Seismic Velocity Distribution Research Articles

Determination of the seismic structure of the Earth based on joint analysis of gravimetric and seismometric data – a case study

The evolution of the Earth’s surface is driven by external and internal forces, the latter of which can only be studied indirectly. Knowledge about the structure of the Earth’s interior is very important for modeling and predicting the processes occurring at the surface. This study presents a new concept of joint analysis of the gravimetric and seismometric recordings of earthquakes for determining the seismic structure of the Earth down to the depth of 1250 km. The proposed method allows the use of gravimetric data without the known full transfer function of the instrument. Group velocity dispersion curves of the fundamental mode of Rayleigh waves up to the period of 550 s are measured based on the joint analysis of the recordings of superconducting gravimeter and broadband seismometers operating at the same location in five testing sites in Europe, allowing for the exploration of a broader response for incoming seismic waves. Averaged dispersion curves for earthquakes around the world for each site are inverted by the weighted linear inversion and Monte Carlo methods to estimate the distribution of shear-wave seismic velocity in the Earth’s mantle. A comparison of the deterministic and probabilistic inversion methods can excellently demonstrate surface waves’ ability to determine the Earth’s mantle structure. The inversion results are compared with the global ak135 seismic model (Kennett et al. 1995) to verify the proposed method.

Imaging the Crustal and Upper Mantle Structure of the North Anatolian Fault: A Transmission Matrix Framework for Local Adaptive Focusing

AbstractImaging the structure of major fault zones is essential for our understanding of crustal deformations and their implications on seismic hazards. Investigating such complex regions presents several issues, including the variation of seismic velocity due to the diversity of geological units and the cumulative damage caused by earthquakes. Conventional migration techniques are in general strongly sensitive to the available velocity model. Here we apply a passive matrix imaging approach which is robust to the mismatch between this model and the real seismic velocity distribution. This method relies on the cross‐correlation of ambient noise recorded by a geophone array. The resulting set of impulse responses form a reflection matrix that contains all the information about the subsurface. In particular, the reflected body waves can be leveraged to: (a) determine the transmission matrix between the Earth's surface and any point in the subsurface; (b) build a confocal image of the subsurface reflectivity with a transverse resolution only limited by diffraction. As a study case, we consider seismic noise (0.1–0.5 Hz) recorded by the Dense Array for Northern Anatolia that consists of 73 stations deployed for 18 months in the region of the 1999 Izmit earthquake. Passive matrix imaging reveals the scattering structure of the crust and upper mantle around the North Anatolian Fault zone over a depth range of 60 km. The results show that most of the scattering is associated with the Northern branch that passes throughout the crust and penetrates into the upper mantle.

Neotectonic Dislocations on the Barents Sea Shelf and Their Origin on the Basis of Morphometry of the Seafloor Relief, Seismic Survey, and the Deep Structure of the Mantle

A comprehensive analysis of the morphometric attribute “general curvature” for the Barents Sea bathymetry and reports on the seismic and seismoacoustic data on tectonic dislocations in a wave field, the fault network of the sedimentary cover, and seismotomographic data on the heterogeneous deep pattern of the distribution of seismic wave velocities in the upper mantle are provided. The analysis showed that mobile blocks of the upper mantle, exhibiting heterogeneous rheological properties, are associated with the consolidated part of the Earth’s crust fault network of deep-seated origin. The fault network emerges on the seafloor and, becoming a relief-forming factor, forms specific domains with different textures displayed in the morphometric attribute “general curvature.”

NEOTECTONIC DISLOCATIONS AT THE BARENTS SEA SHELF AND THEIR GENESIS ACCORDING TO THE MORPHOMETRY OF BATHYMETRY, SEISMIC SECTIONS AND THE DEEP MANTLE STRUCTURE

The paper provides a comprehensive analysis of the morphometric attribute “general curvature” for the bathymetry of the Barents Sea, seismic and seismoacoustic data containing in the wave field patterns tectonic faults emerging to the surface, a fault network of the sedimentary cover confirmed by seismic surveys, and seismotomographic data showing the heterogeneous deep structure by the distribution of seismic wave velocities in the upper mantle. The analysis of these data showed the presence of a causal relation between rheologically heterogeneous and mobile blocks of the upper mantle and the crystalline part of the Earth’s crust with the deep fault network, emerging on the bottom surface and being a relief-forming factor that forms characteristic domains with different textures performed by the morphometric attribute “general curvature”.

Near-Surface Geophysical Characterization of Lithologies in Corfu and Lefkada Towns (Ionian Islands, Greece)

Lefkada and Corfu old towns are located in the western part of Greece, in the Ionian Sea. Their proximity to the Hellenic subduction zone (HSZ) is the reason for their intense seismicity. The main goal of this study was the estimation of the geotechnical characteristics of the subsurface, with the contribution of applied geophysical techniques. Therefore, seismic refraction tomography (SRT) and multichannel analysis of surface waves (MASW) were applied. A total of thirty-three (33) seismic and geoelectrical profiles were performed in both towns in order to evaluate the geotechnical characteristics of the subsurface formations. Additionally, subsurface resistivity distributions were investigated with the application of electrical resistivity tomography (ERT). Some important elastic moduli were calculated through the combination of estimated seismic wave velocities and laboratory density measurements. The horizontal distribution of seismic velocities and mechanical properties (σ, E, K, G) of Corfu town was illustrated in maps, for the depth of 5 m. The geophysical interpretation also revealed that Lefkada’s subsurface consists of only one compact geological formation, with little or no variation of its geophysical-geotechnical characteristics. Beyond that, the ground type classifications for the two towns were determined according to the European Committee for Standardization Eurocode 8, based on VS30 values.

Chronicle of Processes Leading to the 2018 Eruption at Mt. Etna As Inferred by Seismic Ambient Noise Along With Geophysical and Geochemical Observables

AbstractThis work analyzes temporal variations of seismic velocities at Mt. Etna from August 2018 to February 2019. During this time period, a strong flank eruption accompanied by intense seismicity and ground deformation took place along a fracture that opened on 24 December 2018 at the base of the New South‐East summit crater. Furthermore, two moderate earthquakes—the 6 October 2018 ML 4.7 and 26 December 2018 ML 4.8—associated with the volcanic activity were recorded. In this study, we computed cross‐correlation functions (CCFs) between windows of seismic ambient noise to identify seismic velocity variations within the volcano edifice. We calculated daily CCFs at 16 stations for 120 interstation pairs using the vertical component in the 1.0–1.5 Hz frequency band. We observe that dv/v starts to decrease rapidly from the beginning of October 2018 and reaches approximately −0.45% in the pre‐eruption period. The spatio‐temporal distribution of seismic velocities shows that the reduction of dv/v mostly occurs in the vicinity of the summit and close to the flank area and is interpreted to be affected by magmatic intrusion at 0–3 km depth. To infer the source mechanism of this eruption, we compared these observations with volcano‐tectonic earthquakes, volcanic tremor, volcanic degassing, gravity, and ground deformation data. Our study suggests that a relationship between magma intrusion and associated crack opening is responsible for the decrease of dv/v.

The influence of the circular distribution of radial and tangential seismic velocity on the structural safety of residential buildings

The article presents the results of conditions of circular distribution of seismic vibrations generated during rock blasting. The Scales of Dynamic Influences [SWD] of the harmfulness of the vibration velocity on a two-storey building [SWDI] and a five-storey building[SWDII] are given. The value of seismic velocity safe for the construction of the buildings has been determined. They are in Zone II of the Dynamic Influence Scale [SWD] SWD I and SWD II. The magnitude of the components of the vibration velocity at the same distance from the source of vibration depends on the directional angle between the line of blast holes and the line connecting the centre of the surface of the mined rock block and the place of measurement. For the circular distribution of the vibration velocity, a theoretical analysis of the change in the radial value Vx and the tangential Vy of the vibration velocity depending on the change in the directional angle was conducted. Graphs of vibration velocity for circular distribution, measured during mining of rocks and complying with theoretical predictions were presented. For residential buildings, with SWDI and SWDII, the limit value of the safe vibration velocity for the building structures is given.

Underground Rock Mass Behavior Prior to the Occurrence of Mining Induced Seismic Events

The variations of seismic velocity prior to the occurrence of major seismic events are an indicator of the rock mass performance subjected to mining-induced stress. There have been no prior field-scale studies to examine stress change within the rockmass volume immediately prior to potentially damaging mining-induced seismicity. Monitoring stress change is critical for mine stability and operation safety and eventually improves production by optimizing mine designs and mining practices. In this study, five major seismic events that occurred in a narrow-vein mine were used as case studies in order to investigate any significant changes in P-wave velocity distribution, on a daily basis, within a week of seismic events with Mw > 1; if observed, such changes could provide a warning to mine engineers and workers. It was observed there was no consistent significant velocity change of more than 1% within 200 m of the hypocenters within 6 days prior to the events. Additionally, the influence of blasting in the week of the occurrence of events was investigated however no recognizable trend was observed between blasting and changes in the seismic velocity distribution within the rock mass on the day of a blast or the following day.

Low-degree mantle melting controls the deep seismicity and explosive volcanism of the Gakkel Ridge

The world’s strongest known spreading-related seismicity swarm occurred in 1999 in a segment of the Gakkel Ridge located at 85°E as a consequence of an effusive-explosive submarine volcanic eruption. The data of a seismic network deployed on ice floes were used to locate hundreds of local earthquakes down to ∼25 km depth and to build a seismic tomography model under the volcanic area. Here we show the seismicity and the distribution of seismic velocities together with the 3D magmatic-thermomechanical numerical model, which demonstrate how a magma reservoir under the Gakkel Ridge may form, rise and trigger volcanic eruptions in the rift valley. The ultraslow spreading rates with low mantle potential temperatures appear to be a critical factor in the production of volatile-rich, low-degree mantle melts that are focused toward the magma reservoirs within narrow magmatic sections. The degassing of these melts is the main cause of the explosive submarine eruptions.

Arclogite nature of the Colombian Andes magmatic arc root: A receiver-function approach

Arclogites are an important part of the continental arc root architecture. When overlooked, the crustal structure could be misinterpreted, assuming that the seismically defined Moho necessarily coincides with the crust-mantle boundary. Evaluating the possible presence of lower crustal rocks below the seismic Moho with physical properties resembling those of the underlying mantle, is therefore, of major importance. Through a multi-approach receiver function assessment, which includes shear wave velocity inversion, and determination of plausible crustal seismic velocity distributions from forward modeling, we present new constraints on the seismic structure of the arc root beneath the Colombian Andes magmatic arc. Results suggest a high-velocity arc root with a latitudinally variable thickness, from 13.5 to 20 km. The arc root is composed of a 4 to 6 km thick upper domain with seismic wave speeds 5% slower, and 8.5 to 14 km thick lower domain 5% faster, relative to the underlying uppermost mantle. This lower crustal structure, coupled with documented garnet pyroxenitic xenoliths, supports an arclogite nature for the Colombian Andes arc root.

Three-Dimensional Modeling of the Xichang Crust in Sichuan, China by Machine Learning

Seismicity and distribution of earthquakes can provide active fault structural information on the crust at a regional scale. The morphology of faults can be derived from the epicentral distribution of micro-earthquakes. In this study, we combined both the relocated earthquake catalogue and related preliminary geophysical information for 3D modeling of the crust in the Xichang area, Sichuan province, China. The fault morphology and deep crustal structure were automatically extracted by the machine learning approach, such as the supervised classification and cluster analysis methods. This new 3D crustal model includes the seismic velocity distribution, fault planes in 3D and 3D seismicity. There are many earthquake clusters located in the folded basement and low-velocity zone. Our model revealed the topological relation between the folded basement and faults. Our work show the crustal model derived is supported by the earthquake clusters which in turn controls the morphological characteristics of the crystalline basement in this area. Our use of machine learning techniques can not only be used to predict the refined fault geometry, but also to combine the seismic velocity structure with the known geological information. This 3D crustal model can also be used for geodynamic analysis and simulation of strong motionseismic waves.

ЗАСТОСУВАННЯ ПОЛІНОМІАЛЬНИХ ПОПРАВОК ДЛЯ ПОБУДОВИ ОПТИМАЛЬНОЇ ОДНОВИМІРНОЇ МОДЕЛІ ГУСТИНИ МАНТІ

In this work, an optimal one-dimensional density model was obtained, corresponding to the velocity curve for one of the mantle domain under the Ukrainian shield. When obtaining a one-dimensional density model, only the Earth's mass and seismic velocities are known physical parameters. The density is obtained by solving the Adams-Williamson equation, the use of which is possible under the assumption that the density is created only by the weight of the upper layers, with a homogeneous composition of the mantle. Some approximation to the real density distribution gives a seismic parameter that scales the obtained densities in accordance with the geometry of the seismic velocity distribution, while, as shown by our studies, the obtained density values are not absolute, but only an approximation corresponding to the equation is used. In order to obtain a density distribution we transform the first approximation obtained from the Adams-Williamson equation. This paper shows several options for transformation; based on the arithmetic mean correction for 5 reference mantle models (PEMC, PEMA, PREM, AK135, IASP91); using control points representing seismic boundaries to determine the intervals for computation of density using the Adams-Williamson equation; when introducing corrections in the form of the difference between the polynomials for the theoretical density curve and that obtained by the Adams-Wilmson equation for the IASP91 model. The density curve obtained by the last method is not distorted by the introduced density jumps from the IASP91 model, correspond to positions of seismic boundaries along the inflections of the P-velocity curve. The density curve obtained from the Adams-Williamson equation is transformed into a curve that is as close as possible to the geometry of the inherent curve seismic velocity of P and S waves. In our opinion, the density curve obtained using the difference polynomial shows the most approximate solution to the optimal density model for a given seismic velocity distribution, in our case, for the mantle domain under the Ukrainian shield with center coordinates 28.25Å 49N.

Site-Selective Magnetic Moment Collapse in Compressed Fe5O6 Supported by the National Natural Science Foundation of China (Grant Nos. 11604255 and U1930401), and the Natural Science Basic Research Program of Shaanxi (Grant No. 2021JM-001).

Iron oxide is one of the most important components in the Earth’s mantle. The recent discovery of the stable presence of Fe5O6 in the Earth’s mantle environment has stimulated significant interests in understanding of this new category of iron oxides. We report the electronic structure and magnetic properties of Fe5O6 calculated by the density functional theory plus dynamic mean field theory (DFT + DMFT) approach. Our calculations indicate that Fe5O6 is a conductor at ambient pressure with dominant Fe-3d density of states at the Fermi level. The magnetic moments of iron atoms at three non-equivalent crystallographic sites in Fe5O6 collapse at significantly different rates under pressure. This site-selective collapse of magnetic moments originates from the shifting of energy levels and the consequent charge transfer among the Fe-3d orbits when Fe5O6 is being compressed. Our simulations suggest that there could be high conductivity and volume contraction in Fe5O6 at high pressure, which may induce anomalous features in seismic velocity, energy exchange, and mass distribution in the deep interior of the Earth.

Seismic data inversion with acquisition adaptive convolutional neural network for geologic forward prospecting in tunnels

Seismic forward prospecting is essential because it can identify the velocity distribution in front of the tunnel face and provide guidance for safe excavation activities. We have developed a convolutional neural network (CNN)-based method to invert forward-prospecting data recorded in tunnels for accurate and rapid estimation of seismic velocity distribution. Targeting the unusual seismic acquisition setup in tunnels, we design two separate encoders to extract features from observation data recorded on both tunnel sidewalls. Subsequently, these features are concatenated to a decoder for velocity prediction. Considering the various acquisition setups used in different tunneling projects, the deep-learning inversion network must be flexible in terms of the seismic source/receiver positions for practical application. We have generated two auxiliary feature maps that can be used to feed acquisition information to our network. Our network, acquisition adaptive CNN ([Formula: see text]-CNN), can be trained by defining the loss function based on the [Formula: see text]-norm and multiscale structural similarity. Compared with traditional CNNs, our method has superior performance on data sets with fixed and random acquisition setups and also demonstrates certain robustness when handling synthetic data with field noise. Finally, we test how the network performs when feeding the modified acquisition setup information. It turns out that the inversion result will demonstrate a shift when the provided acquisition setup information shift, which verified the validity of the network and its use of acquisition information.

P-velocity of the upper mantle

The authors have constructed models featuring seismic P-wave velocity distribution in the upper mantle beneath oceanic, continental and transition regions, such as mid-ocean ridges, basins, trenches, island arcs, and back-arc troughs, Atlantic transitional zones, flanking plateaus of mid-ocean ridges, platforms, geosynclines, rifts, recent activation zones. The models are in agreement with the deep-seated processes in the tectonosphere as predicted in terms of the advection-polymorphism hypothesis. The models for areas of island arcs and coastal ridges are similar to those for alpine geosynclines disturbed by recent activation. The models for areas of mid-ocean ridges and back-arc troughs are identical. They fit the pattern of recent heat-and-mass transfer in the case of rifting, which, given the basic crust with continental thickness, leads to oceanization. The model for the basin reflects the effect of thermal anomalies smoothing beneath mid-ocean ridges or back-arc troughs about 60 million years later. The model for the trench and flanking plateau reflects the result of lateral heating of the mantle’s upper layers beneath the quiescent block from the direction of the island arc and basin (trench) and mid-ocean ridge and basin (flanking plateau).
 A detailed bibliography on regions covered by studies was presented in the authors’ earlier publications over past eight years. There are quite significant differences between models for regions of the same type that are described in publications of other authors. This is largely due to the fact that individual authors adopt a priori concepts on the velocity structure of the upper mantle.
 High variability of seismic P-wave velocities within the subsurface depth interval has been detected as a result of all sufficiently detailed studies. This variability is responsible for the sharp increase in the scatter of arrival times of waves from earthquakes at small angular distances. The corresponding segments of travel-time graphs were simply ignored, and the graphs started from about 3° after which the scatter of arrival time acquired a stable character. Accordingly, velocity profiles were constructed, as a rule, starting from depths of about 50 km. The constructed velocity profiles vary little from region to region with the same type of endogenous regimes. This enables us to maintain that the models represent standard (typical) VP distributions in the mantle beneath the regions, just as presumed in terms of the theory.

New insights into the High Agri Valley deep structure revealed by magnetotelluric imaging and seismic tomography (Southern Apennine, Italy)

We present an electrical resistivity model obtained from a 2D Magnetotelluric survey across a large sector of the Southern Apennine in the High Agri Valley (HAV), a NW-SE trending intra-mountain basin, with a high seismogenic potential. The intensive hydrocarbon exploitation (Val d'Agri oilfield) makes this area also affected by induced seismicity. In this HAV sector, the injection of salt-water in an unproductive disposal well (Costa Molina 2) causes localized swarms of microearthquakes; a second cluster of continuous induced seismicity is also observed SW of the Pertusillo Lake and it is associated to the seasonal fluctuations of the reservoir's water level.The major insight inferred from this study concerns a better understanding of the geological and tectonic framework in the HAV. The electrical resistivity model images the subsurface as conductive sedimentary sequences (Allochthonous Units) upon the carbonate Apulian Platform Unit characterized by higher resistivity values. Both these units appear composed of thrust-and-fold system deepening with larger wavelength anticlines N-E toward. Most of the structures identified in the magnetotelluric model are rather superficial and confined within the Allochthonous Units. A sudden break of the Apulian platform under the central part of the MT profile defines a conductive zone possibly associated to a major SW-dipping reverse fault or to several branches, as closely spaced thrust-sheets cutting eastern flanks of the Agri Valley.Additional information on the HAV deep structures comes from the joint interpretation of the resistivity model and a 3D seismic tomographic model obtained from the inversion of passive seismic data collected in the period 2002–2018. The availability of this elastic representation of the subsurface allowed us to perform a cluster analysis on the electrical resistivity and seismic P-wave velocity distribution within the subsoil. This joint quantitative interpretation unveiled new insights, otherwise hidden by individual models, on the subsurface structure distinguishing some rheological zones in terms of barriers and asperities.

Interpretation of aeromagnetic data for the geothermal properties in the northwestern part of Turkey

The northwestern part of Turkey is a significant area for its location on the accretionary region of several micro-continents and blocks. Therefore, there are several suture zones and western segments of the “North Anatolian Fault Zone” are scattered in the study area. Due to trans-tensional stresses, there are several pull-apart basins and/or depressions within the fault zone in the study area. As a consequence, it is also possible to observe magmatic intrusions into the fault segments. All these events and magmatic emplacements in the tectonic framework provide significant magnetic anomalies throughout the region.In this study, geothermal potential of the region is investigated based on the spectral analysis of the magnetic data, together with the seismic velocity (VP) distribution in the area using the log-linear relationship between the seismic velocity and the radiogenic heat production. Although two regions are emerging in the Curie Point Depth (CPD) and geothermal gradient maps, only one anomalous area from Bursa to Tekirdag may be considered as a promising region for the further geothermal energy exploration in the heat-flow maps prepared according to two different thermal conductivity coefficients. Seismic velocities are also low in this anomalous region decreasing down to 6.40 km/s around the shoreline of Tekirdag. Heat-flow values are increasing up to 110 mW/m2 in this particular area where the CPDs are around 14 km.

Determination of the structure of the earth's crust by changing the velocity of seismic waves from the earthquake data

Objective. A study of the structure of the earth's crust by determining the velocities of seismic waves from previously broken sections of the earth's crust.Methods. Two mathematical methods are proposed for determining the seismic wave velocities. The first is based on the weighted average method, and the second is based on the matrix method for solving systems of linear algebraic equations. The initial parameters used in the calculations are data from earthquakes that occurred – the coordinates of earthquake centers of origin and seismic sensors, as well as the travel times of seismic waves from the earthquake center of origin to the seismic sensors.Results. The problem of determining the propagation velocity of seismic waves in different parts of the earth's crust is solved by two different methods. The distribution densities of errors in determining the seismic wave velocities are obtained for the weighted average method, in six different situations, for the distribution of seismic wave velocities on the ground, and the matrix method, in the case of distribution of seismic wave velocities on the ground in staggered order.Conclusion. The proposed methods allow refining the coordinates of the centers of the occurred earthquakes while using the iteration method can significantly improve the accuracy of determining the coordinates of the earthquake center of origin and the speeds of seismic waves in various areas. The presence of seismic wave velocities in various parts of the earth's crust during calculations allows determining the coordinates of the earthquake source using new methods based on the use of second-order hyperboloid figures, which were not previously used by seismologists.

Anatomy of the Bezymianny volcano merely before an explosive eruption on 20.12.2017

Strong explosive eruptions of volcanoes throw out mixtures of gases and ash from high-pressure underground reservoirs. Investigating these subsurface reservoirs may help to forecast and characterize an eruption. In this study, we compare seismic tomography results with remote sensing and petrology data to identify deep and subaerial manifestations of pre-eruptive processes at Bezymianny volcano in Kamchatka shortly before its violent explosion on December 20, 2017. Based on camera networks we identify precursory rockfalls, and based on satellite radar data we find pre-eruptive summit inflation. Our seismic network recorded the P and S wave data from over 500 local earthquakes used to invert for a 3D seismic velocity distribution beneath Bezymianny illuminating its eruptive state days before the eruption. The derived tomography model, in conjunction with the presence of the high-temperature-stable SiO2 polymorph Tridymite in juvenile rock samples , allowed us to infer the coexistence of magma and gas reservoirs revealed as anomalies of low (1.5) and high (2.0) Vp/Vs ratios, respectively, located at depths of 2–3 km and only 2 km apart. The reservoirs both control the current eruptive activity: while the magma reservoir is responsible for episodic dome growth and lava flow emplacements, the spatially separated gas reservoir may control short but powerful explosive eruptions of Bezymianny.

Building Complex Seismic Velocity Models for Deep Learning Inversion

Training a deep learning inversion network usually requires hundreds of thousands of complex velocity models, which is labor-intensive and expensive to acquire. In this work, we develop a new framework to automatically generate various velocity models with common geological structures, such as folding layers, faults and salt bodies. There are three main modules in the proposed framework. The first module generates a folded model with a given number of layers; the other two modules can add faults and salt bodies onto the folded model to form a fault or salt model, respectively. To best simulate the shape of subsurface geological structures while ensuring a good application effect in deep learning inversion, we generate the structural model based on composition of several basic functions with recommended parameter ranges. Then the generated structural model will be assigned with velocity values based on assumptions of underground seismic velocity distribution. To investigate the application effect of the generated 3D models, we conduct a deep learning inversion test. Since currently there is no 3D deep learning inversion algorithms available, the latest 2D inversion network called SeisInvNet is used to test the feasibility of the randomly generated velocity models. Through the experiments, we can see that the 2D inversion results are consistent with the true models from the aspects of velocity values and geological structure shapes, which demonstrates the rationality of the designed 3D models. In the end, we further discuss the feasibility of applying the proposed 3D model dataset to train a 3D inversion network. This work paves the way for the development of 3D deep learning inversion methods.