%, at the mineralogical phase boundary by applying inversions of amplitude variations with offset to underside S reflections. Our observations reveal a global average velocity jump of 4.1% and a density jump of 5.3%, favoring a pyrolitic bulk composition. Near major subduction zones, most notably the western Pacific and South America, we identify reduced density jumps in regions of depressed 660‐km discontinuity, consistent with basalt fractions greater than 30% in a mechanically mixed mantle. The causal association between density jump and compositional changes is further supported by a moderate correlation between lateral variations in density jump and those of water content anomalies.

After the occurrence of the 2008 Wenchuan Mw 7.9 earthquake, the Longmen-Shan fault zone exhibits highly active seismicity. In the years between 2008 and 2022, successive moderate magnitude aftershocks followed in the region. In this study, we used high-quality seismic data from the National Earthquake Data Center of China to estimate the source parameters of 184 moderate magnitude events (Mw 3.9–6.2) in the Longmen-Shan Fault zone. The dataset comprised the aftershocks of two sequences that occurred in the area, including the 2008 Wenchuan earthquake sequence and the 2013 Lushan earthquake sequence. They have been classified according to their focal mechanism. First, our results suggested that the values of apparent stress varied from approximately 0.1–11 MPa, which are higher than the global average. The strike-slip earthquakes have the highest average apparent stress among the types of earthquakes. The above results indicate that the moderately high levels of energy release from earthquakes in this region could result in high ground shaking potential. This behavior may be associated with the high stress accumulation and the strong deformation characteristics in the intraplate environment of the Longmen-Shan Fault zone. The values of the stress drop vary from approximately 0.24–39 MPa and appear to show a slightly increasing trend with the seismic moment. The stress drop level of the Lushan earthquake sequence is higher than the mean value of intraplate earthquakes, while the Wenchuan earthquake sequence is typical of intraplate earthquakes. Second, we investigate the radiated energy enhancement factor (REEF) to quantify the source complexities. The results show that the earthquakes have REEF values between 1.2 and 7.1, and most events are consistent with expectations based on the Brune ω-square model. There exist differences in the source complexity of earthquakes within the source area, and the average ω-square source model could result in an unreliable estimate of the radiated seismic energy of some events. The observation of moderate magnitude earthquakes suggests that the availability of different source parameters will help us to assess the seismic hazard and predict the shaking potential in the earthquake area.

This work is the mantle component of constructing the Seismological Reference Earth Model in South China (SREM-SC). Although there has been a wide range of research for imaging the upper mantle structures beneath South China, most of them focus on the large-scale features of the upper mantle, and the depth resolution is insufficient for existing surface wave tomography models to distinguish anomalies below 200 km. This study aims to develop a 3-D upper mantle Seismological Reference Earth Model in South China based on the prior tomography models. The shear wave velocity model comes from the analysis of several seismic surface wave tomography, supplemented by body wave tomography and the P-wave velocity model is constructed by the conversion from S-wave velocity. The radial anisotropy model is calculated from the SV-wave and SH-wave velocity. The Density model of the upper mantle is derived using the empirical relationship linking the density to the shear-wave velocity. The model is grid with 0.5° × 0.5° in latitude and longitude and 5 km interval in depth from 60 to 300 km. The mantle component of Seismological Reference Earth Model in South China is expected to provide a good representation of the upper mantle structures for further detailed studies. The mantle component of Seismological Reference Earth Model in South China provides new insights into upper mantle structures that should be meaningful to reveal the dynamic mechanism and tectonic evolution of South China.

The South China Block is located on the eastern margin of the Eurasian Plate and the western margin of the Pacific Plate. The South China Block is currently in a tectonically compressed environment, while the Tibetan Plateau is moving eastward and the Philippine Sea Plate is moving westward from geodetic observations. The South China Block is an ideal place to revisit tectonic history from the Archean to Cenozoic, where its information could be well preserved in the crust. In this study, we aim to build the crustal and uppermost mantle component of the Seismological Reference Earth Model in South China (SREM-SC) to provide a background velocity model for geological interpretations and fine-scale velocity inversion. The S-wave velocity model comes from combining models inverted by ambient noise tomography and surface wave tomography. The P-wave velocity model is obtained from converted S-wave velocity and joint inversion tomography. The density model is inferred from an empirical relationship with P-wave velocity. The Moho depth is obtained by a weighted averaging scheme of previously published receiver function results. The P-wave and S-wave velocity models have a grid interval of 0.5° in both latitude and longitude, and with a vertical sampling interval of 5 km down to the 60 km depth. This work provides the 3-D crust and uppermost mantle structures and a representative reference model beneath South China.

The exploration of the deep structure of the earth is the core part of geophysical research. Seismic body waves can penetrate deep into the earth with a high resolution and it is an indispensable technical means for studying the internal structure of the earth. The rapid development of technology for extracting high signal-to-noise ratio body wave signals based on ambient noise has greatly promoted the development and application of seismology, making it increasingly play an important role in the imaging of deep earth structures and shallow urban space exploration. This article reviews in detail how to use seismic interferometry and array processing technology to extract various types of body wave signals for research on different detection scales (local, regional, global). Among them, the seismic interferometry cross-correlates the waveform signals recorded by the seismic stations to cancel out the coincident ray paths, and finally the seismic records between the station pairs are obtained; and the array processing method is based on the data developed by the receiver array processing means. This technology can not only further improve the signal-to-noise ratio (SNR), but also obtain position information. Generally speaking, since the energy of the body wave signal contained in the ambient noise is much weaker than that of the surface wave signal, it is difficult to extract. This article focuses on the Bin stack method, the double beamforming method (DBF) and the phase-weighted stacks method (PWS), and concludes the applicable conditions of the three methods.

The Central-Western Mediterranean (CWM) is one of the most complex tectonic setting on Earth. Episodes of slab rollback, break-off and tearing, the opening of back-arc extensional basins (i.e., Liguro-Provencal, Alborean, Algerian and Tyrrhenian basins), the presence of large mountain ranges, active volcanoes and violent earthquakes have made the Mediterranean an ideal environment to study a wide range of geodynamic processes and an important target for seismological studies (e.g, seismic tomography). Here we build a geodynamic model which, although it does not reproduce its exact tectonic structure (e.g., due to the limits of the numerical method, approximations in the initial setup, etc), presents multiple and geometrically complex subduction systems analogous to those found in the CWM. The tectonic evolution of this model is estimated with petrological-thermo-mechanical 3D simulations, then, we dynamically compute the upper mantle fabrics and seismic anisotropy as a function of the strain history and local P-T conditions. After comparing the model with SKS splitting observations in order to quantify the discrepancies with the true Central-Western Mediterranean, we use the elastic tensors predicted for the modeled configuration to perform 3D P-wave anisotropic tomography by inverting synthetic P-wave delay times. Using the geodynamic model as reference, we evaluate the capabilities of a recently developed seismic tomography technique to recover the isotropic anomalies and anisotropy patterns related to a complex subduction environment in different conditions, such as poor data coverage and bad data quality. We observe that, although P-wave tomography still remains a powerful tool to investigate the upper mantle, the reliability of the retrieved structures strongly depends on data quality and data density. Furthermore, the recovered anisotropic patterns are consistent with those of the target model, but in general an underestimation of the anisotropy magnitude in the upper mantle is observed. In the light of future developments, our study suggests that by combining micro- and macro-scale geodynamic simulations and seismological modeling of seismic anisotropy it will be possible to reproduce, at least to a first order, the tectonic evolution of real study regions (e.g., the Mediterranean) thus providing fundamental constraints on the processes that have contributed in shaping their current geological scenario.

Abstract 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.

The vertical-vertical noise cross-correlation functions (NCFs) between two seismic arrays, the Japan F-net and ChinArray phase II, are calculated using continuous recordings during 2013–2016. After array interferometry to obtain bin stacked NCFs, clear body waves are retrieved at different period bands. Teleseismic direct P waves for distance 15–40 degrees are observed between short period 3–10 ​s while core reflected PcP/ScS waves are more obvious for longer period 30–60s. The signal-to-noise-ratio (SNR) of the short period P waves reaches its highest point with bin widths around 20 ​km while SNRs of PcP and ScS increase slowly with bin width. All those body waves demonstrate clear directivity with strong signals traveling from the east. The time-lapse SNR variations for the PcP and ScS show correlation with the occurrence of major earthquakes, while the P-wave SNR demonstrates seasonal variations with additional contribution from major earthquakes. The present results suggest teleseismic body waves can be retrieved through bin stacking, though further processing is still necessary to obtain finer waveforms such as P wave triplications.

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.

In seismic exploration, obtaining accurate bandwidth of the reflected signal is essential for determining seismic resolution. A portion of the stored energy is attenuated during signal propagation, narrowing the received seismic signal bandwidth. Therefore, compensating the energy attenuation is important for improving the seismic resolution. The current method of compensating absorption and attenuation based on a single channel can only compensate the post-stack data (self-exciting and self-receiving), whereas in practice, seismic waves do not propagate along the self-exciting and self-receiving seismic wave path; the propagation path is complex. The absorption and attenuation depend on the propagation path. The primary methods used for Q-compensation along the propagation paths are one-way wave extrapolation in the Cartesian coordinate system and Gaussian beam Q-compensation migration in the ray-centered coordinate system. However, the large angle limits the one-way wave method, and the Gaussian beam method refers to the high-frequency approximation solution of the two-way wave equation. Therefore, a 15-degree equation in the ray-centered coordinate system is proposed. Seismic waves extrapolate along the ray, which compensates the absorption and attenuation along the real propagation path. The 15-degree equation in the ray-centered coordinate system does not perform high-frequency approximation in the ray beam and has no large angle limit, facilitating the accurate description of local wavefields in the ray beam.

Abstract Recent seismic results from the Cascadia Initiative indicate that heterogeneity in the oceanic asthenosphere affects subduction dynamics. Accurate characterization of the oceanic upper mantle is thus necessary to fully understand subduction processes, including the behavioral segmentation of the megathrust. A key challenge is integrating onshore and offshore datasets, which span large variations in near‐surface features that teleseismic body wave tomography is ill‐posed to resolve. Here, we perform a series of P and S forward modeling predictions to better understand the relative contribution of elevation, crustal thickness, offshore sedimentation, and near‐surface velocity structure to teleseismic delay times. Crustal thickness and elevation variations dominate the signal, contributing ∼1 s of delay time difference for P‐waves (roughly double for S). We test several inversion strategies to account for near‐surface features, identifying potential artifacts and causes of imaging errors. Undamped station statics are found to absorb mantle structures and introduce low‐velocity artifacts beneath the forearc. Our preferred inversion strategy utilizes a three‐dimensional starting model (including elevation) of the upper 50 km and heavily damped station statics, which we find leads to better resolution of mantle structure, particularly at asthenospheric depths. These insights guide inversions of observed delay times from the Cascadia subduction zone and Juan de Fuca plate system. We present a new onshore‐offshore S model and an updated P model. Major features are common to both models, including localized subslab low‐velocity anomalies, along‐strike variations in slab structure, and offshore heterogeneity, while regional differences may reflect changes in Vp/Vs.

Abstract Seismic signals can be extracted from ambient noise wavefields by the correlation technique. Recently, a prominent P‐type phase was observed from teleseismic noise correlations in the secondary microseism period band. The phase is named Pdmc in this paper, corresponding to its origin from the interference between the direct P waves transmitting through the deep mantle and the core (P and PKPab waves). We extract the phase by correlating noise records from two seismic networks in the Northern Hemisphere and locate the microseism sources that are efficient for the Pdmc construction in the South Pacific. We investigate the spatiotemporal links of the Pdmc signal with global oceanic waves and microseism sources. Interestingly, the correlation with wave height is higher in several regions surrounding the effective source region, rather than in the effective source region. The Pdmc amplitude is highly correlated with the power of the effective microseism sources. Also, it is apparently correlated with ineffective sources in the Southern Hemisphere and anticorrelated with sources in the Northern Hemisphere. We ascribe the correlation with the ineffective southern sources to the spatiotemporal interconnections of the southern sources. The anticorrelation with northern sources can be explained by the reverse seasonal patterns between the southern and northern sources and by that the northern sources impede the signal construction. The signal construction from noise correlations relies on the competition between the effective and ineffective sources, not just on the power of the effective sources. This principle should be valid in a general sense for noise‐derived signals.

Abstract 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.