Internal Structure Of Earth Research Articles

Computation and analysis of surface wave dispersion and attenuation in layered viscoelastic-vertical transversely isotropic media by the generalized R/T coefficient method

SUMMARY In this study, we propose a systematic and effective method, that is, an extended version of the generalized reflection/transmission (R/T) coefficient method, for computing the phase-velocity (${c}_r$) dispersion curves, attenuation coefficient ($\alpha $) curves, and eigenfunctions of both Rayleigh and Love waves as well as the ellipticity of Rayleigh waves in layered viscoelastic-vertical transversely isotropic (VTI) media. The numerical scheme of combining the root-searching method with the local optimization method is designed for determining the complex-valued modal solutions (i.e. complex wavenumber $k = {\omega {/ {\vphantom {\omega {{c}_r - i\alpha }}}} {{c}_r - i\alpha }}$) of surface waves. The near-surface sedimentary geological environment is taken as the model example because it is typical viscoelastic-VTI media. Besides the anisotropic-viscoelastic (AV) media, our algorithm can also compute surface waves in isotropic-elastic (IE), isotropic-viscoelastic (IV) and anisotropic-elastic (AE) media by resetting the corresponding parameters. Using the six-layer half-space models and in these four media, we verify the correctness of our algorithm by benchmarking the modal solutions against those from other methods. In the four-layer half-space model, by comparing the results of IE, IV, AE and AV media, we analyse the effects of velocity anisotropy, viscoelasticity and attenuation anisotropy on the dispersion and attenuation characteristics of both Rayleigh and Love waves in detail. Our study can provide a theoretical basis and useful tool for surface wave imaging considering the anisotropy and/or viscoelasticity of the medium, which has the potential to better investigate the solid Earth's internal structure.

Rational function neural networks for learning rock physics models from field data

Abstract Seismic wave velocity estimation is critical for understanding Earth's internal structure. Traditional rock physics models require careful physical assumptions and mathematical derivations, often facing challenges when applied to complex field data. Empirical formulas, while simple, lack a solid physical foundation. To address these limitations, we propose a data-driven approach using rational function neural networks (RafNN) for rock physics modelling. By analysing logging data, RafNN establishes a rational equation capturing the interdependencies among rock modulus, matrix stiffness, porosity, and fluid. The results show that RafNN accurately extracts the Gassmann's equation when the training data adheres to its constraints. Moreover, RafNN can derive general models from logging data that deviate from the Gassmann's equation. These data-driven models exhibit lower prediction errors while maintaining consistency with Gassmann's model. RafNN's adaptability to field data variability is a key advantage, facilitating better comprehension of the underlying mathematical and physical principles. Additionally, we explore the relationship between modulus, porosity, and compressibility, shedding light on the physical interpretation of RafNN models. Notably, RafNN derives analytical models directly from field data, reducing reliance on mathematical derivations and physical assumptions. Although further research is needed to understand the convergence theory of RafNN, this study presents a promising approach for data-driven rock physics modelling. It contributes to the exploration of Earth's heterogeneous structure and advances the field of seismic wave velocity estimation.

Extraction of Mantle Discontinuities From Teleseismic Body‐Wave Microseisms

AbstractOcean swell activities excite body‐wave microseisms that contain information on the Earth's internal structure. Although seismic interferometry is feasible for exploring structures, it faces the problem of spurious phases stemming from an inhomogeneous source distribution. This paper proposes a new method for inferring seismic discontinuity structures beneath receivers using body‐wave microseisms. This method considers the excitation sources of body‐wave microseisms to be spatially localized and persistent over time. To detect the P‐s conversion beneath the receivers, we generalize the receiver function analysis for earthquakes to body‐wave microseisms. The resultant receiver functions are migrated to the depth section. The detected 410‐ and 660‐km mantle discontinuities are consistent with the results obtained using earthquakes, thereby demonstrating the feasibility of our method for exploring deep‐earth interiors. This study is a significant step toward body‐wave exploration considering the sources of P‐wave microseisms to be isolated events.

Revisiting the period and quality factor of the Chandler wobble and its possible geomagnetic jerk excitation

The period and quality factor of the Chandler wobble (CW) are useful for constraining the Earth's internal structure properties, such as the mantle elasticity. It has been shown that the CW is mainly excited by a combination of atmospheric and oceanic processes; hence based on a deconvolution method, we can remove them from the excitation sequence of the CW to estimate its period P and quality factor Q. We finally re-estimate P = 432.3 ± 0.8 days and Q = 85 ± 15 for the CW. Based on those two estimates, we investigate the relationship between the geomagnetic jerks and the excitation sequences of the CW. The geomagnetic jerks or jerk bounds are well consistent with the sudden changes of the excitation sequences of the CW. This demonstrates that the geomagnetic jerks could be a possible excitation source of the CW. It is crucial for understanding the excitation of the CW and for deeper geophysical insights into the geomagnetic jerks.

Mapping of fracture zones and structural lineaments of the Gulf of Guinea passive margins using marine gravity data from CryoSat-2 and Jason-1 satellites

Satellite gravity datasets have been widely used in understanding of the Earth's internal structure and processes. These datasets are effective in providing insights into the lithospheric structure of the Earth. Gravity data from the satellite products CryoSat-2 and Jason-1 have been used to investigate the Gulf of Guinea sedimentary basins and structural lineaments in this under-explored marginal sea region through the application of various edge filters. Present-day filtering methods such as the analytical signal, tilt angle of the gradient amplitude, NTilt gradient amplitude and softsign function were evaluated for their performance on synthetic gravity anomalies with and without noise prior to their application to gravity data covering the Gulf of Guinea. The softsign filter outputs obtained from synthetic examples result in higher resolution and more explicit edges while preventing fictitious edges production in the findings. The results of the data interpretation disclosed deep-seated and shallow structural features within the Gulf of Guinea oriented mainly in the WNW-ESE, NW-SE, NE-SW and NNE-SSW directions and seen to be concentrated in the upper slope domain. This observation is in good agreement with the occurrence of drag folds and the apparent concentration of wrench faulting features across the ridge top and bordering the upper slope. These results bring new facts ruling our understanding of this atypical regional tectonic setting in close agreement with the NE–SW-striking Pelusium Megashear Zone.

ГЕОДИНАМІКА

Purpose. To investigate the features of the algorithm implementation for finding the derivatives of the spatial distribution function of the planet's masses with the use of high-order Stokes constants and, on the basis of this, to find its analytical expression. According to the given methodology, to carry out calculations with the help of which to carry on the study of dynamic phenomena occurring inside an ellipsoidal planet. The proposed method involves the determination of the derivatives of the mass distribution function by the sum, the coefficients of which are obtained from the system of equations, which is incorrect. In order to solve it, an error-resistant method for calculating unknowns was used. The implementation of the construction is carried out in an iterative way, while for the initial approximation we take the three-dimensional function of the density of the Earth's masses, built according to Stokes constants up to the second order inclusive, by dynamic compression by the one-dimensional density distribution, and we determine the expansion coefficients of the derivatives of the function in the variables to the third order inclusive. They are followed by the corresponding density function, which is then taken as the initial one. The process is repeated until the specified order of approximation is reached. To obtain a stable result, we use the Cesaro summation method (method of means).. The calculations performed with the help of programs that implement the given algorithm, while the achieved high (ninth) order of obtaining the terms of the sum of calculations. The studies of the convergence of the sum of the series have been carried out, and on this basis, a conclusion has been made about the advisability of using the generalized finding of the sums based on the Cesaro method. The optimal number of contents of the sum terms has been chosen, provides convergence both for the mass distribution function and for its derivatives. Calculations of the deviations of mass distribution from the mean value ("inhomogeneities") for extreme points of the earth's geoid, which basically show the total compensation along the radius of the Earth, have been performed. For such three-dimensional distributions, calculations were performed and schematic maps were constructed according to the taken into account values of deviations of three-dimensional distributions of the mean ("inhomogeneities") at different depths reflecting the general structure of the Earth's internal structure. The presented vector diagrams of the horizontal components of the density gradient at characteristic depths (2891 km - core-mantle, 700 km - middle of the mantle, also the upper mantle - 200, 100 km) allow us to draw preliminary conclusions about the global movement of masses. At the same time, a closed loop is observed on the “core-mantle” edge, which is an analogy of a closed electric circuit. For shallower depths, differentiation of vector motions is already taking place, which gives hope for attracting these vector-grams to the study of dynamic motions inside the Earth. In fact, the vertical component (derivative with respect to the z variable) is directed towards the center of mass and confirms the main property of mass distributions - growth when approaching the center of mass. The method of stable solution of incorrect linear systems is applied, by means of which the vector-gram of the gradient of the mass distribution function is constructed. The nature of such schemes provides a tool for possible causes of mass redistribution in the middle of the planet and to identify possible factors of tectonic processes in the middle of the Earth, i.e indirectly confirms the gravitational convection of masses. The proposed technique can be used to create detailed models of density functions and its characteristics (derivatives) of the planet's interior, and the results of numerical experiments - to solve tectonics problems.

CCREM: New Reference Earth Model From the Global Coda‐Correlation Wavefield

AbstractThe existing reference Earth models have provided an excellent one‐dimensional representation of Earth's properties as a function of its radius and explained many seismic observations in a broad frequency band. However, some discrepancies still exist among these models near the first‐order discontinuities (e.g., the core‐mantle and the inner‐core boundaries) due to different data sets and approaches. As a new paradigm in global seismology, the analysis of coda‐correlation wavefield is fundamentally different from interpreting direct observations of seismic phases or free oscillations of the Earth. The correlation features exist in global correlograms due to the similarity of body waves reverberating through the Earth's interior. As such, there is a great potential to utilize the information stored in the coda‐correlation wavefield in constraining the Earth's internal structure. Here, we deploy the global earthquake‐coda correlation wavefield as an independent data source in the 15–50 s period interval to increase the Earth's radial structure constraints. We assemble a data set of multiple pronounced correlation features and fit both their travel times and waveforms by computing synthetic correlograms through a series of candidate models. Misfit measurements for correlation features are then computed to search for the best‐fitting model. The model that provides an optimal representation of the correlation features in the coda‐correlation wavefield is CCREM. It displays differences in radial seismic velocities, especially near the first‐order discontinuities, relative to previously proposed Earth‐reference models. This is the first application of the earthquake‐coda correlation wavefield in constraining the whole Earth's radial velocity structure.

Magma plumbing system and origin of the intraplate volcanoes in Mainland China: an overview of constraints from geophysical imaging

Abstract Volcanic eruptions, despite causing large-scale disasters, also provide important natural resources and are an effective way to understand the Earth's internal structure and its evolution. Herein, a comprehensive review is presented on recent progress in geophysical imaging of the structure and origin of intraplate volcanoes in Mainland China. We primarily focus on the Changbaishan, Wudalianchi, Tengchong, Hainan and Ashikule volcanoes as they are currently active and hence pose potential hazards during future eruptions, particularly the Changbaishan volcano. The Changbaishan and Wudalianchi volcanoes are widely believed to be caused by the dehydration of the stagnant Pacific slab in the mantle transition zone along with wet upwelling in the big mantle wedge. There are a number of different views regarding the formation mechanism of the Tengchong volcano. Some studies suggest that a big mantle wedge structure is also present under eastern Tibet, and the Tengchong volcano has a deep origin, similar to volcanism in NE China. Others suggest that the Tengchong volcano is caused by a local and shallow process. Most tomographic studies suggest that the Hainan volcano is a hotspot, and its track has been located in SE China by combining seismological, geochemical and numerical modelling data. A gap exists between the subducted Indian Plate and the Tarim lithosphere beneath the Ashikule volcano, which provides a channel for asthenospheric upwelling to give rise to intraplate volcanism in the Ashikule basin. The interactions of lithospheres may produce shear heating of the subcontinental lithospheric mantle, which can generate localized melting. This process has been proposed as an explanation for the intraplate volcanism in Ashikule.

Amplitude and decay of long-period coda of great earthquakes

The earthquake coda is the residual vibration energy long after the main arrivals of an earthquake. Previous studies of coda waves have focused on high frequencies and small earthquakes, which are generally attributed to the scattering from the Earth's internal heterogeneities. Here we examined the early coda of great earthquakes at long periods (50 to 300 s) at time window of 10,000 to 50,000 s after the origin time. We used 10 major earthquakes (magnitude > 8.0) in recent decades that were recorded at global seismic networks. We found that the coda energy can be well represented by a simple exponential decay model for the early coda. The variation of the initial coda energy with distance is dominated by the effect of geometric spreading, suggesting that the scattering of the Earth's internal heterogeneities contributes little to the long-period early coda. The energy decay rate is largely insensitive to earthquake source (location, magnitude, focal mechanism, or rupture process), station location, or 3D variation of the earth's internal structure, making it ideal to constrain the average global attenuation structure. A small modification of the quality factor based on a global reference model can fit the coda energy decay well. The long-period coda energy may potentially provide a new way to determine the seismic moment of a major earthquake.

The Influence of Water Storage in Marine Sediment on Sea‐Level Change

AbstractSea‐level changes are of wide interest because they provide information about Earth's internal structure and the sensitivity of ice sheets to climate change. Here we illustrate the sensitivity of sea level to marine sedimentary water storage by modeling sea‐level responses to a synthetic global sediment redistribution history in which rates and patterns of erosion and deposition are similar to those at present and steady in time from the Last Interglacial to present. Our simulations show that if sediment redistribution were accounted for but sedimentary water storage were neglected, modeled sea‐level changes could be overestimated by ~2 ± 1 m of global mean sea‐level equivalent, a significant fraction of published estimates of 6–9 m of global mean sea‐level change since the Last Interglacial. These results show that sedimentary water storage may significantly contribute to changes in Earth's long‐term seawater budget over >105 year timescales and underscore the importance of accounting for it in modeling long‐term sea‐level changes.

Inquiry learning for gender equity using History of Science in Life and Earth Sciences’ learning environments

<p>The main objective of the present work is the selection and integration of objectives and methods of education for gender equity within the Life and Earth Sciences’ learning environments in the current portuguese frameworks of middle and high school. My proposal combines inquiry learning-teaching methods with the aim of promoting gender equity, mainly focusing in relevant 20th century women-scientists with a huge contribute to the History of Science.</p><p>The hands-on and minds-on activities proposed for high scholl students of Life and Earth Sciences onstitute a learnig environment enriched in features of science by focusing on the work of two scientists: Lynn Margulis (1938-2011) and her endosymbiosis theory of the origin of life on Earth and Inge Leehman (1888-1993) responsible for a breakthrough regarding the internal structure of Earth, by caracterizing a discontinuity within the nucleus, contributing to the current geophysical model. For middle scholl students the learning environment includes Inge Leehman and Mary Tharp (1920-2006) and her first world map of the ocean floor. My strategy includes features of science, such as: theory-laden nature of scientific knowledge, models, values and socio-scientific issues, technology contributes to science and feminism. </p><p>In conclusion, I consider that this study may constitute an example to facilitate the implementation, by other teachers, of active inquiry strategies focused on features of science within a framework of social responsibility of science, as well as the basis for future research. </p>

Visualization and dissemination of global crustal models on virtual globes

Global crustal models, such as CRUST 5.1 and its descendants, are very useful in a broad range of geoscience applications. The current method for representing the existing global crustal models relies heavily on dedicated computer programs to read and work with those models. Therefore, it is not suited to visualize and disseminate global crustal information to non-geological users. This shortcoming is becoming obvious as more and more people from both academic and non-academic institutions are interested in understanding the structure and composition of the crust. There is a pressing need to provide a modern, universal and user-friendly method to represent and visualize the existing global crustal models. In this paper, we present a systematic framework to easily visualize and disseminate the global crustal structure on virtual globes. Based on crustal information exported from the existing global crustal models, we first create a variety of KML-formatted crustal models with different levels of detail (LODs). And then the KML-formatted models can be loaded into a virtual globe for 3D visualization and model dissemination. A Keyhole Markup Language (KML) generator (Crust2KML) is developed to automatically convert crustal information obtained from the CRUST 1.0 model into KML-formatted global crustal models, and a web application (VisualCrust) is designed to disseminate and visualize those models over the Internet. The presented framework and associated implementations can be conveniently exported to other applications to support visualizing and analyzing the Earth's internal structure on both regional and global scales in a 3D virtual-globe environment.

Oil, Earth mass and gravitational force

Fossil fuels are intensively extracted from around the world faster than they are renewed. Regardless of direct and indirect effects of such extractions on climate change and biosphere, another issue relating to Earth's internal structure and Earth mass should receive at least some interest. According to the Energy Information Administration (EIA), about 34billionbarrels of oil (~4.7trillionmetrictons) and 9billiontons of coal have been extracted in 2014 worldwide. Converting the amounts of oil and coal extracted over the last 3 decades and their respective reserves, intended to be extracted in the future, into mass values suggests that about 355trilliontons, or ~5.86∗10−9 (~0.0000000058)% of the Earth mass, would be ‘lost’. Although this is a tiny percentage, modeling the potential loss of Earth mass may help figuring out a critical threshold of mass loss that should not be exceeded. Here, I briefly discuss whether such loss would have any potential consequences on the Earth's internal structure and on its gravitational force based on the Newton's law of gravitation that links the attraction force between planets to their respective masses and the distance that separate them.

The gravity tunnel in a non-uniform Earth

This paper examines the gravity tunnel using the internal structure of Earth as ascertained from seismic data. Numerically, it is found that the time taken to fall along the diameter is 38 min, compared to 42 min for a planet with uniform density. The time taken to fall along a straight line between any two points is no longer independent of distance but interpolates between 42 min for short trips and 38 min for long trips. The brachistochrone path (minimizing the time between any two points) is similar in shape to the uniform-density solution but tends to reach a greater maximum depth and takes less time to traverse. Although the assumption of uniform density works well in many cases, the simpler assumption of a constant gravitational field serves as a better approximation to the true results.

Appreciative GIS and Strength‐Based Community Change

AbstractProblem‐solving is embedded deeply in the digital DNA of GIS. Most projects involving GIS 1.0 and 2.0 raise community weaknesses, failures and other problems and use them to motivate change. The projects often create a future that does not differ greatly from the past or last long. Although looking into community problems is important and sometimes necessary, the steps of problem‐solving do not always leave people feeling happy. To inspire communities to create change based on their strengths and achievements, and to provide better participant experience, this article develops the concept of a new form of GIS called Appreciative GIS (AGIS). AGIS are grounded in the affirmative premise that every community, however challenged, is gifted with certain assets, potentials and strengths which, when effectively exploited, can take that community to a better and sustainable future. Based on Earth's internal structure, the article develops a useful layered community model where crust, mantle and core strata represent the community's body, life‐draining and life‐giving elements in that order. The model helps explain the different views of the community from current GIS and AGIS perspectives. A Total GIS (TGIS) cube is also developed and used to indicate that current GIS commonly focus on elements of the mantle or what we do not have, what we cannot do and what makes us weak. AGIS, on the other hand, dig up the core and cultivate what we have, what we can possibly do and what makes us strong. In a significant departure from the linear approach of problem‐solving, the article suggests implementing AGIS through a 7G cyclical method. The article contends that although AGIS may not be the whole enchilada, they can serve as a beacon of hope and a powerful source of inspiration especially for individuals and groups who are perceived or made to perceive themselves as dysfunctional, weak or poor.

Observation of Earth's free oscillation by dense GPS array: After the 2011 Tohoku megathrust earthquake

Day-scale Earth's free oscillation after large earthquakes has been detected by underground instruments such as strainmeters, gravimeters and seismometers, to investigate Earth's internal structure, geodynamics, and source properties of earthquakes. Here we show that Global Positioning System (GPS) can also detect the signals of the Earth's free oscillation. A dense GPS array in Japan (GEONET) recorded the surface deformation following the 2011 Tohoku megathrust earthquake. A simple array analysis over 300 stations reduces local noise in GPS time series. We find that the dense GPS array truly detected both spheroidal and toroidal fundamental modes in three-direction displacement. This new tool has a strong potential to investigate the free oscillations particularly in low-frequency bands.

Joint inversion of long-period magnetotelluric data and surface-wave dispersion curves for anisotropic structure: Application to data from Central Germany

[1] Geophysical datasets sensitive to different physical parameters can be used to improve resolution of Earth's internal structure. Herein, we jointly invert long-period magnetotelluric (MT) data and surface-wave dispersion curves. Our approach is based on a joint inversion using a genetic algorithm for a one-dimensional (1-D) isotropic structure, which we extend to 1-D anisotropic media. We apply our new anisotropic joint inversion to datasets from Central Germany demonstrating the capacity of our joint inversion algorithm to establish a 1-D anisotropic model that fits MT and seismic datasets simultaneously and providing new information regarding the deep structure in Central Germany. The lithosphere/asthenosphere boundary is found at approx. 84 km depth and two main anisotropic layers with coincident most conductive/seismic fast-axis direction are resolved at lower crustal and asthenospheric depths. We also quantify the amount of seismic and electrical anisotropy in the asthenosphere showing an emerging agreement between the two anisotropic coefficients.

Study of Normal Modes of Earth's Free Oscillation Based on Changes of Stratified Structure Parameters

AbstractResearch on Earth's free oscillations generated by large earthquakes gives important support to the study of Earth's deep internal structure. Comparing calculated normal modes' eigenperiods with observed predications can offer an efficient constraint on the study of Earth's internal structure and new earth models. Using the numerical integration method and earth model G‐D1066A, we obtain 187 eigenperiods of 0 to 48 degrees. The relative errors between our results and previous three theoretical or four observed predictions are less than 0.3%. The analysis shows that because the effects of density and Lame's parameters in different layers of the earth's interior are considered, the eigenperiods we obtained have good precision. The theoretical results presented in this paper can give an effective reference for revising the parameters of the real Earth's interior.

Estimation of Earth interior parameters from a Bayesian inversion of very long baseline interferometry nutation time series

Among the variations in the rotation of the Earth, the nutation is the most suitable for studying the Earth's internal structure. The nutation is mainly driven by the gravitational torque of the Moon, the Sun and the planets acting on its equatorial bulge. The Earth response to this external forcing is influenced by its internal structure. Because the gravitational forcing is known to a very good accuracy, the high precision nutation observations, using the very long baseline interferometry (VLBI) technique, allow to estimate Earth interior parameters. The nutational response of the nonrigid Earth to the gravitational forcing has previously been modeled by a semianalytic model which depends on parameters, related to the Earth interior, that are adjusted on the nutation observations. Those parameters are the dynamical ellipticities of the whole Earth and fluid core, compliances describing the deformability of the whole Earth and fluid core, and coupling constants related to the torques generated by the differential rotation of the mantle, fluid core and solid inner core. Most of the nutation models are frequency domain models so that, in previous studies, the time series of observations are processed before the fit in order to get data in the frequency domain. Because the parameters are fit only on the twenty dominant terms in the frequency domain, this fit leads to a loss of information. In this paper, we present a new fit procedure of the nutation model to the observations, which estimates the Earth's parameters directly from the time series of nutation data. This allows to use all the information of the time domain data and to account for the time variable uncertainties on the data (this time variability is due to the improvement of the measurement techniques). Rather than the linearized least squares method used in previous studies, we use a probabilistic (Bayesian) inversion method. This method does not rely on the assumption that the model is linear in its parameters, it is thus particularly well‐suited for the highly nonlinear nutation model. In addition, we include an additional parameter to allow for uncertainties in the nutation model itself. This parameter is estimated jointly with the geophysical parameters. As result, we obtain a probability distribution on the parameters. The numerical results are compared with those obtained from previous studies.

Using seismic waves to image Earth's internal structure

Seismic waves generated in Earth's interior provide images that help us to better understand the pattern of mantle convection that drives plate motions.