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We reply to the comments of De Natale and Pino (2013) on the paper ‘Are the source models of the M 7.1 1908 Messina Straits earthquake reliable? Insights from a novel inversion and sensitivity analysis of levelling data’ by Aloisi et al. (2012, Geophys. J. Int., 192, 1025–1041). We entirely reject their speculative comments and confirm our viewpoint about the impossibility of discriminating between the two oppositely dipping fault models on the basis of the levelling data alone; we state again that their role as a keystone for modellers is untenable. The comments of De Natale and Pino (2013) are welcomed insofar they give us the possibility to improve our previous analysis, and to criticize the mainstream hypothesis favouring to a low-angle East dipping fault in the Sicilian side of the Messina Straits as responsible of the 1908 destructive earthquake. [ABSTRACT FROM AUTHOR]

In situ stress monitoring of crustal rocks is desirable as it yields insights into earthquake mechanisms, volcanic eruptions and changes in hydrocarbon reservoirs. Shear wave splitting, induced by stress-controlled cracks in the shallow crust, provides a way to infer this stress. Temporal variations in these observations can be difficult to quantify due to scatter in the data and discontinuous observations. Here we present evidence of temporal variations in shear wave splitting from a continuous time-series with a high occurrence of microseismic events recorded in a borehole. We interpret these observations in terms of stress-controlled cracks and are able to infer changes in stress and, via modelling, suggest the cause of the anisotropy. Possible origins of the temporal variation in per cent anisotropy are tidal or oil-field production processes. Our results suggest that shear wave splitting is a viable probe for inferring changes in crustal stress in cracked rock. [ABSTRACT FROM AUTHOR]

Concerns raised by Okazaki & Ueda (2022) on the paper by Sambridge et al. (2022) are addressed. Two issues are discussed and some new numerical results presented. The first concerns whether the properties of the Wasserstein time-series misfit introduced in our earlier paper will translate to model space non-uniqueness in a seismic waveform inversion setting. It is argued that this is unlikely, given the special conditions, which must exist between all observed/predicted seismic waveform pairs for non-uniqueness to result. The second issue discussed is the efficacy of using the Sliced Wasserstein algorithm of Bonneel et al. (2015) as an alternate to the marginal Wasserstein algorithm, as proposed by Okazaki & Ueda (2022). It is argued that for optimization-based waveform fitting, the Sliced Wasserstein algorithm is a viable alternate provided care is taken to ensure that conditions arise which do invalidate analytical derivative expressions of the resulting Wasserstein misfit. In practice, this would likely mean recasting the 2D Optimal Transport problem posed in our earlier paper onto unstructured grids. [ABSTRACT FROM AUTHOR]

More accurate inversion of source fault geometry and slip parameters under the constraint of the Bayesian algorithm has become a research hotspot in the field of geodetic inversion in recent years. In nonlinear inversion, the determination of the weight ratio of the joint inversion of multisource data is more complicated. In this context, this paper proposes a simple and easily generalized weighting method for inversion of source fault parameters by joint geodetic multisource data under the Bayesian framework. This method determines the relative weight ratio of multisource data by root mean square error (RMSE) value and can be extended to other nonlinear search algorithms. To verify the validity of the method in this paper, this paper first sets up four sets of simulated seismic experiment schemes. The inversion results show that the joint inversion weighting method proposed in this paper has a significant decrease in the large residual value compared with the equal weight joint inversion and the single data source joint inversion method. The east–west deformation RMSE is 0.1458 mm, the north–south deformation RMSE is 0.2119 mm and the vertical deformation RMSE is 0.2756 mm. The RMSEs of the three directions are lower than those of other schemes, indicating that the proposed method is suitable for the joint inversion of source parameters under Bayesian algorithm. To further verify the applicability of the proposed method in complex earthquakes, the source parameters of the Maduo earthquake were inverted using the method of this paper. The focal depth of the inversion results in this paper is closer to the focal depth released by the GCMT agency. In terms of strike angle and dip angle, the joint inversion in this paper is also more inclined to the GCMT results. The joint inversion results generally conform to the characteristics of left-lateral strike-slip, which shows the adaptability of this method in complex earthquakes. [ABSTRACT FROM AUTHOR]

A robust, in situ estimate of shear-wave velocity VS and the small-strain damping ratio DS (or equivalently, the quality factor QS) is crucial for the design of buildings and geotechnical systems subjected to vibrations or earthquake ground shaking. A promising technique for simultaneously obtaining both VS and DS relies on the Multichannel Analysis of Surface Waves (MASW) method. MASW can be used to extract the Rayleigh wave phase velocity and phase attenuation data from active-source seismic traces recorded along linear arrays. Then, these data can be inverted to obtain VS and DS profiles. This paper introduces two novel methodologies for extracting the phase velocity and attenuation data. These new approaches are based on an extension of the beamforming technique which can be combined with a modal filter to isolate different Rayleigh propagation modes. Thus, the techniques return reliable phase velocity and attenuation estimates even in the presence of a multimode wavefield, which is typical of complex stratigraphic conditions. The reliability and effectiveness of the proposed approaches are assessed on a suite of synthetic wavefields and on experimental data collected at the Garner Valley Downhole Array and Mirandola sites. The results reveal that, under proper modelling of wavefield conditions, accurate estimates of Rayleigh wave phase velocity and attenuation can be extracted from active-source MASW wavefields over a broad frequency range. Eventually, the estimation of soil mechanical parameters also requires a robust inversion procedure to map the experimental Rayleigh wave parameters into soil models describing VS and DS with depth. The simultaneous inversion of phase velocity and attenuation data is discussed in detail in the companion paper. [ABSTRACT FROM AUTHOR]

This paper deals with in situ characterization of the small-strain shear-wave velocity VS and damping ratio DS from an advanced interpretation of Multi-channel Analysis of Surface Waves (MASW) surveys. A new approach based on extracting Rayleigh wave data using the CFDBFa method has been discussed in the companion paper. This paper focuses on mapping the experimental Rayleigh wave phase velocity and attenuation into profiles of VS and DS versus depth, which is achieved through a joint inversion procedure. The joint inversion of phase velocity and attenuation data utilizes a newly developed Monte Carlo global search algorithm, which implements a smart sampling procedure. This scheme exploits the scaling properties of the solution of the Rayleigh eigenvalue problem to modify the trial earth models and improve the matching with the experimental data. Thus, a reliable result can be achieved with a limited number of trial ground models. The proposed algorithm is applied to the inversion of synthetic data and of experimental data collected at the Garner Valley Downhole Array site, as described in the companion paper. In general, inverted soil models exhibit well-defined VS profiles, whereas DS profiles are affected by larger uncertainties. Greater uncertainty in the inverted DS profiles is a direct result of higher variability in the experimental attenuation data, the limited wavelength range at which reliable values of attenuation parameters can be retrieved, and the sensitivity of attenuation data to both DS and VS. Nonetheless, the resulting inverted earth models agree well with alternative in situ estimates and geological data. The results stress the feasibility of retrieving both stiffness and attenuation parameters from active-source MASW testing and the effectiveness of extracting in situ damping ratio estimates from surface wave data. [ABSTRACT FROM AUTHOR]

It is a well-accepted hypothesis that deep-mantle primary plumes originate from a buoyant source layer at the core–mantle boundary (CMB), where Rayleigh–Taylor instabilities (RTIs) play a key role in the plume initiation process. Previous studies have characterized their growth rates mainly in terms of the density, viscosity and layer-thickness ratios between the denser overburden and the source layer. The RTIs, however, develop in the presence of global flows in the overlying mantle, which can act as an additional factor in the plume mechanics. Combining 2-D computational fluid dynamic (CFD) model simulations and a linear stability analysis, this paper explores the influence of a horizontal global mantle flow in the instability dynamics. Both the CFD simulation results and analytical solutions reveal that the global flow is a dampening factor in reducing the instability growth rate. At a threshold value of the normalized global flow velocity, short- as well as long-wavelength instabilities are completely suppressed, allowing the entire system to advect in the horizontal direction. Using a series of real-scale numerical simulations, this paper also investigates the growth rate  as a function of the density contrast, expressed in Atwood number |${A}_T = ({{{\rho }_1 - {\rho }_2}})/({{{\rho }_1 + {\rho }_2}})$|⁠ , and the viscosity ratio |$\ {\mu }^* = \ {\mu }_1/{\mu }_2$|⁠ , where |${\rho }_1,\ {\mu }_{1\ }$| and |${\rho }_{2,}\ {\mu }_{2\ }$| are densities and viscosities of the overburden mantle and source layer, respectively. It is found that increase in either |${A}_T$| or |${\mu }^*$| promotes the growth rate of a plume. In addition, the stability analysis predicts a nonlinearly increasing RTI wavelength with increasing global flow velocity, implying that the resulting plumes widen their spacing preferentially in the flow direction of kinematically active mantle regions. The theory accounts for additional physical parameters: source-layer viscosity and thickness in the analysis of the dominant wavelengths and their corresponding growth rates. The paper finally discusses the problem of unusually large inter-hotspot spacing, providing a new conceptual framework for the origin of sporadically distributed hotspots of deep-mantle sources. [ABSTRACT FROM AUTHOR]

During tunnel construction, accurately ascertaining the adverse geological condition in front of the tunnel face is essential to ensure construction safety and decrease economic loss. As an accurate wave velocity modelling method, seismic full-waveform inversion (FWI) still faces many difficulties when applied in a tunnel observation environment because of fewer observation data and smaller offset than ground seismic detection. This paper analysed the FWI for tunnel active seismic forward prospecting with a single shot. We adopted the N -order time integral wavefield and normalized integration objective function method to improve the stability and reduce the dependence of FWI on the initial model. Based on two assumptions on the wave velocity distribution in the tunnel, we proposed a 1-D velocity structure correction method to reduce the multiplicity of inversion. Then the improved tunnel FWI method based on these two inversion strategies was applied to a synthetic model with a rectangular anomaly and a lithology interface, verifying the method's effectiveness. Based on typical adverse geological bodies during tunnelling, three additional adverse geological models were built and verified the reliability of the methods in tunnel detection environments. There are still some false anomalies interference in the inversion results of the synthetic models from the inversion results. However, these problems are acceptable in limited tunnel observation space. Then the method was applied in a field example in the Yinsong water supply project in Jilin Province, China, and verified the effect on field data. Finally, the influence of the shot number on the inversion effect and the method's robustness are discussed. [ABSTRACT FROM AUTHOR]

The article lists the papers that will be published in the journal, including "A Non-Linear Geodetic Data Inversion Using ABIC for Slip Distribution on a Fault With an Unknown Dip Angle," by Yukitoshi Fukahata and Tim J. Wright, and "Non-Linear Conjugate Gradient Inversion for Global EM Induction: Resolution Studies ," W. Heise, T. G. Caldwell, H. M. Bibby and S. C. Bannister, and the paper "Global Upper-Mantle Tomography With the Automated Multimode Inversion of Surface and S-Wave Forms," by Sergei Lebedev and Rob D. van der Hilst.

In this paper, physics-based numerical simulation (PBS) is employed to render a broad-band (0–10 Hz) realization of the near-field seismic response of the experimental nuclear site of Cadarache, located nearby the active Middle Durance Fault (southeastern France). The sensitivity of the earthquake numerical model to geological features is investigated by comparison with geophysical measurements and past aftershock and by highlighting the amplification induced by the soft sediments below Cadarache. The blind prediction of an M W6 target earthquake is approached by synthesizing four different finite-fault scenarios. The outcome is compared to the standard ground motion prediction equations (GMPEs), unveiling a possible GMPE overestimation of the pseudospectral acceleration ordinates at short natural periods, supporting the actual need to integrate synthetic and empirical predictions when direct observations are not available. [ABSTRACT FROM AUTHOR]

This paper proposes a procedure of forward modelling 3-D frequency-domain wire-source electromagnetic data using the meshless generalized finite-difference (MGFD) method. This method is based on Taylor series expansions and the weighted least-squares method, and its basic principle is to express the partial derivatives of the unknown function on a particular central point by a linear combination of function values on the adjacent points. The advantages of the method over mainstream forward-modelling methods, for example, the regular finite-difference (FD) method, or the finite-element (FE) method, is that mesh generation is not needed: a discretization in the form of just points is applied instead. This allows the points to be distributed freely to fit the arbitrary shape of the structures in the model, which is helpful in the modelling of complex earth structures. It makes the MGFD method more suitable to deal with complex model than FD method. Also, unlike that in the FE method, interpolation functions are not required and no integral needs to be calculated in MGFD method. This results in high computational efficiency and a concise forward-modelling process. In this paper, the particulars of the MGFD method are introduced, the discretized MGFD system of equations (for an |${\boldsymbol{A}} - {\rm{\ }}\varphi $| potential decomposition of the fields, with the Coulomb gauge condition enforced and a primary–secondary separation approach to deal with the singularity of the source) are solved using a direct solver, and the forward-modelling code are programmed. To test the method and code, we compare the MGFD solutions for three 3-D earth models with the equivalent solutions calculated by other methods, and verify the correctness of the MGFD solution by the good agreement between the corresponding results (with relative error of the electric field |${{\boldsymbol{E}}}_{\boldsymbol{x}}$| smaller than 4.89 per cent). We also investigate the performance of this method when applying different discretizations of points, and when using different weighting functions, to assess the influence of these two factors on the forward-modelling accuracy and efficiency. Results indicate that denser point distributions and straightforward weighting functions result in better accuracy and efficiency. [ABSTRACT FROM AUTHOR]

The aim of this paper is gravitational inversion, that is, the determination of the mass density distribution of a central body when given the observed external gravitational field that is produced by the mass itself. This inversion for a 3-D mass distribution is known to be grossly non-unique; whereas that for a 2-D mass distribution on a spherical surface is known to be unique. The latter justifies the surface 'mascon', or equivalent-water-thickness, solutions when not considering any interior mass transports for the Earth's time-variable gravity as observed by the GRACE satellite. In this paper, using the gravitational multipole formalism cast in the framework of linear Hilbert space with the notion of inner product, we do two things further: (i) we prove mathematically that the 2-D gravitational inversion on an arbitrary surface is unique; and (ii) we derive the algorithm that leads to the unique exact 2-D mass distribution solution, up to a truncated finite spherical harmonic degree. This pertains directly to reaching refined GRACE mascon solutions that account for the actual Earth surface shape including the ellipsoidal figure and the topography. In the process, we also (re)formulate the exact equations, for spherical or arbitrary shape, unifying the formulae that have appeared in the GRACE-relevant literature. [ABSTRACT FROM AUTHOR]

High-resolution 3-D seismic simulation imposes severe demands for computational memory, making low-storage seismic simulation particularly important. Due to its high-efficiency and low-storage, the half-precision floating-point 16-bit format (FP16) is widely used in heterogeneous computing platforms, such as Sunway series supercomputers and graphics processing unit (GPU) computing platforms. Furthermore, the low-storage Runge–Kutta (LSRK) technique requires lower memory resources compared with the classical Runge–Kutta. Therefore, FP16 and LSRK provide the possibility for low-storage seismic simulation. However, the orders of magnitude of the physical quantities (velocity, stress and Lamé constants) in the elastic wave equations are influenced by the P -wave and S -wave velocities and the densities of the elastic media. This results in a huge order of magnitude difference between the stored velocity and stress values, which exceed the range of the stored values of FP16. In this paper, we introduce three dimensionless constants, Cv , Cs and Cp , into elastic wave equations, and new elastic wave equations are derived. The three constants, Cv , Cs and Cp , keep the orders of magnitude of the velocity and stress at a similar level in the new elastic wave equations. Thus, the stored values of these variables in new equations remain within the range of the stored values of FP16. In addition, we introduce the use of the LSRK due to its low-storage characteristic. In this paper, based on the FP16 and LSRK low-storage techniques, we develop 3 optimized multi-GPU solvers for seismic simulation using the curved grid finite-difference method (CGFDM). Moreover, we perform a series of seismic simulations to verify the accuracy, stability, and validity of the optimized solver coupled with the two techniques. The verifications indicate that through maintaining the calculation accuracy, the computational efficiency of the solver is significantly optimized, and the memory usage is remarkably reduced. In particular, under the best conditions, the memory usage can be reduced to nearly 1/3 that of the original CGFDM solver. [ABSTRACT FROM AUTHOR]

Plenty of studies have suggested that pore fluid may play an important role in earthquake rupture processes. Establishing numerical models can provide great insight into how pore fluid may affect earthquake rupture processes. However, numerical simulation of 3-D spontaneous ruptures in poroelastic mediums is still a challenging task. In this paper, it is found that a closed-form time-domain Green's function of Biot's poroelastodynamic model can be constructed when the source frequency and source-field distance are within a certain range. The time-domain Green's function is validated by being transformed into the frequency domain and comparing with the frequency-domain Green's functions obtained by former papers. Poroelastic wave propagation phase diagrams for various two-phase poroelastic mediums are then plotted to show the applicable range of frequency and source-field distance for the new time-domain Green's function. It is shown that the applicable range not only include the frequency and spatial range of concern in seismology but also overlap that in acoustics. Based on the time-domain Green's function, the boundary integral equations (BIEs) for modelling dynamic ruptures in elastic mediums are extended to fluid-saturated mediums. In the meantime, a functional relationship between the effective stress tensor and the total stress tensor in fluid-saturated mediums is also obtained, which allows us to directly obtain the effective stress by BIEs. The spontaneous rupture processes controlled by the slip-weakening friction law on faults in elastic mediums and in fluid-saturated mediums are compared. It is found that under the same conditions, fluid-saturated rocks are more prone to supershear rupture than dry rocks. This result suggests that pore fluid may promote the excitation of supershear rupture. The poroelastic wave propagation phase diagrams also suggest that simulating a coseismic phase in the real scale requires a certain sample length in laboratories. They also suggest that an undrained governing equation is suitable for seismic wave propagation simulation in poroelastic media. [ABSTRACT FROM AUTHOR]

A list of forthcoming papers is presented including "Full three-dimensional tomography: a comparison between the scattering-integral and adjoint-wavefield methods," "Estimating the model resolution matrix for large seismic tomography problems based on Lanczos bidiagonalization with partial reorthogonalization," and "Abrupt changes in crustal structure beneath the Coast Ranges of northern California—developing new techniques in receiver function analysis."

This article presents a list of papers on earthquakes, to be published in the periodical "Geophysical Journal International," as of October 2003.

SUMMARY: In the past three decades, an immense amount of magnetic field data (with periods of measurements from months to years) has been collected worldwide, either inland or at the sea bottom. This makes it tempting to estimate from these data magnetotelluric (MT) vertical transfer functions—tippers—and further probe with them the 3-D distribution of electrical conductivity on a continental or even global scale. Such a problem setup requires modelling tippers in spherical geometry. It is known that MT impedances in spherical coordinates can be modelled using different polarizations of a uniform external magnetic field. As for tippers, one needs another type of excitation because the uniform external magnetic field of any polarization contains a non-zero radial component. In the paper, we elaborate a source model, which leads to valid tippers on a whole sphere. We also present a novel, accurate and computationally efficient solver to calculate the electromagnetic field in a spherical shell or a part thereof. To illustrate the workability of both the introduced source model and the developed new solver, we present the results of modelling the ocean induction effect in tippers on a global scale. We performed computations in the models with and without oceanic sediments and show that the sediments substantially influence the results in the coastal regions where the thickness of sediments is comparable with or even exceeds the ocean's bathymetry. [ABSTRACT FROM AUTHOR]

This paper presents a strategy for extending scalar (P – V – T) equations of state to self-consistently model anisotropic materials over a wide range of pressures and temperatures under nearly hydrostatic conditions. The method involves defining a conventional scalar equation of state (V (P, T) or P (V, T)) and a fourth-rank tensor state variable |$\boldsymbol {\Psi }(V,T)$| whose derivatives can be used to determine the anisotropic properties of materials of arbitrary symmetry. This paper proposes two functional forms for |$\boldsymbol {\Psi }(V,T)$| and provides expressions describing the relationship between |$\boldsymbol {\Psi }$| and physical properties including the deformation gradient tensor, the lattice parameters, the isothermal elastic compliance tensor and thermal expansivity tensor. The isothermal and isentropic stiffness tensors, the Grüneisen tensor and anisotropic seismic velocities can be derived from these properties. To illustrate the use of the formulations, anisotropic models are parametrized using numerical simulations of cubic periclase and experimental data on orthorhombic San Carlos olivine. [ABSTRACT FROM AUTHOR]

SUMMARY: In this paper, the 3-D theoretical seismogram at the free surface of an elastic half-space with 'vertical transverse isotropy (VTI)' and an 'elliptic anisotropy' is synthesized via semi-analytical formulation. To this end, the time-domain solution is obtained via the Hankel and Laplace integral transforms and the Cagniard contour integration scheme. The formulations include complete Green's functions expressed in compact and elegant formulations in terms of some elementary line integrals over the finite interval (0, π/2), due to the general point force and point double-couple source of arbitrary orientation, varying with time as Heaviside step function. This solution is further implemented to model seismic waves generated from moment tensor source with a ramp-type slip function. The solution clearly delineates compressional P waves, shear SV waves and SH waves, diffracted SPwaves and surface Rayleigh waves, with two critical distances in which the mode conversion happens. The first marks the conversion of the total reflection of SV wave into the diffracted SPwave travelling on the free surface with the velocity of compressional wave. The second, however, marks a location where the order of arrival times of SP and SH waves is reversed. The interesting phenomenon of shear wave splitting by the apparent time lag between the arrivals of SV and SHwaves is further demonstrated. Studied also is the effect of non-double-couple (non-DC) components of the moment tensor of the shear fault. Particularly, it is shown that non-DC components may lead to large amplitude of the P wave in the presence of anisotropy, resulting in the changed polarity of P wave at the onset of the motion. It is found that for the elliptic anisotropy, and for oblique-thrust faults, the isotropic moment (ISO) is always negative if the elastic constants satisfy C11 > C33 and C44 > C66, whereas it is always positive if C11 < C33 and C44 < C66 (for oblique-reverse faults, the situation is reversed). It is shown that for some sedimentary rocks, the decomposition of moment tensor in the general VTI and in its elliptic approximation are close to each other, and that the moment tensor decomposition in VTI media is very sensitive to the elastic coefficients, such that a small change in the elastic constants may lead to a remarkable change in the moment tensor decomposition. Particularly, the effect of elastic constant C13 is shown to be significant. While the time-domain solution obtained in this paper can be also degenerated to the solution in the corresponding isotropic half-space, the Mathematica code of the solution provided can be served as benchmark to other numerical solutions, applicable for the computation of the theoretical seismogram in the involved media. [ABSTRACT FROM AUTHOR]

Full waveform inversion (FWI) is an important and popular technique in subsurface Earth property estimation. In this paper, several improvements to the FWI methodology are developed and demonstrated with numerical examples, including a simple two-layer seismic velocity model, a cross borehole Camembert model and a surface seismic Marmousi model. We introduce an unbalanced optimal transport (UOT) distance with Kullback–Leibler divergence to replace the L2 distance in the FWI problem. Also, a mixed L1/Wasserstein distance is constructed that preserves the convex properties with respect to shift, dilation, and amplitude change operation. An entropy regularization approach and convolutional scaling algorithms are used to compute the distance and the gradient efficiently. Two strategies of normalization methods that transform the seismic signals into non-negative functions are discussed. The numerical examples are then presented at the end of the paper. [ABSTRACT FROM AUTHOR]

Lists several articles to be published in the forthcoming issues of the periodical 'Geophysics.' 'Imaging of a seismic fault slip transients recorded by dense geodetic networks,' by J.J. MacGuire and P. Segall; 'What can GPS data tell us about the dynamics of post-seismic deformation?,' by E.H. Hearn.

We present a solution for long-term direct current resistivity and time-domain induced polarization (DCIP) monitoring, which consists of a monitoring system and the associated software that automates the data collection and processing. This paper describes the acquisition system that is used for remote data collection and then introduces the routines that have been developed for pre-processing of the monitoring data set. The collected data set is pre-processed using digital signal processing algorithms for outlier detection and removal; the resulting data set is then used for the inversion procedure. The suggested processing workflow is tested against a simulated time-lapse experiment and then applied to field data. The results from the simulation show that the suggested approach is very efficient for detecting changes in the subsurface; however, there are some limitations when no a priori information is used. Furthermore, the mean weekly data sets that are generated from the daily collected data can resolve low-frequency changes, making the approach a good option for monitoring experiments where slow changes occur (i.e. leachates in landfills, internal erosion in dams, bioremediation). The workflow is then used to process a large data set containing 20 months of daily monitoring data from a field site where a pilot test of in situ bioremediation is taking place. Based on the time-series analysis of the inverted data sets, we can detect two portions of the ground that show different geophysical properties and that coincide with the locations where the different fluids were injected. The approach that we used in this paper provides consistency in the data processing and has the possibility to be applied to further real-time geophysical monitoring in the future. [ABSTRACT FROM AUTHOR]

The effects of pore fluids on the elastic properties of sedimentary rocks can be broadly categorized into two mechanisms: variations in pore compressibility due to the physical properties of the fluids and alterations in the stiffness of the rock frame resulting from rock–fluid interactions. Particularly, as rock–fluid interactions alter the stiffness of contacts between mineral grains, changes in the fluid properties around grain contacts can induce volumetric deformation of the rock as well as in variations of the associated elastic coefficients. Even though many previous studies have explored the influence of swelling of clay minerals, the relationships between changes in elastic moduli and deformation in rocks, which hardly contain swellable minerals, remain to date enigmatic. In this paper, to evaluate quantitatively these effects, drying rates, strains and ultrasonic velocities of small cylindrical Berea sandstone samples were measured as their water saturation was decreased by evaporative drying. The measurements clearly showed drying shrinkage and drastic increases in shear and compressional moduli for all the samples under an almost fully dried condition. Previous studies considered that the alteration in surface energy of grain minerals between wet and dry states affected the contact stiffness between them. Some of them used micromechanical model, uniting the Digby grain contact model with the effective medium theory, to interpret the changes in elastic moduli observed in the non-swellable sandstones during water adsorption on the grain surface. The conventional micromechanical model assumes that a mineral grain is a pure sphere and that the number of contacts between the grains is one. However, grains in a sedimentary rock are generally not purely spherical, and the contact surface is composed of several adhesive asperities. We therefore modified the conventional model by introducing the curvature radius and the number of asperities per contact surface. The modified model well reproduced the shear moduli under wet conditions using the strain and moduli measured under dry conditions. On the other hand, the predictions of the compressional moduli using the model were partially in agreement with the experimental results. Therefore, we attempted to qualitatively interpret the relationship by combining the model with the viscoelastic effect associated with wave-induced fluid flow. The deformation and changes in the elastic moduli of rocks resulting from multiple pore fluids within them can be better understood by the present combined model. [ABSTRACT FROM AUTHOR]

The M w 6.8 Murghob earthquake is the third earthquake in an M w 6.4+ sequence occurring in the Pamir initiated by the 2015 Sarez M w 7.2 earthquake. It is of great significance to investigate their interactions and to assess future seismic hazards in the region. In this paper, we use Sentinel-1 radar interferometric data to retrieve coseismic deformation, invert for the slip distributions of the four events, and then investigate their interactions. The cumulative Coulomb failure stress changes (ΔCFS) suggest that the 2023 Murghob earthquake was promoted by the three prior earthquakes in the sequence. Pre-stress from historical earthquakes is a key factor in explaining the triggering mechanism of the two 2016 M w 6.4+ earthquakes. Stress loading and unloading effects on major faults in the region indicate that future attention should be paid in (1) the segment of the Sarez-Karakul fault north of the Kokuibel Valley, (2) the segment of the Sarez-Murghab thrust fault west of the Sarez-Karakul fault and (3) the east segments of the Pamir thrust fault system, all with a large positive ΔCFS. [ABSTRACT FROM AUTHOR]