Search

Geologically complex Proterozoic Cuddapah basin and its environs, have played a vital role in the geodynamic evolution of the India-east Antarctica terrain during Columbia, Rodinia and Gondwana supercontinental assembly periods. In the present study, we investigate subsurface crustal seismic structure of the north Cuddapah basin, by reprocessing seismic data, acquired along 325 km long Alampur–Koniki–Ganapeswaram profile. The study depicted a thin veneer of alluvium (2.00–2.90 km/s), underlain by the Gondwana (4.00 km/s) and upper (5.00–5.20 km/s) and lower (5.50–5.70 km/s) Proterozoic sediments in East Coast Sedimentary Basin. Similarly, in the fault-bounded Cuddapah basin, upper and lower Cuddapah sediments are characterised by velocities 5.05–5.10 km/s and 5.50–5.70 km/s with combined thickness of 2.8 km in Kurnool basin and 1.9 km in Nallamalai basin. They rest directly over crystalline basement (5.90–6.05 km/s), underlain further by mid-crustal layer (6.20–6.30 km/s) located at depths of 5–15 km. Massive mid-crustal exhumation in the form of a prominent hidden ridge structure below Darsi and Addanki regions of Nellore Schist and Eastern Ghats Belts is imaged, which is ascribed to India-East Antarctic collision. The studied region coincides with anomalous thermal field, shallow LAB, up-dipping of over-thrusted layers and active seismicity. Possibly, a marginal ocean basin existed in this region during the Columbia and Rodinia supercontinent assembly periods, which underwent successive rifting episodes during Paleo-Mesoproterozoic Era. This study confirms that the Proterozoic sedimentation was much more intense and more widespread, than presently known. [ABSTRACT FROM AUTHOR]

In our study, we estimated earthquake source parameters and spectral decay characteristics of 378 seismograms corresponding to 80 small earthquake events with magnitudes ranging from 1.5 to 3.5. These earthquakes occurred between July 2011 and May 2012 in the Siang region of the Arunachal Himalaya which is not well-studied. To estimate source parameters, the classical Brune model is employed and through the analysis, a scaling relationship is established between the estimated corner frequency ( f c ) and seismic moment ( M 0 ), which can be expressed as M 0 = 1 × 1 0 22 f c - 3.18 . This relationship is in close agreement with previous studies conducted in the Arunachal and Himachal Himalaya regions. It provides support for deviations from self-similarity in the study area, offering valuable insights into the tectonics and structural heterogeneity beneath the Arunachal Himalaya. Our analysis revealed variations in source radius, ranging from 154.4 m to 312.6 m, and seismic moment, spanning from 2.37 × 1011 N-m to 9.32 × 1013 N-m. Interestingly, we observed an increasing trend in stress drop, ranging from 0.013 MPa to 3.26 MPa, within the same range of seismic moment. This significant variation in stress drop primarily occurs in the upper 10–15 km of the Earth's crust, indicating shear brittle failure in this upper crustal region. Furthermore, we conducted an in-depth examination of spectral parameters, including f c high-cut frequency (fmax), and high-frequency spectral decay parameter kappa (κ). Our study highlighted the dependence of κ and fmax estimates on both source characteristics and propagation path, with the source having the most substantial influence. This observation was substantiated through statistical analyses. Additionally, we explored the effect of recording site characteristics on κ and observed a significant contribution of shallow geology at the recording site. This was evident through a negative correlation between the site component of κ (κ0) and VS30, indicating that the local geology of the recording site plays a significant role in spectral parameter estimation. Based on our comprehensive data analysis and statistical observations, we conclude that the variations in source and decay parameters for earthquakes of different magnitudes are attributed to the diverse structural heterogeneities and complex seismotectonic processes underlying the Arunachal Himalaya region. [ABSTRACT FROM AUTHOR]

Surface wave dispersion curves are widely used to constrain earth velocity structures and are important to compute theoretical synthetic seismograms with a mode-summation approach. While the computation of dispersion curves requires searching roots of nonlinear functions, some high-mode may be missed with improper choice of searching steps. The asymptotic distribution of eigenvalues of the elastic wave equation can be used as auxiliary information to design a sophisticated scheme to compute the surface wave dispersion curves. In this study, we show the Weyl's law, combined with the Liouville transformation, can be exploited to derive asymptotic eigenvalue counting functions of elastic surface waves in a horizontally stratified or radially heterogeneous medium. We also show the derived formulation according to the Weyl's law, in its simple case, agrees with previous studies. The derived asymptotic eigenvalue counting functions are validated by comparison with numerical results. This study demonstrates the Weyl's law can be used to derive eigenvalue counting functions of surface waves in elastic media, and it is also possible to be applied to more complex media. [ABSTRACT FROM AUTHOR]

The Revell Site, located in Northwestern Ontario within the Canadian Shield, is being assessed as a potential Deep Geological Repository (DGR) for Canada's used nuclear fuel. Effective DGR establishment requires comprehensive subsurface assessment, particularly in evaluating structural stability, hydrogeological attributes, geological composition, and geochemical properties. Key among these considerations is understanding the three-dimensional characteristics of structural features to ensure the site's suitability for long-term containment of radioactive materials. This case study focuses on imaging and characterizing structures within the predominantly intact biotite granodiorite-tonalite host rock at the Revell Site. Borehole data reveals these structures as mostly sub-horizontal mafic intrusions, with thicknesses of up to 3.5 m. They appear either as separate, discrete entities or in stacked configurations. Despite their limited thickness, most of these intrusions exhibit discernible attributes in surface seismic images due to their distinct physical properties. Some, however, exhibit velocity variations that decrease their overall reflectivity properties. An integrative approach employing surface seismic, Vertical Seismic Profiles (VSP), and borehole data facilitates the spatial identification of over 30 of these mafic structures. This comprehensive characterization not only lays the foundational framework for future discrete fracture network models but also provides important support for simulations related to fluid flow dynamics, groundwater behavior, contaminant dispersion, and heat transport mechanisms within the Revell Site. This study underscores the critical role of in-depth subsurface characterization in ensuring the secure, long-term management of radioactive materials in DGRs. [ABSTRACT FROM AUTHOR]

Viscoelastic seismic modeling is essential to incorporate intrinsic attenuation effects in the wave propagation. Attenuation and dispersion can be quantified in terms of the quality factor Q. The consideration of these phenomena is crucial to improve the seismic processing and imaging, mainly in very deep reservoirs, having a thick salt layer above it. This work aims to evaluate the seismic attenuation effects through the 3D finite-difference viscoelastic seismic modeling using the three classical rheological models - Maxwell (M), Kelvin-Voigt (KV) and Standard Linear Solid (SLS). These viscoelastic models are analyzed comparatively considering the seismic attributes: waveform, amplitude and phase spectra. In addition, seismic attenuation of the real seismic data from Búzios field are evaluated through a frequency analysis. Synthetic seismograms of each rheological model are generated for a multi-layered geological model considering two approaches of Q distribution: a constant Q for the entire model and different Q values per layer. The amplitude spectra of real data confirmed that the shallowest post-salt package is more dissipative than deeper regions. The analysis of synthetic data shows that KV and SLS models proved to be effective to mimic seismic wave propagation at Búzios field. Thus, the KV model can be a good approach to simulate seismic waves in anelastic media due to its lower computational cost when compared to SLS. Our results also suggest that a Q distribution per layer can yield a seismic attenuation behavior similar to that observed in the seismic data of the Búzios field. [ABSTRACT FROM AUTHOR]

Conventional impedance inversion from post-stack zero-offset seismic data is usually based on the convolution model, and wave-equation based inversion is rarely used, although it is capable to precisely describe seismic wave propagation and invert impedance model with higher resolution. That is because there are more than one physical parameters involved in the conventional wave equation, making impedance inversion complicated. In this study, a one-dimensional (1D) wave equation, containing only the impedance parameter, is adopted and applied for the inversion of 1D impedance model by seismic waveform tomography. However, for a three-dimensional (3D) model, direct application of the 1D waveform tomography may lead to lateral discontinuities. Therefore, we propose to utilize a truncated Fourier series to parameterize the 3D impedance model, and then invert for the Fourier coefficients. With this parameterization scheme, the large- and small-scale components of the impedance model can be reconstructed stepwise by gradually increasing the number of Fourier coefficients. To efficiently and effectively invert the coefficients for the 3D model with salt structure, we propose a joint strategy, in which the low-frequency seismic data is used to invert for the Fourier coefficients representing the large-scale components of the model, while the high-frequency seismic data is applied to invert for the Fourier coefficients representing the small-scale components of the model. Tests on a 3D impedance model with salt structure result in models with high resolution and good spatial continuity, proving the feasibility and stability of the impedance inversion procedure. [ABSTRACT FROM AUTHOR]

A trend in the development of geophysics is to seek wave theory closer to the physical reality and derive corresponding wave equations to achieve highly accurate forward modeling, imaging, and inversion of complex structures. The generalized continuum mechanics (GCM) theory enriches the context of the conventional continuum mechanics theory by introducing the additional characteristic length scale parameters to represent the microstructural properties of the medium, and the asymmetric elastic wave equations derived from GCM theory can handle the influence of heterogeneity of a medium caused by the microstructural interactions on the propagation of seismic waves. To date, there are few studies on the numerical and analytical solutions of the elastic wave equations derived from the GCM theory, especially in the frequency band of seismic exploration. In addition, there are few studies in the existing literature that incorporate multiple theories and methods of the GCM theory into an integrated frame. In this paper, we introduce the concept of the multi-scale microstructural interactions and construct the quantitative relationship between the characteristic length scale parameter of the medium and the characteristic length scale parameter of the micro-pore reflecting the micro-pore structures, and then integrate the modified couple stress theory and the one-parameter second strain gradient theory into a unified framework for numerical modeling and analysis. [ABSTRACT FROM AUTHOR]

Rock damage from underground nuclear explosions (UNEs) has a strong influence on sub-surface gas movement and on seismic waveform characteristics, both of which are used to detect UNEs. Although advanced numerical simulation capabilities exist to predict rock damage patterns and corresponding detection signals, those predictions are dependent on (generally) unknown properties of the host rock. For example, the effects of in-situ mechanical heterogeneities on the explosively generated damage/fractures that provide gas flow pathways to the surface are not well understood, due largely to the difficulty in accessing and characterizing the near-source region. In this paper we demonstrate the emerging use of electrical resistivity tomography (ERT) for imaging rock damage and gas flow patterns resulting from two relatively small-scale underground chemical explosions. Pre-explosion ERT and crosshole seismic imaging revealed a natural fracture zone within the test bed. Post-explosion imaging revealed that the damage zone was non-symmetric and was focused primarily within the pre-existing fracture zone, located 10 m above the first explosion and 5 m above the second explosion. Time-lapse ERT imaging of heated air injected into the detonation borehole revealed the primary gas flow paths to be within the upper margin of the same primary damage zone. These results point to the utility of ERT imaging for understanding rock damage and gas flow patterns under experimental conditions, and to the importance of understanding the effects of geologic heterogeneity on UNE detection signals, particularly gas surface breakthrough times. [ABSTRACT FROM AUTHOR]

Sequences of buried basalt lavas found along continental margins may be associated with hydrocarbon deposits. However, they have heterogeneous velocity and density structures that degrade seismic imaging of basalt and sub-basalt structures and make them difficult to interpret. Hence, ray-tracing tomography is commonly performed on long-offset data to provide a velocity model as an aid, for advanced processing such as pre-stack migration, or as a starting point for full-waveform inversion. This is based on the assumption that at low-frequencies, the basalt sequence may be approximated by a single layer with a simple velocity gradient, which generates a turning wave refraction whose travel-time can be inverted to give both the velocity and thickness of the basalt layer. In this study, we generate synthetic long-offset data from a geologically constrained basalt model and perform subsequent ray-tracing inversion to provide an insight into the uncertainties that may be present by following this tomographic approach. By using Monte-Carlo analysis, we show that the thickness of the basalt layer may be under or over-estimated when compared to the true thickness, indicating that simple ray-tracing based on the assumption of turning rays is not appropriate when considering long-offset refractions from data over regions affected by extrusive basalt sequences. [ABSTRACT FROM AUTHOR]

Geophysical methods are instrumental in characterizing lithospheric-scale heterogeneity beneath continental rifts and collisional zones. Seismic imaging techniques, in particular have played a significant role in imaging lithospheric discontinuities within the crust as well as its Moho boundary with the underlying sub-continental lithospheric mantle (SCLM). However, in geodynamic settings where there have been significant mafic magmatic underplating, the seismic Moho and the petrological Moho become distinct, making gravity methods more effective to image the compositional variation between the crust and the SCLM. Our work assesses one of the gravity modeling methods that can be applied to image the petrological Moho by introducing a new approach, which is referred to as the spectral analysis with piecewise regression (SAPR). To test the effectiveness of SAPR, we used the World Gravity Model 2012 (WGM 2012) to calculate the depth to the petrological Moho and the depth to the top of the Precambrian crystalline basement beneath the Lake Turkana rift within the East African Rift System in southern Ethiopia and northern Kenya. Subsequently, the results of the Moho depths from the SAPR were compared with previous Moho depth estimates using other gravity methods as well as controlled-source seismic techniques. [ABSTRACT FROM AUTHOR]

The viscoelasticity of the subsurface media varies spatially, and such viscoelasticity can be represented concisely by a wave equation in the form of fractional temporal derivative (FTD). We have developed a strategy for simulating seismic waves propagating through a heterogeneous viscoelastic model. The FTD is transferred to fractional spatial derivatives (FSDs), and the FSDs are implemented through the fast Fourier transform (FFT), for improving the computational efficiency. However, the FFT implementation is not rigorously applicable to the heterogeneous model. In this paper, we have reformulated the FSD wave equation by introducing a spatial-position dependent filter. This spatial filter corrects the errors that are caused by the assumption of non-heterogeneity in the FFT implementation. This formulation appropriately represents the viscoelastic effect in seismic wave propagation, leading to the improvement on the accuracy of numerical simulation. [ABSTRACT FROM AUTHOR]

The effect of mantle viscosity structures on numerical simulations of geoid anomalies in the Ross Sea is investigated. Four relatively new S-wave seismic tomography models, SEMUCB_WM1, TX2019slab, S40RTS, and SEISGLOB2, are applied to convert seismic wave velocity anomalies into density anomalies. These density anomalies constitute the buoyancy-driven terms in mantle convection equations used to numerically simulate geoid anomalies corresponding to structures with different upper-to-lower mantle viscosity ratios. When the simulated geoid anomalies fit the observations best globally, the correlation coefficients between the geoid anomalies in the Ross Sea and the observed values are 0.56, 0.57, 0.57, and 0.46, respectively, for the four tomographic models. The best fits between the simulated geoid anomalies and observations in the Ross Sea area were obtained for upper-to-lower mantle viscosity ratios of 1:35, 1:45, 1:30, and 1:30 using the SEMUCB_WM1, TX2019slab, S40RTS, and SEISGLOB2, respectively. Within the range of the mantle viscosity structures selected for the simulation experiment, as the viscosity of the lower mantle gradually increases from the value at which the numerical results best fit the observations to the maximum value, the correlation coefficient between the simulated geoid anomalies and observations decreases faster for the Ross Sea area than on the global scale. We speculate that the lower mantle below the Ross Sea area may contain materials with lower viscosity than those in other mantle regions at the same depth. [ABSTRACT FROM AUTHOR]

We test a deep learning based denoising autoencoder algorithm on regional and teleseismic seismological and hydroacoustic datasets, which we compile from the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organisation. We focus on stations which can be relevant to investigate North Korean nuclear tests. Denoising of waveform records using autoencoder techniques potentially enables improved signal detection and processing due to lowered signal-to-noise ratios. We train and compare the performance of several different denoising autoencoder models, for short- and long waveform periods, trained on the complete station network as well as on individual stations. We investigate if the denoised waveform signals are useful for seismic source analysis and if they can still be reliably used in downstream analysis for further inferences on the seismic source type, i.e. seismic moment tensor analysis. The declared North Korean nuclear tests are a suitable benchmark test set, as they have extensively been researched and their source type and location might be assumed known. Verification of the source type is of particular interest for potential nuclear tests under international law. We find that care needs to be taken using the denoised waveform data, as a slight bias is introduced in the seismic moment tensor analysis. However we also find promising results hinting at possible future use of the technique for standard analyses, as it improves the investigation of smaller events. Autoencoder based denoising techniques could be employed in future routine frameworks to increase earthquake catalog completeness and possibly aid in detecting smaller potential treaty relevant events. [ABSTRACT FROM AUTHOR]

In this study, a novel strategy is presented for fault enhancement in seismic images, specifically in a time slice, through a combination of seismic attribute image reprocessing by a morphology algorithm, image lineament enhancement filters and image properties integration techniques. In the method presented herein, the most applicable seismic attributes for fault enhancement are initially selected based on their frequency of use. The selected attributes for fault enhancement are the similarity, the semblance, the variance and the chaos. Images of the extracted seismic attributes are processed by the morphology algorithms and the faults are enhanced. The applied morphology algorithm proposed here is the erosion algorithm. This algorithm boosts the background components of the image and also its random noise content. However, it is purposely used to be subtracted later from the original attribute image, which will then result in proper enhancement of the faults. Afterwards, edge detection filters are applied on this enhanced image. The selected edge enhancement filters here are the Sobel and the Laplacian of Gaussian. Image processing by these filters, which are applied separately on images, not in sequence, will further enhance faults in images. Nevertheless, it should be noted that at this point, the proposed method is not capable of enhancing all faults of any size and length and contrasting their properties with the surrounding media. To overcome this obstacle, information for the images processed so far is then integrated and illustrated by RGB image combination. There are various possibilities for that, but only the most appropriate combinations are presented. The proposed method is applied on 3D marine seismic data which contain several fault sets in different sizes and azimuths. The results of the proposed method are also compared with the conventional fault enhancement methods using seismic attributes. The results show that the proposed method can be considered as an alternative to the commonly used fault enhancement methods. [ABSTRACT FROM AUTHOR]

Even though micropolar theories are widely applied for engineering applications such as the design of metamaterials, applications in the study of the Earth's interior still remain limited and in particular in seismology. This is due to the lack of understanding of the required elastic material parameters present in the theory as well as the eigenfrequency ω r which is not observed in seismic data. By showing that the general dynamic equations of the Timoshenko's beam is a particular case of the micropolar theory we are able to connect micropolar elastic parameters to physically measurable quantities. We then present an alternative micropolar model that, based on the same physical basis as the original model, circumvents the problem of the original eigenfrequency ω r laking in seismological data. We finally validate our model with a seismic experiment and show it is relevant to explain observed seismic dispersion curves. [ABSTRACT FROM AUTHOR]

Throughout its history, South America has experienced megathrust earthquakes which have produced large tsunamis, devastating coastal cities in the near and far field. Studying these phenomena is important for tsunami hazard mitigation in this region. We propose 10 earthquake scenarios along the South American subduction zone on the basis of the seismic history of each region, seismic-geodetic coupling, general scaling relationships, among others. Tsunami run-up (coastal amplification) is then estimated using 200 nonuniform stochastic sources for each scenario, totaling 2000 simulations. Our results show great variability in run-up distribution along the Nazca-South America subduction zone, with some of the most affected areas being Valparaiso in Chile, with a most likely scenario of 20 m of run-up and a maximum scenario of 33 m; and Lima in Peru, with 25 and 40 m for the most likely and maximum scenarios, respectively. Similar results are seen in Iquique and Huasco in Chile. We have also identified 17 coastal locations with a higher vulnerability due to local amplification of tsunami run-up and two instances in which a regional amplification can occur due to tsunami directivity and coastal barriers. We conclude that tsunami hazard remains high along the coast of South America, even in areas where great earthquakes have recently occurred. [ABSTRACT FROM AUTHOR]

There are only a few analytical 2+1D models for tsunami propagation, most of which treat tsunami generation from a static deformation field isolated from the kinematics of the rupture. This work examines the behavior of tsunami propagation in a simple setup including a source time function which accounts for a time description of the rupture process on the tsunami source. An analytical solution is derived in the wavenumber domain, which is quickly inverted to space with the fast Fourier transform. The solution is obtained in closed form in the 1+1D case. The inclusion of temporal parameters of the source such as rise time and rupture velocity reveals a specific domain of very slow earthquakes that enhance tsunami amplitudes and produce non-negligible shifts in arrival times. The results confirm that amplification occurs when the rupture velocity matches the long-wave tsunami speed, and the static approximation corresponds to a limit case for (relatively) fast ruptures. [ABSTRACT FROM AUTHOR]

The Ethiopian Rift is a unique natural environment to study the different stages of evolution from initial continental rifting to embryonic seafloor spreading. We used transdimensional hierarchical Bayesian seismic ambient noise tomography to first construct group velocity maps across the Afar, Main Ethiopian Rift, and the adjoining plateaus, and then inverted these for a shear wave velocity model. The uppermost mantle shear wave velocity ranges between 3.9 and 4.3 km/s, 5–15% lower than the upper mantle velocity in the PREM model. The combined effect of temperature and partial melt is needed to explain a 15% shear wave velocity reduction in the uppermost mantle. Tectonic and magmatic activities are not limited to the rift center, but instead are widespread within the upper crust beneath the Main Ethiopian Rift and Afar. The Main Ethiopian Rift is dominated by two velocity belts, the Wonji Fault Belt along the rift axis and the Silti-Debre Zeit Fault Zone on the western side of the Central Main Ethiopian Rift; the Boru-Toru structural high appears to serve as a transfer zone between them, exhibiting relatively high crustal velocities (3.6 km/s) at 14 km depth. Low velocities persist in the crust beneath the rift flanks and border faults, indicating that they are still tectonically and magmatically active. The crust beneath the western plateau is characterized by a low-velocity anomaly, implying that the plateau is also active. Low-velocity linear belts are further imaged beneath the western and eastern plateaus, away from the active rift axes. These off-axis belts could represent failed or buried rifts. [ABSTRACT FROM AUTHOR]

Seismic events produced by block rotations about vertical axis occur in many geodynamic contexts. In this study, we show that these rotations can be accounted for using the proper theory, namely micropolar theory, and a new asymmetric moment tensor can be derived. We then apply this new theory to the Kaikōura earthquake (2016/11/14), Mw 7.8, one of the most complex earthquakes ever recorded with modern instrumental techniques. Using advanced numerical techniques, we compute synthetic seismograms including a full asymmetric moment tensor and we show that it induces measurable differences in the waveforms proving that seismic data can record the effects of the block rotations observed in the field. Therefore, the theory developed in this work provides a full framework for future dynamic source inversions of asymmetric moment tensors. [ABSTRACT FROM AUTHOR]

Morlet wavelet analysis is a method of studying the periodic spectrum of non-stationary physical signals and is applied to the Himalayan Tectonic Belt to explore whether there is any seismic periodicity, and to explore the possibility of harmony or commonality of properties among the seismic activities of different zones. The earthquake sequence during 1951–2016 with magnitudes M ≥ 6.0 is analysed. Wavelet non-periodicity for the Centre zone suggests a non-uniform spatial–temporal distribution of earthquake movement between plates which may relate with the rare great earthquakes, while the periodicities for the west and east zones may suggest the concurrence with the adjustment of the tectonic movement of the east- and west-end regions of the Himalayan Tectonic Belt relative to its central core. These three zones collectively form the Himalayan Tectonic Belt. This contains a periodicity of about five years of seismic activity that tests successfully with a 95% confidence statistic. Borrowing from the concept of musical harmony, this is the significant seismic harmonic which reflects the Belt's pervasive tectonic stress and an overall harmony of continent–continent plate convergence. Morlet wavelet analysis also reveals the Himalayan Tectonic Belt and the Pamir–Hindu Kush Tectonic Zone to be engaged as a big new family: the Himalayan Tectonic Belt Plus. It is demonstrated that this new whole also has seismic harmony with the common property again being the 5-year periodicity. This indicates a unified structure of pervading active stress and seismic harmony permeating the overall seismicity. [ABSTRACT FROM AUTHOR]

Three seismic gaps lie along the Pacific Coast of Mexico, the Tehuantepec, Guerrero, and Colima–Jalisco gaps. In the Tehuantepec seismic gap, there has not been a M > 7 earthquake since 1902 until the M w = 8.2 in 2017 which was an intraplate event. Furthermore, there has not been significant seismic activity in the Guerrero gap for over fifty years; therefore, it is considered as potentially likely to produce a major event. Based on historical seismicity and the National Seismological Service of Mexico's earthquakes catalog, the last major earthquake in the Colima–Jalisco gap struck in 1995 with M w = 8.0 . To better understand the tsunami hazards due to the generation of near-field tsunamis in Mexico, in this work we characterize hypothetical events of magnitudes M w = 8.0 , M w = 8.2 , and M w = 8.2 in the Tehuantepec, Guerrero, and Colima–Jalisco gaps, respectively. We generated 99 earthquakes with stochastic k - 2 finite fault slip distributions and one earthquake with uniform slip distribution at each seismic gap. The non-planar geometry of the megathrust for each rupture area was taken into account. For each seismic gap, we compute the vertical co-seismic displacement by adding up the contribution from all point sources distributed over a grid mesh on each of the faults. Under the passive tsunami generation assumption, we simulated the tsunami wave-field propagation to obtain the coastal amplitude along the Pacific coast of Mexico. The numerical results show likely maximum peak amplitudes of ∼ 8 m, ∼ 14 m, and ∼ 15 m, in Tehuantepec, Guerrero, and Colima–Jalisco gaps, respectively. The uniform slip distribution assumption over stochastic scenarios shows an average underestimation factor of 1.3 for the three seismic gaps. While our computations were carried out carefully and accurately, our models have limitations. Thus, our results cannot be used as an authoritative tsunami hazard assessment. [ABSTRACT FROM AUTHOR]

A new algorithm and computer code for seismic tomography in anisotropic inhomogeneous media was developed. The new tomographic approach is a generalization of classical isotropic seismic tomography which introduces spatially and directionally varying slowness. The velocity model was considered as a stack of homogeneous blocks in contact, with each block individually parameterized using background velocities of P- and S-waves and a set of 21 anisotropy parameters. The inverse problem was solved sequentially in five steps, using the velocity model from the previous step as the starting model for the subsequent step. These steps form a chain with increasing complexity: (1) isotropic homogeneous model; (2) isotropic velocity model with vertical velocity gradient; (3) 3-D inhomogeneous isotropic velocity model; (4) 3-D inhomogeneous model with uniform anisotropy; (5) 3-D inhomogeneous generally anisotropic model. The new algorithm was applied to real bulletin data of 18 seismic stations deployed in SW Iceland and operated favourably for the monitoring of local swarm-like seismicity. Next, the resolution, robustness and accuracy of the inversion were discussed using real and synthetic data. Real data inversion revealed a predominantly depth-dependent isotropic velocity background and additional general 3-D anisotropy. The parameterization of the medium was too flexible to allow for a reliable interpretation of the anisotropy inside the elementary blocks and a cluster analysis was applied to stabilize the inversion results. Three important clusters were identified as a result. The orientation of the anisotropy (fast and slow P-wave propagation directions) of two clusters coincided with the strike of the documented faults. The orientation of the anisotropy in the third cluster was interpreted as a consequence of the fluid dynamics around Kleifarvatn Lake. The P-wave anisotropy strength reached a value of ± 5–8%. [ABSTRACT FROM AUTHOR]

In post-mining regions with seismic hazard, timely decision making for risk management faces the challenge of quick and reliable detection and location of seismic events. As a response to the increasing density of monitoring stations, generating large volumes of seismic data, automatic, full waveform-based methods have been developed in recent years in global seismology. Such methods often cannot be directly applied to post-mining monitoring with a limited station coverage, as it is the case when temporarily networks are installed as an emergency response. In this paper we propose a new methodology that bridges this gap and enables the application of a full waveform, backprojection based method (BackTrackBB) to data of sparse network. The methodology was successfully tested on an abandoned and flooded underground coalmine in South-eastern France. Steps preceding BackTrackBB application were implemented in order to remove the coherent noise that otherwise results in numerous false detections. First results indicate that seismic activity in the study area is controlled by water level variation within former room-and-pillar mine works and fault segments (re)activation below them. [ABSTRACT FROM AUTHOR]

Earthquake diffusion and the migration behaviour of seismic clusters are commonly studied to provide insight on the spatiotemporal evolution of seismicity and the interplaying driving mechanisms. Using a high-resolution relocated catalogue, we study the variations of the earthquake diffusion rates in the Western Gulf of Corinth during 2013–2014, a period with abundant local seismicity, including intense microseismic background, seismic swarms and mainshock-aftershock sequences. We treat earthquake occurrence as a point process in time and space and estimate the diffusion rates of the main seismic sequences and the background seismicity in terms of normalized spatial histograms and the evolutions of the mean squared distance of seismicity with time. The statistical analysis of the studied seismic sequences reveals that the mean squared distance of the hypocentres increases slowly with time, at a much lower rate than for a normal diffusion process. Such findings confirm previous results on weak earthquake diffusion, analogous to subdiffusion, in regional and clustered seismicity. In addition, seismic swarms associated with pore fluid pressure diffusion present considerably higher diffusion exponents compared to mainshock-aftershock-type sequences that are consistent with primary or secondary stress triggering effects and stress corrosion. The observed variations of the earthquake diffusion rates indicate the stochastic nature of the phenomenon and may provide novel constraints on the triggering mechanisms of clustered seismic activity in the Western Gulf of Corinth and in other seismically active regions. [ABSTRACT FROM AUTHOR]

All nuclear explosions are banned by the Comprehensive Nuclear-Test-Ban Treaty. In the context of the treaty a verification regime was put into place to detect, locate, and characterize nuclear explosions at any time, by anyone and everywhere on the Earth. The International Monitoring System, which plays a key role in the verification regime, was set up by the Preparatory Commission of the Comprehensive Nuclear-Test-Ban Treaty Organization. Out of the several different monitoring techniques applied in the International Monitoring System the seismic waveform approach is the most effective and reliable technology for monitoring nuclear explosions underground. This study introduces a deterministic method of threshold monitoring that allows to asses a lower body wave magnitude limit of a potential seismic event in a certain geographical region, that can be detected by those seismic stations being part of the International Monitoring System network. The method is based on measurements of ambient seismic noise levels at the individual seismic stations along with global distance corrections terms for the body wave magnitude. The results suggest that an average global detection capability of approximately body wave magnitude 4.0 can be achieved using only stations from the primary seismic network of the International Monitoring System. The incorporation of seismic stations from the auxiliary seismic network leads to a slight improvement of the detection capability, while the use and analysis of wave arrivals from distances greater than 120 ∘ results in a significant improvement of the detection capability. Temporal variations in terms of hourly and monthly changes of the global detection capability can not be observed. Overall, comparisons between detection capability and manually retrieved body wave magnitudes from the Reviewed Event Bulletin suggest, that our method yields a more conservative estimation of the detection capability and that in reality detection thresholds might be even lower than estimated. [ABSTRACT FROM AUTHOR]

As the Treaty Monitoring Community seeks to lower detection thresholds across its sparse sensor network, single-station location estimates and accurate backazimuth predictions become increasingly important. Accurate backazimuth predictions are traditionally limited to array stations, where beamforming provides high-confidence backazimuth prediction that can be reliably passed on to the associator. Three-component stations, on the other hand, rely on polarization analysis for backazimuth prediction, which suffers from both high error and low confidence. As such, very few three-component backazimuth predictions are passed on to the association algorithm. This study presents BazNet, a deep neural-network that takes in a three-component seismogram and produces both a backazimuth prediction and corresponding certainty measure. For existing stations with ample historical training data, the technique achieves an overall median absolute deviation of around 14 ∘ , a modest improvement over the 15 ∘ achieved by polarization. More importantly, each estimate is accompanied by a robust certainty measure, allowing the selection of high-confidence predictions to be passed on to the associator. Using the BazNet certainty measure, roughly 60% of all three-component predictions can be selected with a median absolute deviation of just 6 ∘ , which is on par with the predictions from a full beamformed seismic array. This represents a sevenfold improvement over the 8% of signals similarly selectable via polarization analysis. BazNet performance is demonstrated against 10 years of waveform data from 561,154 cataloged arrivals across nine stations selected from the global IMS Network: STKA, CPUP, VNDA, LPAZ, AAK, BOSA, ULM, BATI, INK. [ABSTRACT FROM AUTHOR]

In this work, we show that large-scale processes of the long-range propagation and scattering of infrasound signals in stratospheric and thermospheric waveguides are to some extent similar to the smaller scale processes of waveguide propagation and scattering of acoustic signals in stably stratified ABL. Moreover, we note a resemblance between the zeroth order tropospheric mode and the Lamb mode that is observed for larger nuclear explosions. The results of physical modeling of the long-range propagation of infrasound signals in the atmosphere by studying the propagation of the acoustic pulses from detonation sources in the stably stratified atmospheric boundary layer (ABL) are presented. Such modeling became possible due to continuous measurement of the wind velocity and temperature profiles in the ABL by Doppler sodar and temperature profiler, respectively. The resemblance between the propagation of the infrasound signals from surface explosions (20–70 t TNT) in the "shallow" tropospheric wave guide and propagation of Lamb mode from nuclear explosions is found. Also, the propagation of infrasound signals in the stratospheric wave guide is modeled by studying the propagation of acoustic pulses in the ABL during morning hours (after sunrise), when an effective sound speed profile in the ABL is similar to that in the stratospheric wave guide. The instantaneous vertical profiles of the effective sound speed and wind velocity fluctuations in the thin layers of the stably stratified ABL located at different heights (up to 700 m) above the ground and at different distances from the source (up to 2.25 km) have been retrieved using the wave forms and travel times of the recorded arrivals of the acoustic pulses from the detonation sources. The effect of scattering of acoustic pulses from the fine-scale layered structure of the stably stratified ABL is studied to model the effect of scattering of infrasound signals from the fine-scale layered structure of the stratosphere. [ABSTRACT FROM AUTHOR]

A research concept that was first presented on a poster at a Comprehensive Nuclear-Test-Ban Treaty (CTBT) conference in 2009 has resulted in a fully-fledged operational software product named NET-VISA (Network Processing Vertically Integrated Seismic Analysis), which performs the Network Processing step of the automatic processing at the International Data Centre (IDC). It has become one of the tools of the waveform analysts to review and improve the IDC standard automatic third Standard Events List (SEL3) bulletin and produce the Reviewed Events Bulletin (REB), the only global seismo-acoustic bulletin. The basic scientific concepts imbedded into NET-VISA are briefly summarized in this paper but the emphasis is on the process of adopting, developing, adapting, testing, and operationalizing the initial prototype. Extensive off-line testing has shown that one of the expected benefits of NET-VISA was the significant reduction in missed events rate compared to the current Network Processing software, Global Association (GA). NET-VISA also finds some events previously missed by manual review. Starting in July 2017, and as of January 2020, NET-VISA generates an automatic bulletin, called VSEL3, in parallel to SEL3, which is also used by analysts since January 2018. Tracing the origin of the REB events has confirmed the significant reduction in missed events when complementary VSEL3 events are reviewed in addition to the SEL3 events. If sufficient confidence is established, NET-VISA has the potential to replace GA in producing the standard automatic SEL3 bulletin. [ABSTRACT FROM AUTHOR]

We present our analysis of the detections from the Piszkés-tető, Hungary infrasound array that has been operational since May 2017. We introduce an automatic search method to associate infrasound detections with a preliminary list of explosions detected by the seismic network. Once infrasound detections are associated with seismic events, we relocate the events using both the seismic arrival time and infrasound azimuth observations. We validate our methodology on ground truth events, i.e., explosions confirmed by the mine operators. We demonstrate that seismo-acoustic locations are able to identify the source of explosions even for closely spaced quarries. The discriminative power of the infrasound azimuth is strongest at near-field distances, where local and regional seismic discrimination methods have the most difficulties. [ABSTRACT FROM AUTHOR]

High-angle or steeply-dipping faults, salt flanks, and subsalt sediments are important structures for many applications such as oil/gas and geothermal energy exploration. However, conventional seismic imaging has poor illuminations in these areas because of using singly scattered waves or primary reflections. By using high-order scattered waves in the reverse time migration (RTM), particularly the secondary scattered waves, we can enhance the seismic illumination for imaging the steep faults and subsalt areas. In our method, we first identify reflection features such as geological layer boundaries and discontinuities, based on the conventional RTM images. We then modify the migration model by adding potential point scatterers into the reflection features. These artificial point scatterers could be randomly placed on and around the reflection features to illuminate the structures where primaries cannot reach. The random placement of the point scatterers differentiates our method from previous methods using duplex or prism waves to image high-angle structures where true reflector locations are particularly identified. In addition, we assign the artificial point scatterers as density perturbations without modifying the velocity model so that they do not affect the spatial position of the imaged reflectors. The proposed method of high-order scattering imaging only requires one additional implementation of RTM but can substantially improve the image quality around high-angle faults and subsalt regions. We verify the effectiveness of our method using two synthetic models, a trapezoidal model and the Sigsbee2B model. And the results demonstrate that our new method produces better images for steep faults and subsalt areas compared with those obtained using conventional RTM. [ABSTRACT FROM AUTHOR]

Seismic risk analyses derive from earthquake catalogs a recurrence relation linking earthquake activity rate to magnitude. The most widely employed model is the log-linear Gutenberg–Richter relation (Gutenberg and Richter, Science, 83, 183–185, 1936; Gutenberg and Richter, Bulletin of the Seismological Society of America, 46(3), 105–145, 1945), with modifications at larger magnitudes (Cosentino et al., Bulletin of the Seismological Society of America, 67, 1615–1623, 1977; Kijko and Sellevoll, Bulletin of the Seismological Society of America, 79(3), 644–654, 1989; Page, Bulletin of the Seismological Society of America, 58, 1131–1168, 1968; Pisarenko and Sornette, Pure and Applied Geophysics, 160, 2343–2364, 2003; Turcotte, Physics of the Earth and Planetary Interiors, 111, 275–293, 1999). This relation leads to exponentially distributed magnitudes truncated to a maximum magnitude, a priori fixed under geophysical considerations. In this paper, we assume seismic events occur according to a Poisson distribution, but we propose to model the tail distribution of magnitudes with a generalized Pareto distribution (GPD). The GPD parameters are estimated with a maximum likelihood procedure. This GPD-based model gives rise to a new recurrence model that differs from the Gutenberg–Richter Law. It eliminates the need to introduce a maximum magnitude in the analysis that is difficult to determine. This paper details the expression of the estimators of the GPD parameters and the asymptotic normal distribution when the shape parameter ξ > - 1 . This asymptotic distribution yields confidence intervals for all parameters. The GPD parameter estimators account for the following features of the data set: (a) seismic events are collected on periods whose span depends on their magnitudes; (b) magnitudes are imprecisely known: each magnitude is supposed to uniformly belong to an interval of length 0.5. Our new model is estimated from information coming from the FCAT17 catalog. This catalog collects seismic events from the Alps region in France. We conduct an uncertainty analysis, and we quantify the impact of estimation uncertainty on the recurrence model. [ABSTRACT FROM AUTHOR]

Migration deconvolution is an image domain approach to least-squares migration, which is considered the state-of-the-art algorithm for obtaining seismic reflectivity models of the earth from seismic acquisition results. Seismic imaging is an active research field with the development over the last few years of several techniques that have mitigated imaging issues. Ongoing research aims to improve image resolution and thus provide a more reliable seismic amplitude for the interpreter. Migration deconvolution can be framed as an inverse problem in the image domain to mitigate image resolution problems and reduce migration artifacts. This paper presents a migration deconvolution method via deep learning based on the Hessian filter least-squares migration (HF-LSM) algorithm. The idea is to use deep learning techniques to model the inverse operator instead of directly estimating the inverse Hessian matrix. A data set is generated from a given velocity model by applying Born modeling to the migrated image, followed by application of the reverse time migration algorithm. The resultant data set is then used to train several neural network models. The networks learn the blurring operator that describes the image degradation due to the effects of acquisition geometry. Three different network topologies were developed to handle this problem: a simple fully convolutional neural network, a U-Net and a generative adversarial network. Our results show that the proposed approach provides images of higher resolution and superior quality than the traditional HF-LSM workflow. [ABSTRACT FROM AUTHOR]

This paper studies the propagation of SH wavelet intensity through the random distribution of aligned line cracks in a two-dimensional (2-D) homogeneous medium as the simplest mathematical model of the heterogeneous Earth medium. The scattering process of a single line crack is described by Mathieu functions. The random distribution of cracks is well represented by the scattering coefficient, that is, the scattering power per unit area. Two methods are proposed for the derivation of the space–time distribution of the intensity Green function for the unit isotropic radiation from a point source: one is the single scattering approximation and the other is the Monte Carlo simulation. The former and the latter are deterministic and stochastic methods, respectively. In the case that the wavenumber is larger than the reciprocal of the crack length, synthesized time traces show that direct wavelets near the ray direction parallel to the line cracks decrease according to the geometrical decay with increasing travel distance as if there exists a transparent channel along the crack line direction; however, those near the direction normal to the line cracks decrease more rapidly because of strong scattering attenuation. At large lapse times, multiple scattering produces a swelling around the source location, which is prolonged to the crack line direction. Those characteristics well reflect the anisotropy of the line crack scattering. The Monte Carlo simulation presented here could be a fundamental base for the study of more realistic crack scattering problems. [ABSTRACT FROM AUTHOR]

In this article, we describe a method for predicting of the epicenter location region of Kamchatka and Commander earthquakes (EQs). It is based on the properties of the precursory atmospheric ULF/ELF (1–30 Hz) radiation and the spatial statistics of local EQs in relation to the Kuril-Kamchatka and Aleutian trenches. From this statistics, it follows that more than 90% of events with magnitude (ML) of more than 5 occur in the gap ~ 150 km west of the Kuril-Kamchatka trench and north-east of the Aleutian trench. Additionally, we obtain the approximate location of the epicenter by determining the position of the radiation source. We suppose that it is caused by gas eruption from the hearth of the EQ to the trench and has the nearest location to the EQ epicenter. Further, we obtain other parameters of supposed position of the epicenter from spatial statistics of EQs relative to the projection of radiation source, which approximately coincides with gas emanation area in the ocean surface. The main drawbacks of the method are the dependence of its accuracy on the industrial interferences and the ambiguity of determining the epicenter location in the case of single-point registration of radiation when the main lobe of the azimuthal distribution of radiation crosses both trenches. [ABSTRACT FROM AUTHOR]

Large impact structures are characterized by peak ring and central uplifts with lateral/vertical mass transport during late formation stages. Here we investigate the Chicxulub crater, which has been surveyed by an array of marine, aerial and land-borne geophysical methods. Seismic reflection surveys in its central sector have shown lack of resolution, making it difficult to image the central uplift. We develop an integrated seismic and gravity model for the structural elements, imaging the central uplift and melt and breccia units. The 3D velocity model built from interpolation of seismic data is validated using perfectly matched layer seismic acoustic wave propagation modeling, optimized at grazing incidence using the shift in the frequency domain. Modeling shows that lack of illumination relates to seismic energy that remains trapped in an upper low-velocity zone corresponding to the carbonate sediments, upper melt/breccias and surrounding faulted blocks. After conversion of seismic velocities into a volume of density values, we apply parallel forward gravity modeling to constrain the size and shape of the central uplift, which has a ~ 40 km diameter concave upwards top lying at ~ 3.5–4.5 km depth. The preferred model provides a high-resolution image of crater units and structure. The gravity response of modeled units shows asymmetries in structure and the distribution of breccias, melt and target carbonates. Finally, we apply an adjoint reverse time migration approach for seismic imaging using the density and velocity models built for the acoustic wave propagation and gravity modeling, which allows improved modeling of the crater structure. [ABSTRACT FROM AUTHOR]

Pakistan and its surroundings are among the most seismically active regions in the world. Occurrence of major earthquakes in Pakistan is a real hazard for life. In this study, an attempt has been made to estimate the seismic hazard in terms of earthquake occurrence conditional probabilities in different zones of Pakistan. We assumed that the temporal data of earthquakes, that occurred in different regions of Pakistan follow a certain probability distribution. In present study, the 10 probability models that include the exponential, gamma, generalized Pareto, inverse Pareto, kappa, log-logistic, lognormal, Pareto, transformed gamma and Weibull are used to analyze the interevent times of the earthquakes with M w ≥ 5.5 in different regions of Pakistan. In current study, Pakistan territory has been divided into five different seismic zones based on their tectonic settings. Parameters of the considered distributions are estimated by using the maximum likelihood method using the reviewed earthquake catalogue compiled by Centre for Earthquake Studies (CES), Pakistan. The best suitable model in each seismic zone is selected through various model selection criteria. The results show that the log-logistic distribution provides the best representation of earthquake temporal data of plate convergence zones; while the gamma distribution best explains the temporal seismicity of other tectonic regimes including the transform plate boundary, fold and thrust belts and stable plains. The most suitable gamma and log-logistic models are used for the earthquake occurrence probabilities in different regions of Pakistan. The highest (> 96 %) and the least (> 30 %) earthquake occurrence conditional probabilities are respectively observed for Zones I and V among five seismic zones for different combinations of elapsed time (τ) and time interval (υ) . The estimated probability of M w ≥ 5.5 event reaches (60–98)% in next (3–9) years time interval without any elapsed time (τ = 0) in Zones II to IV (the time periods corresponding to 3–9 years time interval are 2016–2022, 2020–2026 and 2019–2025 in Zones I, II and III respectively) since the last event occurrence time in each of these seismic zone. The mean values of the interevent time for an earthquake occurrence of M w ≥ 5.5 in Zone-I to Zone-V are respectively calculated as 0.90, 1.78, 4.52, 3.35 and 11.76 years, respectively. The results of the present study indicate that Zone-I has potential capability of producing an earthquake of magnitude 5.5 or higher in next 3 years (2019–2021) time interval since the last event occurrence in May 2018. The earthquake hazard curves associated with earthquake occurrence probabilities are also developed in order to understand the current and future earthquake mechanism in the study region. [ABSTRACT FROM AUTHOR]

The intraplate and interplate seismic catalogues of Iquique (Chile) area were investigated by using seismological (b value of the Gutenberg–Richter law), fractal (Allan Factor and detrended fluctuation analysis) and topological (Horizontal Visibility Graph) methods. The two catalogues show different stress state indicated by the different b value, larger for the intraplate seismicity. The interplate seismicity shows significant time-clustering behavior of the earthquake occurrence times at large time scales, while the time distribution of the intraplate earthquakes seems to be featured by random fluctuations without any significant sign of time-clustering. Different role seems to be played by aftershocks in the two investigated seismic zones, revealed by the different time distribution of the aftershock-depleted catalogue of the intraplate zone, which remains Poissonian for most of the time scales, and the interplate zone that still keeps its clustering phenomenon but at higher timescales, thus shortening its clustering time scale range. Furthermore, the interevent times of the interplate catalogue are characterized by persistent dynamics even after removing the aftershocks, while uncorrelated fluctuations characterize the behavior of interevent intervals of the whole as well as aftershock-depleted intraplate catalogue. Additionally, the magnitudes of both catalogues are characterized by time-reversibility before and after the aftershock removal; while the interevent times of intraplate catalogue change their status from time-irreversibility to time-reversibility after removing the aftershocks. The obtained results seem to evidence a different role played by the aftershocks in the intraplate and interplate zones, that needs to be further explored. [ABSTRACT FROM AUTHOR]

Passive seismic source imaging can extract geophysical information from underground noise and has been widely utilized in geophysical research. Compared with conventional active seismic exploration, it is low-cost and eco-friendly; however, the application of passive seismic data is limited by coherent noise in the virtual-shot gathers. An approach involving direct denoising in the virtual-shot gathers has not previously been discussed; therefore, we present an iterative denoising strategy for passive seismic data. The reflection-preserving characteristic of focal transformation is adopted in the virtual-shot gathers to eliminate the coherent noise, and L1-norm sparse inversion is utilized to obtain a more accurate solution during focal transformation. A key aspect of this strategy is clean focal operator building at high noise levels. We apply local similarity as the criterion for extracting the majority of reflection energy for the focal operator. Because of strong coherent noise, a clean focal operator cannot be obtained in one iteration. We therefore obtain both denoised passive seismic data and a clean focal operator by denoising using a cleaner focal operator and operator building using updated denoising results. The presented approach can overcome the limits of coherent noise in virtual-shot gathers, which is significant for subsequent data processing and wider application. Synthetic examples achieve excellent performance in coherent noise attenuation and reflection energy reconstruction, especially in far-offset sections. [ABSTRACT FROM AUTHOR]

The quality of seismic migration images strongly depends on accurate velocity models. However, migration velocity models often contain errors causing artifacts which degrade seismic resolution and fidelity. Here, we propose a method to enhance seismic migration images using processes in angle-domain common-image-gathers (ADCIGs), without requiring modification to the velocity model. The ADCIGs impacted from velocity errors show non-flat gathers and scattered noise. To enhance the imaging quality in ADCIGs, these problems need to be treated prior to stacking. Ideally, layers in ADCIGs should be flat without smearing amplitudes through all angles. Using this principle, we iteratively process one reflection at a time by isolating it into a local window and then flattening it. Also, we apply internal processing steps to enhance the signal. The algorithm for segregating a reflection, referred to as the connected-component labeling (CCL) method, is the primary method for extracting any feature that has a continuous form within the ADCIGs. Moreover, this method is insensitive to scattered noise so it is a suitable approach for classifying reflections or noise. To test the efficiency of this algorithm, we create ADCIGs containing relatively poor-quality images using an inaccurate migration velocity model. The primary objective of this test is to show how well the CCL method can select various forms of reflections in the presence of various levels of noise. The results show that the CCL method is capable of individually decomposing a reflection, which can facilitate processing in the internal workflow. Finally, we evaluate the quality of processed migration images by comparing our processed Poynting-vector reverse time migration (PVRTM) images with reverse time migration applied Laplacian filter and partial stacked PVRTM. These comparable migration images are benchmarked against a baseline synthetic reflector model. The essential improvements of our method are addressed by the migration sections, namely the removal of diffraction artifacts at both small and large scales and the improvement of phase coherency. The final stacked migration image produced with our methods shows clearer geological features that are capable of delivering a more reliable seismic interpretation. [ABSTRACT FROM AUTHOR]

An improved pattern informatics (PI) method, inspired by the soup-of-groups model, is developed in this study. This improved method reveals its potential in reducing the noise that, when the classical PI is applied, can yield misleading conclusions about the existence of "hotspots" in the vicinity of the epicenter of a strong earthquake. The application of this new method to Italian seismicity from 1985 to 2018 properly reveals "hotspots" around the two strongest earthquakes that occurred in 2009 (L'Aquila earthquake) and 2016 (Norcia earthquake), suggesting its utility in studies devoted to earthquake prediction. [ABSTRACT FROM AUTHOR]

Variations in the topography and thickness of the near-surface low-velocity weathering zone negatively impact the quality of onshore seismic data and images. Such impact can be mitigated using accurate near-surface velocity models. We propose to estimate near-surface velocities using a layer-cell tomography method which sequentially combines the model parameterizations of multiscale deformable-layer tomography (DLT) and cell tomography. We first estimate the long-wavelength velocity variations using multiscale DLT, which maps the undulating geometry of the weathering base where the angular coverage of first-arrival raypaths is limited. Then we convert the solution model of multiscale DLT into a cell model, and further determine the short-wavelength velocity variations using cell tomography. We test the proposed layer-cell tomography method using synthetic datasets and a field dataset. The results indicate that the new method improves the resolution of the velocity model from the multiscale DLT, and reduces the artifacts in the velocity solution compared to those observed in the cell tomography method. Reverse time migration of the synthetic data shows that using the velocity model from the layer-cell tomography method gives more accurate images of reflectors. For the field dataset test, the layer-cell tomography method yields a high-resolution near-surface velocity model that could be validated by the improvements in the tomostatic correction results. [ABSTRACT FROM AUTHOR]

In the present paper, a new method referred to as the Poisson probability map (PMAP) method is presented for identifying and visualizing seismic quiescence. With the PMAP, the P-value is defined as the probability that consecutive earthquakes occur according to a homogeneous Poisson process: the smaller the P-value, the less frequently the longer time interval is observed, i.e. the more significant the seismic quiescence. The PMAP method was applied to the sequence which preceded the Kurile Islands earthquake that occurred on 15 November 2006 [Mw = 8.3 and the centroid = (154.33 °E, 46.71 °N)]. The seismic quiescence is identified by a small P-value of 9.0 × 10–5 that was found to start in 1990.1, which lasted for 15.4 years and ended in 2005.5 within a circular area centered at (153.8 °E, 47.1 °N) and with a radius of 26 km. This seismic quiescence has not previously been recognized using any other method. [ABSTRACT FROM AUTHOR]

The Kokkola–Kymi Deep Seismic Sounding profile crosses the Fennoscandian Shield in northwest-southeast (NW–SE) direction from Bothnian belt to Wiborg rapakivi batholith through Central Finland granitoid complex (CFGC). The 490-km refraction seismic line is perpendicular to the orogenic strike in Central Finland and entirely based on data from quarry blasts and road construction sites in years 2012 and 2013. The campaign resulted in 63 usable seismic record sections. The average perpendicular distance between these and the profile was 14 km. Tomographic velocity models were computed with JIVE3D program. The velocity fields of the tomographic models were used as starting points in the ray tracing modelling. Based on collected seismic sections a layer-cake model was prepared with the ray tracing package SEIS83. Along the profile, upper crust has an average thickness of 22 km average, and P-wave velocities (Vp) of 5.9–6.2 km/s near the surface, increasing downward to 6.25–6.40 km/s. The thickness of middle crust is 14 km below CFGC, 20 km in SE and 25 km in NW, but Vp ranges from 6.6 to 6.9 km/s in all parts. Lower crust has Vp values of 7.35–7.4 km/s and lithospheric mantle 8.2–8.25 km/s. Moho depth is 54 km in NW part, 63 km in the middle and 43 km in SW, yet a 55-km long section in the middle does not reveal an obvious Moho reflection. S-wave velocities vary from 3.4 km/s near the surface to 4.85 km/s in upper mantle, consistently with P-wave velocity variations. Results confirm the previously assumed high-velocity lower crust and depression of Moho in central Finland. [ABSTRACT FROM AUTHOR]

The earthquakes in the western East Sea (Sea of Japan) mostly occur in the continental margin off the east coast of the Korean Peninsula. The seismic hazard potentials in and around the western Ease Sea are studied based on analyses of tectonic structures, seismicity features, earthquake source properties, Coulomb stress changes, and strong ground motions. The earthquake source mechanisms suggest that paleo-rifting structures in the western East Sea were activated by the current stress field. A low stress cumulation rate results in the occurrence of earthquakes with long recurrence intervals. The background seismicity suggests that earthquakes with magnitudes M w 5.0, 6.0, and 7.0 may occur within every ∼ 44, ∼ 336, and ∼ 2550 years at 95 % confidence level. The spatial distribution of earthquakes changes with time. Most earthquakes are clustered within ∼ 60 km from the coast. The seismicity analysis indicates an apparent increase of moderate-size ( M w 3–5) earthquakes since the 2011 M w 9.0 Tohoku-Oki megathrust earthquake. Static stress changes by moderate-size inland earthquakes induce offshore events. The seismicity and Coulomb stress changes suggest high seismic potentials around the western margin of the Ulleung basin. Earthquakes with magnitudes M w 6.0–7.0 in the western East Sea may produce peak ground accelerations of 0.2 g within the distance of ∼ 40–80 km, which includes the coastal regions. [ABSTRACT FROM AUTHOR]

In seismic hazard studies, the Horizontal-to-Vertical Spectral Ratio (HVSR) is nowadays routinely considered as a quick way to assess possible amplification effects. However, because of several issues that can affect the data, the HVSR cannot be considered as valid per se, and a careful data evaluation is necessary. In this study, a series of HVSR curves are evaluated in order to highlight industrial components that can significantly alter the natural HVSR. First, a controlled-source experiment is carried out in order to define the characteristics of spurious industrial signals. Data analysis shows that the coherence functions and the mildly smoothed amplitude spectra plotted with linear scales can help significantly in identifying industrial components that can be otherwise difficult to highlight. Coherence functions appear particularly interesting because of their ability to reveal the presence of industrial components independently of their amplitude. Field data from three sites are then analyzed on the basis of the evidence obtained through the controlled-source experiment. For the third site, data recorded on two different days are considered. While in the first data set no significant industrial component is present, in the second and third data sets, a series of remarkable industrial signals that severely alter the natural HVSR are identified. The assessment of the coherence functions and mildly smoothed amplitude spectra is therefore suggested as a valuable support to avoid pitfalls in the interpretation of the experimental HVSR. Finally, two quick and dirty procedures aimed at reducing the effect of industrial components are also presented. [ABSTRACT FROM AUTHOR]

Crustal faults at subduction zones show evidence of activity over geological time, but at the scale of earthquake cycle the mechanical behavior of these faults is not fully understood. Here we construct 2-D viscoelastic models constrained by both horizontal and vertical GPS-derived interseismic velocities to investigate the contribution and interrelation between subduction zone locking, viscous mantle flow, and upper plate faulting on surface deformation in Central Andes. Main pattern of horizontal velocities can be explained by a combination of locking degree and viscous flow, whereas vertical signal is found essential for estimating the locking depth. A sharp deformation gradient near the major back-arc fault suggests an active interseismic shorting across this structure. We further conduct mechanical viscoelastic models with a frictional back-arc fault to analyze its displacement and activation conditions. Our results suggest that the back-arc fault is creeping at ~ 3 mm/year and its motion is mainly driven by the interseismic viscous mantle flow, which spreads plate tectonic stresses broadly across the continent. Moreover, the frictional strength of the back-arc fault must be remarkably weak and its mechanics re-distributes the interseismic deformation and shortens the continental plate in Central Andes. Geological estimates suggest that the long-term shortening rate at the back-arc fault is ~ 10 mm/year, suggesting that this structure can accumulate ~ 7 mm/year of slip deficit, confirming the seismic potential of this structure. [ABSTRACT FROM AUTHOR]

Machine learning using convolutional neural networks (CNNs) is investigated for the imaging of sparsely sampled seismic reflection data. A limitation of traditional imaging methods is that they often require seismic data with sufficient spatial sampling. Using CNNs for imaging, even if the spatial sampling of the data is sparse, good imaging results can still be obtained. Therefore, CNNs applied to seismic imaging have the potential of producing improved imaging results when spatial sampling of the data is sparse. The imaged model can then be used to generate more densely sampled data and in this way be used to interpolate either regularly or irregularly sampled data. Although there are many approaches for the interpolation of seismic data, here seismic imaging is performed directly with sparse seismic data once the CNN model has been trained. The CNN model is found to be relatively robust to small variations from the training dataset. For greater deviations, a larger training dataset would likely be required. If the CNN is trained with a sufficient amount of data, it has the potential of imaging more complex seismic profiles. [ABSTRACT FROM AUTHOR]

The dynamic properties of mining induced seismic activity with respect to production rate, depth and size are studied in seven orebodies in the same underground iron ore mine. The objective is to understand the relationship between the measured seismic activity and the: seismic decay time, planned production rate, production size and mining depth. This relationship is the first step to individually customise the production rate for each orebody in the mine, make short-term predictions of future seismicity given planned productions, and to find out in what way the available predictors affect the seismicity. The seismic response with respect to the dependent variables is parametrised and the estimated decay times for each orebody, which are of particular interest here, are compared. An autoregressive model is proposed to capture the dynamic relationship between the induced seismic activity, the current production rate and the past seismic activity. Bayesian estimation of the parameters is considered and parameter constraints are incorporated in the prior distributions. The models for all orebodies are tied together and modelled hierarchically to capture the underlying joint structure of the problem, where the mine-wide parameters are learnt together with the individual orebody parameters from the observed data. Comparisons between the parameters from the hierarchical model and independent models are given. Group-level regressions reveal dependencies on size and mining depth. Model validation with posterior predictive checking using several discrepancy measures could not detect any model deficiencies or flaws. Posterior predictive intervals are evaluated and inference of model parameters are presented. [ABSTRACT FROM AUTHOR]

Power spectra of shear-waves for eighteen earthquakes from the Anza-Imperial Valley region were inverted for source, mid-path Q, site attenuation and site response. The motivation was whether differences in site attenuation (parameterized as t*, r/cQ, where r is distance along ray path near the site, c is shear velocity and Q is the quality factor that parameterizes attenuation) and site response could be correlated with residuals in peak values of velocity or acceleration after removing the affect of distance-dependent attenuation. We decomposed spectra of S-waves from horizontal components of 18 earthquakes from 2010 to 2018 into a common source for each event with ω−2 spectral fall-off at high frequencies and then projected the residuals onto path and site terms following the methodology of Boatwright et al. (Bull Seismol Soc Am 81:1754–1782, 1991). The site terms were constrained to have an amplification at a particular frequency governed by VS30 at two of the sites which had downhole shear-wave logs. The 18 events, 3 < M < 4, had moments between approximately 1020 and 1022 dyne-cm, and stress drops between 1 and 100 bars. Average mid-crust attenuation had a Q of 844 reflecting the average path through the crystalline rock of the San Jacinto Mountains. t* for each station corresponded to the geologic environment such that stations on hard rock had low t* (e.g. stations KNW, PFO and RDM) a station in the San Jacinto fault zone (station SND) had a moderate t* of 0.035 s and stations in the Imperial Valley usually had higher t*s. Generally t* correlated with average amplification suggesting that sites characterized by low surface velocities and higher attenuation also have more amplification in the 1–6 Hz band. Residuals of peak values were determined by subtracting the prediction of Boore and Atkinson (2008). There is a correlation between average amplification and peak velocity, but not peak acceleration. Interestingly, there is less scatter at high values of amplification although there is also less data. Scatter in values of peak velocity and peak acceleration are higher at shorter compared to longer durations. When using a frequency-dependent form for Q, variances are higher, sometimes much higher; the dataset does not support frequency-dependent Q, which is not similar to results from the Imperial Valley and northeastern North America. [ABSTRACT FROM AUTHOR]

In recent years the International Atomic Energy Agency (IAEA) has been closely following and supporting the use of physics-based rupture models for ground motion prediction (e.g. IAEA Safety Standards Series No. SSG–9 and Safety Report Series No. 85) as well as for fault displacement prediction (IAEA-TECDOC, in preparation), respectively for applications in Seismic Hazard Analysis (SHA) and Fault Displacement Hazard Analysis (FDHA) for nuclear installations. Further strengthening of this effort and dissemination in practices for SHA, FDHA and engineering issues have been done through different international working group activities, being the most outstanding two international workshops on Best Practices in Physics-based Fault Rupture Models for Seismic Hazard Assessment of Nuclear Installations (BestPSHANI) in 2015 and 2018. A PAGEOPH topical volume for the BestPSHANI 2015 was published in Dalguer et al. (Pure Appl Geophys 174:3325–3329, 2017). Now, in this PAGEOPH topical volume we collect several articles from the BestPSHANI 2018 workshop as well as several new contributions. The issue also covers further topics on the assessments of engineering issues that rely on ground motion estimates for the evaluation of structures oriented to full seismic risk analysis. A total of twenty-nine papers have been selected covering topics ranging from the seismological aspects of earthquake source studies, ground motion and fault displacement modeling to the engineering application of simulated ground motion for the analysis of soil structure interaction, structural response and fragility curve analysis for the quantification of seismic vulnerability of structures and their seismic performance. Collectively, the seismological papers discuss several current issues of source characterization and ground motion prediction for SHA, highlighting the usefulness of physics-based models for future applications in practice. The engineering papers describe methodologies to develop integral models from source-to-structures that consider the developments of synthetic seismograms as input for structural response and fragility curves estimation for seismic vulnerability assessment. Therefore, this issue contents advanced seismological and engineering resources that might be useful to scientists, engineers, students and practitioners involved in all aspects of SHA, FDHA and vulnerability analysis of engineering structures for seismic risk. [ABSTRACT FROM AUTHOR]