The response of the local geomagnetic field to geodynamic processes during the preparation of the 1988 Spitak earthquake
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Актуальность работы. Изучение изменений локального геомагнитного поля с целью выявления предвестников сильных землетрясений, особенно в сейсмоактивных регионах, где расположены большие города и объекты особо важного значения (АЭС, водохранилище и т.п.) остается одной из главных задач современной науки. В разных странах мира, используя магнитометрические методы, проводятся исследования по поиску предвестников сильных землетрясений. Цель. Однако, за первую половину XX века, несмотря на отдельные попытки ученых Японии и других стран, серьезных результатов достичь не удалось. Установлено, что с развитием геодинамических процессов в земной коре, особенно при подготовке сильных землетрясений, происходят изменения в магнитных свойствах горных пород (электропроводности, диэлектрической и магнитной проницаемости). Геомагнитные вариации, создаваемые внешним источником, несут в себе важную информацию об изменениях в физических свойствах в земной коры и верхней мантии, а так же позволяют оценить эти изменения. Методы. Представлена методика, которая позволяет с помощью изучения вариаций локального геомагнитного поля, создаваемых внешним источником, выявить изменения в электропроводности на разных глубинах земной коры и верхней мантии, связанные с развитием геодинамических процессов. С этой целью использован расчетный параметр N(A), который является отношением амплитуд вариаций геомагнитного поля внешнего происхождения, измеренных синхронно на разных парах станций. Изучены вариации с периодами 1025, 3060 минут и Sq-вариации. Метод применяется в низкоширотных областях Земли, где вариации переменного геомагнитного поля хорошо выделяются. Результаты. Используя предлагаемую методику, на территории Армении были выявлены аномальные изменения локального отклика геомагнитного поля перед Парванийским 1986 г. (М5,4) и Спитакским 1988 г. (М7,0) землетрясениями. Предполагается, что причинами изменений в физических свойств геологической среды в частности электропроводности, являются дегазация Земли и вертикальная фильтрация флюидов в верхние слои земной коры Relevance. The study of local geomagnetic field changes in order to identify harbingers of strong earthquakes, especially in seismically active regions where large cities and especially important objects (nuclear power plants, a storage reservoir, etc.) are located remains one of the main tasks of modern science. In different countries studies are being conducted to search for precursors of strong earthquakes, using magnetometric methods. Aim. However, for the first half of the 20th century, despite some attempts by scientists from Japan and other countries, no serious results were obtained. It has been established that with the progress of geodynamic processes in the earths crust, especially during the preparation of strong earthquakes, changes in the magnetic properties of rocks (electrical conductivity, dielectric and magnetic permeability) occur. However, geomagnetic variations created by an external source carry important information about changes in physical properties, in particular, electrical conductivity in the earths crust to the upper mantle, and make it possible to evaluate these changes. Methods. A technique that allows to identify changes in electrical conductivity at different depths of the earths crust and upper mantle associated with the development of the geodynamic process, using the study of local geomagnetic field variations created by an external source, is presented. For this purpose, parameter N(A), which is the ratio of the amplitudes of variations of the geomagnetic field of external origin, measured synchronously at different pairs of stations, was used. Variations with periods of 10-25, 30-60 minutes and Sq-variations were studied. The method is used in low latitude areas of the Earth, where variations of the variable geomagnetic field stand out well. Results. Anomalous changes in the local geomagnetic field were revealed in Armenia before the Parvania 1986 (M 5.4) and Spitak 1988 (M 7.0) earthquakes, using the proposed methodology. It is assumed that the causes of changes in the physical properties of the geological environment, in particular, electrical conductivity, are most likely to be the degassing of the Earth and the vertical filtration of fluids into the upper layers of the earths crustThe results of instrumental observations of variations of Earth's magnetic field, which have been obtained on the basis of the North Caucasus Geophysical Observatory IPE (Elbrus volcanic area), and the incorporation of observation points Troitsk, located in the European part of Russia. Analyzed abnormal «quasi-harmonic» disturbance marked variations in the Earth's magnetic field at all stages of the seismic process. The experimental data give a general idea of the geomagnetic activity and some characteristics of the induced anomalous geomagnetic disturbances, which can be co-delivered with the development of related geodynamic and geoelectric processes in the subsurface of the focal zone.
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In this article, we study the possibility that Ceres has, or had in the past, a crust heavier than a pure or muddy ice mantle, in principle gravitationally unstable. Such a structure is not unusual in the Solar system: Callisto is an example. In this work, we test how the composition (i.e. the volumetric quantity of ice) and the size of the crust can affect its survival during thermo-physical evolution after differentiation. We have considered two different configurations: the first characterized by a dehydrated silicate core and a mantle made of pure ice, the second with a hydrated silicate core and a muddy mantle (ice with silicate impurities). In both cases, the crust is composed of a mixture of ice and silicates. These structures are constrained by a recent measurement of the mean density by Park et al. The Rayleigh–Taylor instability, which operates in such an unstable structure, could reverse all or part of the crust. The whole unstable crust (or part of it) can interact chemically with the underlying mantle and what is currently observed could be a partially/totally new crust. Our results suggest that, in the case of a pure ice mantle, the primordial crust has not survived until today, with a stability timespan always less than 3 Gyr. Conversely, in the case of a muddy mantle, with some 'favourable' conditions (low volumetric ice percentage in the crust and small crustal thickness), the primordial crust could be characterized by a stability timespan compatible with the lifetime of the Solar system.
Dwarf planet
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The nature of geomagnetic field behavior during polarity transitions is one of the most hotly debated issues in modern geophysics. Dynamo action in the earth's fluid outer core is known to be responsible for generating the earth's main magnetic field, however, the mechanism for polarity transition is still unknown. Polarity transitions are a fundamentally important property of the geomagnetic field, but because they have not taken place in historic time, it is necessary to turn to the paleomagnetic record to understand the mechanism by which they occur.
Polarity (international relations)
Polarity reversal
Geomagnetic secular variation
Ionospheric dynamo region
Geomagnetic pole
Geomagnetic reversal
Outer core
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Geomagnetic secular variation
Ionospheric dynamo region
Geomagnetic pole
Ring current
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When doing experiments about magnetic elements such as parts of magnetic compasses, a weak magnetic test field is often required. Especially, if there is heavy traffic near the test place, the magnetic field varies depending not only on the fluctuation of geomagnetic field, but the heavy magnetic influence caused by the traffic. In the urban area much magnetic effects are superposed. These magnetic disturbances greatly affect upon the weak test field direction. The Authors measured such disturbing magnetic fields, such as the ones caused by the traffic, and tried to cancell them, by using a controll circuit and a Helmholz coil. The result showed that even the fluctuation of geomagnetic field as well as the effect of magnetic disturbances produced by external causes were cancelled to the level of one tenth of the diurnal variation of the geomagnetic field.
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The construction process of Earth crust geophysical model using observed geophysical field leads to solution of the inverse problem, which is classical example of ill-posed problem as its solution is unstable and not unique. It is possible to choose specific variant of density or magnetization distribution if additional information is presented. The purpose of this article is to show the results gained from the study of the structure of the Earth crust in Northern Urals using geophysical methods (seismics, gravics and magnetics).
Exploration geophysics
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The geomagnetic field is generated in the fluid outer core region of the Earth by electrical currents flowing in the slowly
moving molten iron. In addition to sources in the Earth’s core, the geomagnetic field observable on the Earth’s surface has
sources in the crust and in the ionosphere and magnetosphere. The signal from the core dominates, accounting for over 95%
of the field at the Earth’s surface. The geomagnetic field varies on a range of scales, both temporal and spatial; the
description of the variations made here concentrates on the recent spatial and temporal variations of the field with origins in
the Earth’s core that can be surmised from observations made over the last four centuries.
Geomagnetic secular variation
Ionospheric dynamo region
Outer core
South Atlantic Anomaly
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Theoretical work on the magnetohydrodynamics of the earth's liquid core indicates (a) that horizontal variations in the properties of the core-mantle interface that would escape detection by modern seismological methods might nevertheless produce measurable geomagnetic effects; (b) that the rate of drift, relative to the earth's surface, of nonaxisymmetric features of the main geomagnetic field might be much faster than the average zonal speed of hydrodynamic motion of core material relative to the surrounding mantle; and (c) why magnetic astronomical bodies usually rotate. Among the consequences of (a) and (b) are the possibilities that (i) the shortest interval of time that can be resolved in paleomagnetic studies of the geocentric axial dipole component of the earth's magnetic field might be very much longer than the value often assumed by many paleomagnetic workers, (ii) reversals in sign of the geomagnetic dipole might be expected to show some degree of correlation with processes due to motions in the mantle (for example, tectonic activity, polar wandering), and (iii) variations in the length of the day that have hitherto been tentatively attributed to core motions may be due to some other cause.
Geomagnetic secular variation
Outer core
Geomagnetic pole
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The article considers the possibility of predicting the velocities of modern vertical movements of the earth's crust using a complex method of geological interpretation of the geophysical fields of Arkhangelsk – Fedynsk – Fotiadi. The search for links between the velocities of modern vertical movements of the earth's crust with anomalous gravitational and magnetic fields, the topography of the day surface, and the thickness of the earth's crust for one geological structure of the territory of the Republic of Belarus in the form of the Orsha depression was performed. To search for links, the integral relation of Karataev – Vatlin – Zakharova was used. Based on the results of forecasting, a model map of the velocities of modern vertical movements of the Earth's crust was constructed. The map has sufficient accuracy and reliability for practical use.
Earth crust
Earth structure
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The detailed behaviour of the geomagnetic field during reversals is documented by palaeomagnetists to constrain models of the geomagnetic dynamo. Reversals are studied by measuring the magnetic remanence preserved in rocks to obtain both the direction and intensity of the ancient magnetic field.
Polarity (international relations)
Geomagnetic reversal
Geomagnetic secular variation
Magnetostratigraphy
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