Comparison of 3D, 2D, and 1D Magnetotelluric Inversion Results on the Example of Data from Fore-Sudetic Monocline

This study’s main objective is to better define and understand results for the most commonly used inversion algorithms in magnetotelluric data interpretation as part of geological exploration of the region of the Dolsk fault and the Odra fault. The data obtained from the eastern part of Fore-Sudetic Monocline measurements were used to describe the boundaries of lithospheric blocks (terranes) and recognize their origin. The magnetotelluric (MT) soundings were carried out to achieve this goal. There were conducted 51 soundings on five quasiparallel profiles. That allows constructing a quasiregular mesh in the area of the Fore-Sudetic Monocline. This arrangement of the measuring grid allowed reducing the influence of the largest sources of disturbances on MT data. 1D and 2D models were created by using the inverse algorithms. The models were prepared for each profile separately. Further, parallel (ModEM) 3D inversion codes were applied. The area where the investigation was done involves the region of the Dolsk fault and the Odra fault. These zones are essential geologic borders of a regional nature, and they pull apart the crust blocks with different origins. It was vitally needed to correctly identify the crust and upper mantle structure around a part of the Fore-Sudetic Monocline. The paper shows how these key features of the geological structures are revealed using 1D, 2D, and 3D algorithms.

Deformation of the Crust and Upper Mantle beneath the North China Craton and Its Adjacent Areas Constrained by Rayleigh Wave Phase Velocity and Azimuthal Anisotropy

The North China Craton (NCC) has experienced strong tectonic deformation and lithospheric thinning since the Cenozoic. To better constrain the geodynamic processes and mechanisms of the lithospheric deformation, we used a linear damped least squares method to invert simultaneously Rayleigh wave phase velocity and azimuthal anisotropy at periods of 10–80 s with teleseismic data recorded by 388 permanent stations in the NCC and its adjacent areas. The results reveal that the anomalies of Rayleigh wave phase velocity and azimuthal anisotropy are in good agreement with the tectonic domains in the study area. Low-phase velocities appear in the rift grabens and sedimentary basins at short periods. A rotation pattern of the fast axis direction of the Rayleigh wave together with a distinct low-velocity anomaly occurs around the Datong volcano. A NW–SE trending azimuthal anisotropy and a low-velocity anomaly at periods of 60–80 s are observed subparallel to the Zhangbo fault zone. The whole lithosphere domain of the Ordos block shows a high-phase velocity and counterclockwise rotated fast axis. The northeastern margin of the Tibetan plateau is dominated by a low-velocity and coherent NW–SE fast axis direction. We infer that the subduction of the Paleo-Pacific plate and eastward material escape of the Tibetan plateau mainly contribute to the deformation of the crust and upper mantle in the NCC.

The CIELO Seismic Experiment

The Crust and lithosphere Investigation of the Easternmost expression of the Laramide Orogeny was a two-year deployment of 24 broadband, compact posthole seismometers in a linear array across the eastern half of the Wyoming craton. The experiment was designed to image the crust and upper mantle of the region to better understand the evolution of the cratonic lithosphere. In this article, we describe the motivation and objectives of the experiment; summarize the station design and installation; provide a detailed accounting of data completeness and quality, including issues related to sensor orientation and ambient noise; and show examples of collected waveform data from a local earthquake, a local mine blast, and a teleseismic event. We observe a range of seasonal variations in the long-period noise on the horizontal components (15–20 dB) at some stations that likely reflect the range of soil types across the experiment. In addition, coal mining in the Powder River basin creates high levels of short-period noise at some stations. Preliminary results from Ps receiver function analysis, shear-wave splitting analysis, and averaged P-wave delay times are also included in this report, as is a brief description of education and outreach activities completed during the experiment.

Geoelectric heterogeneities of the Kerch iron ore basin

The three-dimensional geoelectric model of the Earth’s crust and upper mantle of the Kerch Peninsula has been built for the first time based on the results of experimental observations of the Earth’s low-frequency electromagnetic field, carried out in 2007—2013 by the Institutes of the National Academy of Sciences of Ukraine. Its physical and geological interpretation and detailing of the near-surface part were carried out according to the data of the audiomagnetotelluric sounding method to study the deep structure of the Kerch iron ore basin. To the east of the Korsak-Feodosiya fault along the southern part of the Indolo-Kuban trough (in the north of the South Kerch and almost under the entire North Kerch zones), a low-resistance anomaly (ρ=1 Ohm∙m) was found at depths from 2.5 km to 12 km about 20 km wide. Its eastern part is located in the consolidated Earth’s crust and is galvanically connected with surface sedimentary strata, while the western part is completely in sedimentary deposits. The anomaly covers the territory of the Kerch iron ore basin and occurrences of mud volcanism. The characteristics of the upper part of the layered section of the Kerch Peninsula in the interval of the first hundreds of meters were obtained from the results of one-dimensional inversion of the audiomagnetotelluric sounding data (frequency range 8—4000 Hz). It is shown that the first 15 m of the section, corresponding to Quaternary deposits, have resistivity values up to 1 Ohm∙m. Below, in the Neogene sediments, the electrical resistance increases to values of 5 Ohm∙m and more. Both horizontally and vertically, the distribution of resistivity values has a variable character, manifesting as a thin-layered structure with low resistivity values. Possibly, such areas have a direct connection with the channel for transporting hummock material and gases. A connection is assumed between the low-resistivity thin-layered near-surface areas, a deep anomaly of electrical conductivity in the upper part of the Earth’s crust, and the likely high electrical conductivity of rocks at the depths of the upper mantle with iron ore deposits, as well as the manifestation of mud volcanism. The heterogeneity of the crustal and mantle highly conductive layers may indicate a high permeability of the contact zones for deep fluids.

Crust and Upper Mantle Inhomogeneities Beneath Western North Island, New Zealand: Evidence from Seismological and Electromagnetic Data.

<p>Three geophysical techniques have been used to investigate the location and the nature of a large-scale change in crust and uppermost mantle properties below the western North Island of New Zealand. Receiver function analysis reveals a step like change in crustal thickness from ~ 25 km below the northwestern North Island to ≥ 32 km in the southwestern North Island. P-wave attenuation is elevated north of this change in crustal thickness (1000/Qp ≈ 1.9 for α = 0) and is compatible with a wet mantle at near solidus temperatures (T ≈0.97 melting temperature). Attenuation decreases by at least a factor of 2 for the southwestern North Island to values closer to those expected for normal continental lithosphere (1000/Qp ≤ 1 for α = 0). A region of extremely high attenuation (1000/Qp ≈ 5 for α = 0) is observed below the Central Volcanic Region. This value of attenuation is compatible with a wet mantle at temperatures just above melting (T ≈ 1.02 melting temperature). Finally 2D modelling of magnetotelluric data reveals a region of low electrical resistivity (100 Ωm) in the mantle below the region of thinned crust. Like the P-wave attenuation, this region of low resistivity can be explained by a water-saturated mantle at near solidus temperatures (T=0.88-0.97 melting temperature). The changes in crustal thickness, attenuation and electrical resistivity are all coincident with the southern limit of volcanism (~ 39.3°S) at a boundary that runs approximately east-west, perpendicular to the present plate boundary. The only surface expressions of this boundary are the termination of volcanism and the dome-like uplift of the North Island, which has previously been explained by the presence of a buoyant low-density mantle beneath the northwestern North Island. Elevated temperatures and water content inferred from this study are in agreement with this explanation. The sudden transition displayed in all three data sets, but particularly the crustal thickness step seen in the receiver function, calls for a special explanation. Thermal processes are too diffuse to explain the step and instead a mechanical process is called for. One possibility is that the step was created by convective removal of thickened lithosphere.</p>

Crust and Upper Mantle Inhomogeneities Beneath Western North Island, New Zealand: Evidence from Seismological and Electromagnetic Data.

<p>Three geophysical techniques have been used to investigate the location and the nature of a large-scale change in crust and uppermost mantle properties below the western North Island of New Zealand. Receiver function analysis reveals a step like change in crustal thickness from ~ 25 km below the northwestern North Island to ≥ 32 km in the southwestern North Island. P-wave attenuation is elevated north of this change in crustal thickness (1000/Qp ≈ 1.9 for α = 0) and is compatible with a wet mantle at near solidus temperatures (T ≈0.97 melting temperature). Attenuation decreases by at least a factor of 2 for the southwestern North Island to values closer to those expected for normal continental lithosphere (1000/Qp ≤ 1 for α = 0). A region of extremely high attenuation (1000/Qp ≈ 5 for α = 0) is observed below the Central Volcanic Region. This value of attenuation is compatible with a wet mantle at temperatures just above melting (T ≈ 1.02 melting temperature). Finally 2D modelling of magnetotelluric data reveals a region of low electrical resistivity (100 Ωm) in the mantle below the region of thinned crust. Like the P-wave attenuation, this region of low resistivity can be explained by a water-saturated mantle at near solidus temperatures (T=0.88-0.97 melting temperature). The changes in crustal thickness, attenuation and electrical resistivity are all coincident with the southern limit of volcanism (~ 39.3°S) at a boundary that runs approximately east-west, perpendicular to the present plate boundary. The only surface expressions of this boundary are the termination of volcanism and the dome-like uplift of the North Island, which has previously been explained by the presence of a buoyant low-density mantle beneath the northwestern North Island. Elevated temperatures and water content inferred from this study are in agreement with this explanation. The sudden transition displayed in all three data sets, but particularly the crustal thickness step seen in the receiver function, calls for a special explanation. Thermal processes are too diffuse to explain the step and instead a mechanical process is called for. One possibility is that the step was created by convective removal of thickened lithosphere.</p>

Preface

VIII International Symposium “PROBLEMS OF GEODYNAMICS AND GEOECOLOGY OF INTRACONTINENTAL OROGENS” (June, 28 – July, 2, 2021; Bishkek, Kyrgyz Republic; online regime via Zoom due to the restrictions associated with the COVID-19 pandemic) was held within the framework of the Year of Science and Technology in Russia and was dedicated to 85th birthday anniversary of Yuri Trapeznikov – the founder and the first director of the Research Station of the Russian Academy of Sciences in Bishkek city (RS RAS). The Symposium became a representative international scientific event held on the basis of the RS RAS and the International Research Center - Geodynamic Proving Ground in Bishkek (IRC-GPG). More than 100 scientists and specialists from seven countries (Russia, Kyrgyzstan, Kazakhstan, Uzbekistan, Tajikistan, Ukraine and Japan) took part in the Symposium with online reports. The basic themes of the Symposium were quite diverse and related to the main directions of fundamental research in the area of Earth sciences: • Deep structure and evolution of the Earth’s crust and upper mantle in the light of modern conceptions of Geodynamics. Instrumental methods of studying of intracontinental orogens lithosphere: heterogeneities, physical nature of boundaries. • Stressed and deformed state of the Earth’s crust, problems of its block structure and selfsimilarity of geodeformation processes. Seismotectonics of intracontinental orogens zones. • Complex monitoring of seismically active zones. Problems of geospheres interaction, including the influence of physical fields on endogenous processes. • Electromagnetic methods in studying of seismically active regions and in monitoring of geodynamic processes. Development of inversion methods of electromagnetic data. • Assessment of seismic risk, regional studies of seismic regime. • Environmental and social consequences of endogenous and exogenous geological processes, prediction of hazardous events (earthquakes, landslides, etc.). Each of the mentioned themes was considered separately during special section. There were about 30 young researchers who took part in the Symposium – this participation allowed them to become involved in the modern scientific achievements in the Earth sciences area, as well as to receive the leading specialists consultations needed for the successful further research activity. The Symposium states that such meetings and forms of scientific and human communication are very useful and contribute to increase the general level and developing various areas of research in the field of Earth sciences. List of ORGANIZING COMMITTEE, Photo, Logos are available in this pdf.

Tomographic Image of Shear Wave Structure of NE India Based on Analysis of Rayleigh Wave Data

The major scientific purpose of this work is to evaluate the geodynamic processes involved in the development of tectonic features of NE India and its surroundings. In this work, we have obtained tomographic images of the crust and uppermost mantle using inversion of Rayleigh waveform data to augment information about the subsurface gleaned by previous works. The images obtained reveal a very complicated tectonic regime. The Bengal Basin comprises a thick layer of sediments with the thickness increasing from west to east and a sudden steepening of the basement on the eastern side of the Eocene Hinge zone. The nature of the crust below the Bengal Basin varies from oceanic in the south to continental in the north. Indo-Gangetic and Brahmaputra River Valleys comprise ∼5–6-km-thick sediments. Crustal thickness in the higher Himalayas and southern Tibet is ∼70 km but varies between ∼30 and ∼40 km in the remaining part. Several patches of low-velocity medium present in the mid-to-lower crust of southern Tibet along and across the major rifts indicate the presence of either partially molten materials or aqueous fluid. Moho depth decreases drastically from west to east across the Yadong-Gulu rift indicating the complex effect of underthrusting of the Indian plate below the Eurasian plate. Crust and upper mantle below the Shillong Massif and Mikir Hills are at a shallow level. This observation indicates that tectonic forces contribute to the uprising of the Massif.

Assessment of seismic tomographic models of the contiguous United States using intermediate-period 3D wavefield simulation

Summary The contiguous United States has been well instrumented with broadband seismic stations due to the development of the EarthScope Transportable Array. Previous studies have provided various 3D seismic wave speed models for the crust and upper mantle with improved resolution. However, discrepancies exist among these models due to differences in both data sets and tomographic methods, which introduce uncertainties on the imaged lithospheic structure beneath North America. A further model refinement using the best data coverage and advanced tomographic methods such as full-waveform inversion (FWI) is expected to provide better seismological constraints. Initial models have significant impacts on the convergence of FWIs. However, how to select an optimal initial model is not well investigated. Here, we present a data-driven initial model selection procedure for the contiguous US and surrounding regions by assessing waveform fitting and misfit functions between the observations and synthetics from candidate models. We use a data set of waveforms from 30 earthquakes recorded by 5,820 stations across North America. The results suggest that the tested 3D models capture well long-period waveforms while showing discrepancies in short-periods especially on tangential components. This observation indicates that the smaller-scale heterogeneities and radial anisotropy in the crust and upper mantle are not well constrained. Based on our test results, a hybrid initial model combining S40RTS or S362ANI in the mantle and US.2016 for Vsv and CRUST1.0 for Vsh in the crust is compatible for future FWIs to refine the lithospheric structure of North America.