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The fluid-saturated porous layered (FSPL) media widely exist in the Earth’s subsurface and their overall mechanical properties, microscopic pore structure and wave propagation characteristics are highly relevant to the in-situ stress. However, the effect of in-situ stress on wave propagation in FSPL media cannot be well explained with the existing theories. To fill this gap, we propose the dynamic equations for FSPL media under the effect of in-situ stress based on the theories of poroacoustoelasticity and anisotropic elasticity. Biot loss mechanism is considered to account for the stress-dependent wave dispersion and attenuation induced by global wave-induced fluid flow. Thomsen’s elastic anisotropy parameters are used to represent the anisotropy of the skeleton. A plane-wave analysis is implemented on dynamic equations yields the analytic solutions for fast and slow P waves and two S waves. Modelling results show that the elastic anisotropy parameters significantly determine the stress dependence of wave velocities. Vertical tortuosity and permeability have remarkable effects on fast and slow P-wave velocity curves and the corresponding attenuation peaks but have little effect on S-wave velocity. The difference in velocities of two S waves occurs when the FSPL medium is subjected to horizontal uniaxial stress, and the S wave along the stress direction has a larger velocity, which implies that the additional anisotropy other than that induced by the beddings appears due to horizontal stress. Besides, the predicted velocity results have the reasonable agreement with laboratory measurements. Our equations and results are relevant to a better understanding of wave propagation in deep strata, which provide some new theoretical insights in the rock physics, hydrocarbon exploration and stress detection in deep-strata shale reservoirs.

We estimate depth‐dependent azimuthal anisotropy and shear wave velocity structure beneath the Alaska subduction zone by the inversion of a new Rayleigh wave dispersion dataset from 8 to 85 s period. We present a layered azimuthal anisotropy model from the forearc region offshore to the subduction zone onshore. In the forearc crust, we find a trench‐parallel pattern in the Semidi and Kodiak segments, while a trench‐oblique pattern is observed in the Shumagins segment. These fast directions agree well with the orientations of local faults. Within the subducted slab, a dichotomous pattern of anisotropy fast axes is observed along the trench, which is consistent with the orientation of fossil anisotropy generated at the mid‐ocean ridges of the Pacific‐Vancouver and Kula‐Pacific plates that is preserved during subduction. Beneath the subducted slab, a trench‐parallel pattern is observed near the trench, which may indicate the direction of mantle flow. Plain Language Summary The azimuthal anisotropy of seismic waves refers to the directional dependence of the seismic wave propagation speed. We present a comprehensive azimuthal anisotropy model of the Alaska subduction zone to a depth of 200 km, revealing anisotropy caused by local faults and fractures, fossil anisotropy inherited from the oceanic plate within the subducted slab, and sub‐slab mantle flow. The along‐strike variation of crustal anisotropy indicates variations in the stress regime in the forearc region. The along‐strike variation of anisotropy within the subducted slab identifies different origins of the subducted slab. Our model contributes to the understanding of the anisotropic structure and the sources of anisotropy in subduction zones. Key Points A new model of depth‐dependent azimuthal anisotropy of the Alaska subduction zone is built based on a new surface wave dataset The along‐strike variation in the azimuthal anisotropy of the forearc crust is caused by faults and fractures Azimuthal anisotropy within the subducted slab is controlled by fossil anisotropy produced at different mid‐ocean ridges

Ordos Block has undergone rapid uplift, and a series of rift basins have been formed around the block since the Cenozoic, but the formation mechanisms remain controversial. High-resolution 3D velocity structure of crust and mantle is important for understanding lithospheric deformation and deep dynamic process Here we present a 3D S-wave velocity structure of the crust and upper mantle in the Ordos Block and surrounding regions by joint inversion of receiver functions and surface wave data from a dense broadband seismic deployment. The lithosphere of the Ordos Block exhibits an obvious high-velocity anomaly. In the east and north of the Ordos and the southwestern part of the Tibetan Plateau, obvious low-velocity anomalies are detected in the upper mantle and extend into the Ordos The lithosphere of the Ordos Block is thick in the center and thin in the edge, while the crust is relatively thin in the center and thick in the southwest and northeast. The crustal thickness of the tensional basin in the north is greater than that in the central Ordos. We suggest that the outward expansion of the mantle thermal materials in eastern Tibet and the upper mantle thermal upwelling in the eastern part of the North China Craton lead to the non-uniform lithospheric thinning, temperature rise and density reduction of the Ordos Block. The additional buoyancy and thermodynamic effects provided by them contributed to the continuous uplift of the Ordos Block since the Cenozoic. Influenced by the extrusion of Tibetan Plateau, the crustal thickening and rapid uplift occur in the southwestern and northern parts of the Ordos Block. The lithospheric structures of the Alxa and Ordos Blocks are different, and they may belong to different independent blocks before the Mesozoic.

Compared with surface wave corresponding to the normal mode, which is widely studied, there is less research on guided-P wave corresponding to the leaking mode. Guided-P wave carries the dispersion information that can be used to construct the subsurface velocity structures. In this paper, to simultaneously estimate P-wave velocity (vP) and S-wave velocity (vS) structures, an integrated inversion method of guided-P and surface wave dispersion curves is proposed. Through the calculation of Jacobian matrix, the sensitivity of dispersion curves is quantitatively analyzed. It shows that the dispersion curves of guided-P and surface waves are, respectively, sensitive to the vP and vS. Synthetic model tests demonstrate the proposed integrated inversion method can estimate the vP and vS models accurately and effectively identify low-velocity interlayers. The integrated inversion method is also applied to the field seismic data acquired for oil and gas prospecting. The pseudo-2D vP, vS and Poisson’s ratio inversion results are of significance for near-surface geological interpretation. The comparison with the result of first-arrival traveltime tomography further demonstrates the accuracy and practicality of the proposed integrated inversion method. Not only in the field of exploration seismic, the guided-P wave dispersion information can also be extracted from the earthquake seismic, engineering seismic and ambient noise. The proposed inversion method can exploit previously neglected guided-P wave to characterize the subsurface vP structures, showing broad and promising application prospects. This compensates for the inherent defect that the surface wave dispersion curve is mainly sensitive to the vS structure.

The blast-induced ground vibrations result from the combined effects of body waves (compressional and shear waves) and surface waves (Rayleigh waves). Understanding the propagation and evolution characteristics of seismic waves could provide new thoughts for controlling blast-induced vibrations. In an earlier work, the evolution of seismic waves induced by a kind of vertical single-blasthole was investigated using a polarization analysis approach. As a follow-up study, this paper extends to analyzing the seismic waves induced by contour blastholes (i.e., smooth and presplitting blastholes) based on two on-site experiments and numerical simulations. Then, the inherent mechanisms of different kinds of blastholes and predictions for seismic components are discussed in conjunction with our earlier work. The results indicate that the relative location of the source-to-site significantly influences the proportions of different seismic waves. Due to differences in source characteristics and attenuation laws, the dominant wave changes with the location and orientation of concerned points. For smooth blasting, only S- and R-waves propagate in the same plane as blastholes, while the influence of the P-wave is negligible. Horizontal vibrations are primarily caused by the R-wave, while the vertical vibrations result from both S- and R-waves. Additionally, presplitting blastholes exhibit similar mechanical mechanisms to smooth blastholes, as both belong to contour blastholes, and thus the seismic components they induced in the plane are the same. However, the P-wave motion outside the contour plane cannot be ignored, particularly along the orientation of the source-to-site. Highlights As a follow-up study, this study improved the understanding of the evolution of seismic components induced by different kinds of blastholes. The relative location of the source-to-site significantly influences the proportions of different seismic waves. Both the source pattern and attenuation laws play important roles in the evolution of seismic components. Building on earlier work, the mechanisms of various blastholes were analyzed, and the seismic components at different locations were predicted.

Antarctic ice sheet history is imprinted in the structure and fabric of the ice column. At ice rises, the signature of ice flow history is preserved due to the low strain rates inherent at these independent ice flow centres. We present results from a distributed acoustic sensing (DAS) experiment at Skytrain Ice Rise in the Weddell Sea sector of West Antarctica, aimed at delineating the englacial fabric to improve our understanding of ice sheet history in the region. This pilot experiment demonstrates the feasibility of an innovative technique to delineate ice rise structure. Both direct and reflected P- and S-wave energy, as well as surface wave energy, are observed using a range of source offsets, i.e. a walkaway vertical seismic profile, recorded using fibre optic cable. Significant noise, which results from the cable hanging untethered in the borehole, is modelled and suppressed at the processing stage. At greater depth where the cable is suspended in drilling fluid, seismic interval velocities and attenuation are measured. Vertical P-wave velocities are high (VINT=3984±218 m s−1) and consistent with a strong vertical cluster fabric. Seismic attenuation is high (QINT=75±12) and inconsistent with previous observations in ice sheets over this temperature range. The signal level is too low, and the noise level too high, to undertake analysis of englacial fabric variability. However, modelling of P- and S-wave travel times and amplitudes with a range of fabric geometries, combined with these measurements, demonstrates the capacity of the DAS method to discriminate englacial fabric distribution. From this pilot study we make a number of recommendations for future experiments aimed at quantifying englacial fabric to improve our understanding of recent ice sheet history.

In the field of seismic exploration, scholars have been working to conduct wave propagation models that are close to physical reality. Researches for high-speed rail seismology show that the microstructural interactions by different scales will trigger the heterogeneous response of the medium, which in turn has an impact on the mechanical behavior of macro-scales. The generalized wave equations enhance the ability to reflect the heterogeneity of the medium by introducing the higher derivative of displacement and the characteristic scale parameters related to the microstructural properties of the medium. In this paper, we introduce the generalized wave equations under the single-parameter second-order strain gradient theory by considering the nonlocal effects, give the decoupled generalized wave equations using the Helmholtz decomposition theorem, and derive the expression of the phase-velocity of the P- and S-wave. Then, we investigate the dispersion characteristics of seismic wave propagation by considering the microstructural interactions in the medium utilizing theoretical dispersion analysis and numerical experiments which can provide a new approach for the establishment and interpretation of wave propagation models under actual medium.

Since S-wave velocity of the subsurface is an important parameter in near surface applications, many studies have been conducted for its estimation. Among the various methods that use surface waves or body waves, Rayleigh wave inversion is the most popular. In practice, the densities and P-wave velocities of different layers are usually assumed to be known to avoid ill-posed problems, as they have less influence on the dispersion curves. However, improper assignment of these two groups of parameters leads to inaccurate estimation of the S-wave velocity profile. In order to address this problem, the all-parameters Rayleigh wave inversion strategy is proposed in which the S-wave velocities, layer thicknesses, densities and P-wave velocities of different layers are included as the unknown parameters for inversion. Meanwhile, the transitional Markov Chain Monte Carlo (TMCMC) algorithm is applied for the implementation of all-parameters Rayleigh wave inversion. One simulated example and two real-test applications are demonstrated to verify the capability of the proposed method in the estimation of the S-wave velocity profile, the densities and the P-wave velocities. Furthermore, it is verified that the proposed method achieved more accurate S-wave velocity profile estimation than the traditional approach.

Near-surface site characterization is of great significance in the fields of geotechnical engineering and resource exploration. In this paper, we propose a near-surface site characterization method based on the joint iterative analysis of first-arrival and surface-wave data (JIAFS). The proposed method combines the advantages of first-arrival traveltime tomography (FATT) and multichannel analysis of surface waves (MASW). First, the 1D S-wave velocity (vS) models obtained by MASW are interpolated to construct the pseudo-2D vS model. According to the available geological survey information and borehole data, the initial Poisson’s ratio (σ) model is estimated. Based on the estimated σ model, the pseudo-2D vS model is converted to a referenced P-wave velocity (vP) model which is utilized to constrain the progress of FATT. This helps FATT overcome the inherent defect that it cannot effectively identify velocity-inversion interfaces and low-velocity zones. On the other hand, the vP model obtained by FATT can provide a favorable priori information to improve the reliability of the results of MASW. Then, the vP and vS models obtained by constrained FATT and MASW are used to update the σ model. In addition, the vP and vS models are also used as initial models in the next iterative analysis. Finally, through the iteration of this process, the two inversion methods can make use of their own advantages to improve each other, so we can establish accurate near-surface vP, vS and σ models under complex geological conditions. A velocity model including low-velocity zone is established for synthetic model test to analyze and verify the advantage of JIAFS. The vP, vS and σ models obtained by JIAFS can accurately identify the low-velocity zone and match the true models well. In addition, the proposed method is applied to the field seismic data acquired for oil and gas exploration in Northwest China. Compared with the results of individual inversions and borehole data, JIAFS can establish more reliable 3D vP, vS and σ models by interpolating the 2D inversion results, which reveals further details and enhances the geological interpretation significantly.

Intracrustal low-velocity zones (LVZs) indicate a mechanically weak crust and are widely observed in the southeast margin of the Tibetan Plateau. However, their spatial distributions and formation mechanisms remain controversial. To investigate their distribution and detailed morphology of the LVZs in the southeastern Tibetan Plateau, here we used teleseismic events and continuous waveform data recorded by 40 broadband seismic stations newly deployed in the Sichuan-Yunnan region from December 2018 to October 2020. A total of 12,924 high-quality P-wave receiver functions and 5–40 s fundamental Rayleigh surface wave phase velocity dispersion curves from ambient noise cross-correlation functions were obtained. The S-wave velocity model at a depth interval of 0–100 km in the study area was inverted by using the trans-dimensional Markov chain Monte Carlo strategy to jointly invert the complementary data of the receiver function waveform and Rayleigh surface wave phase velocity dispersion. Our results show that there are two separate LVZs (∼3.5 km/s) surrounding the rigid Daliangshan subblock at crustal depths of approximately 30–40 km, providing new constraints on the geometry of the LVZs in our study region. The two LVZs obtained in this study may represent the middle crustal flow channels, through which the material in the center of the Tibetan Plateau extrudes to its southeast margin. Blocked by the rigid Sichuan Basin and the spindle-like Daliangshan subblock, the material continues to flow southward through the mechanically weak middle crustal channels surrounding the Daliangshan subblock. In addition, the existence of thin LVZs in the middle crust plays an important role in understanding the decoupling between the upper and lower crust in the study area. It also provides new constraint on the complex tectonic deformation process of the southeastern margin of the Tibetan Plateau caused by the collision and compression of the Indian and the Eurasian plates.