Piezomagnetic Changes Research Articles

Giant self-biased magnetoelectric coupling in co-fired textured layered composites

Co-fired magnetostrictive/piezoelectric/magnetostrictive laminate structure with silver inner electrode was synthesized and characterized. We demonstrate integration of textured piezoelectric microstructure with the cost-effective low-temperature co-fired layered structure to achieve strong magnetoelectric coupling. Using the co-fired composite, a strategy was developed based upon the hysteretic response of nickel-copper-zinc ferrite magnetostrictive materials to achieve peak magnetoelectric response at zero DC bias, referred as self-biased magnetoelectric response. Fundamental understanding of self-bias phenomenon in composites with single phase magnetic material was investigated by quantifying the magnetization and piezomagnetic changes with applied DC field. We delineate the contribution arising from the interfacial strain and inherent magnetic hysteretic behavior of copper modified nickel-zinc ferrite towards self-bias response.

3D integrated geophysical modeling for the 2008 magma intrusion at Etna: Constraints on rheology and dike overpressure

We present a 3D numerical model based on Finite Element Method (FEM) to jointly evaluate geophysical changes caused by dislocation and overpressure sources in volcanic areas. A coupled numerical problem was solved to estimate ground deformation, gravity and magnetic changes produced by stress redistribution accompanying magma migration within the volcano edifice. We successfully applied the integrated numerical procedure to image the magmatic intrusion occurring in the northern flank of Etna during the onset of the 2008 eruption. A multi-layered crustal structure of the volcano constrained by geological models and geophysical data was considered. Geodetic and gravity data provide information on the strain field, while piezomagnetic changes give constraints on the stress field. Therefore, the integrated modeling gives insights on Mt Etna rheology and dike overpressure involved in the magma propagation and improves understanding of dike emplacement in the northern sector of the volcano. Our FEM-based approach improves the reliability of model-based inference of geophysical parameters obtained during monitoring of the onset of Etna lateral intrusions that can prelude to an impending eruption.

Numerical simulation of piezomagnetic changes associated with hydrothermal pressurization

We have developed a computer program to calculate geomagnetic field changes due to the piezomagnetic effect, which is caused by hydrothermal pressurization. The program uses the distribution of pore fluid pressure that is the result of a numerical simulation for hydrothermal fluid circulation after magma intrusion. First, we present a mathematical formulation of three-dimensional (3-D) effective stress equilibrium equations that contain the hydrostatic buoyancy effects of the pore fluid and the seepage force associated with fluid flow. Then, we develop a 3-D finite element program for solving the elastic effective stress field using the spatial distribution of body forces resulting from both gravity and seepage force, with appropriate boundary conditions. On the basis of the solved elastic effective stress field, the distributions of stress-induced magnetization were estimated from the linear piezomagnetism to calculate the piezomagnetic effect on the ground. We examined the results of numerical experiments on the effects of permeability and the influence of a caprock as important factors in changing the physical state within a volcanic edifice. From these evaluations, we found that both effects influence not only the amount of piezomagnetic anomalies, but also the duration time of the anomalous piezomagnetic field. Finally, our method was applied to actual volcanomagnetic field variations observed at the Merapi Volcano in Indonesia. As a result, our model reasonably explained the observed pattern of variations.

The application of computed piezomagnetic models to volcanic phenomena

Piezomagnetic models, suitable for volcanological applications, can be readily calculated by numerical surface integral methods, providing both savings in computational time and an ability to deal with arbitrarily shaped magnetoelastic media. Studies of piezomagnetic changes have traditionally used models based on the Mogi model in which a spherical underground pressure source is included to represent a magma chamber. We extended the Mogi model to include inclined column sources, as in the Walsh and Decker model, and evaluated piezomagnetic changes for vertical, horizontal, and inclined columns. Our analysis method considers any dependency of magnetic changes on the angle of column inclination. This numerical approach allows for construction of piezomagnetic models that closely resemble actual volcanic phenomena.

The pattern and magnitude of rotational piezomagnetic anomalies along the dip-slip Mosha fault, Northern Tehran, Iran

Abstract. Several active faults affect the Central Alborz, Northern Iran. The main active faults in the region are the North Tehran and the Mosha faults and their westward continuation, the Taleghan fault. Among these the Mosha fault is the most active fault in Central Alborz. The piezomagnetic field has been inspired by many investigations in the earthquake prediction studies and has been reported in the literature. Several theories proposed for relating piezomagnetic changes into magnetic signals accompanying earthquakes in the vicinity of a fault. In this study based on an analytical solution for the elementary piezomagnetic potentials due to an inclined rectangular fault within a semi-infinite elastic medium a computer program has been developed. This program has been used for evaluation of the rotational piezomagnetic changes along the Mosha fault using different earthquakes with specific magnitudes and at the presence of three different geomagnetic main fields. The magnetic data acquired recently at the study area has been used during the modeling. The first result that can be inferred from this study is that rotational piezomagnetic anomalies are completely localized at the fault tips. So for detection of this anomaly the magnetic instruments should be installed at these locations. The other important element which has been investigated was the amount of slip along the fault, which shows anomaly intensification by increase in the amount of slip.

Piezomagnetic fields due to an inclined rectangular fault in a viscoelastic half-space: an application to the 2002-2003 Etna eruption

SUMMARY Time-dependent piezomagnetic fields due to inclined rectangular faults embedded in a viscoelastic, homogeneous half-space were investigated. A viscoelastic rheology of the surrounding medium was assumed to relate piezomagnetic changes at the surface to the stress field at depth. The viscosity of the medium strongly influences time-dependent stress changes. Especially in volcanic areas, rocks near magmatic sources are considerably heated. The presence of higher temperatures produces a lower effective viscosity in the crust, making it necessary to consider its inelastic properties. Rocks no longer behave in a purely elastic manner but permanently deform because the viscosity is significantly lowered. To determine the time-dependent piezomagnetic fields in a viscoelastic medium, we applied the Correspondence Principle to the analytical elastic solutions for dislocation sources. Among all the possible rheological models, we investigated three cases in which the bulk modulus is purely elastic and the shear modulus relaxes as for (i) a Maxwell solid, (ii) a standard linear solid (SLS) and (iii) a Kelvin solid. The piezomagnetic field completely vanishes after the relaxation process for a Maxwell rheology, whereas it is found to decrease over time and reach some finite offset values for SLS and Kelvin rheologies. A real case study concerning the magnetic anomalies observed during the 2002–2003 Mt Etna eruption is also investigated. Post-eruptive magnetic variations were in general agreement with a viscoelastic relaxation process of a SLS rheology undergoing in the volcano edifice.

P23 熱水流動に伴うピエゾ磁気効果の数値シミュレーション(2)(ポスターセッション,日本火山学会2007年秋季大会)

P23 熱水流動に伴うピエゾ磁気効果の数値シミュレーション(2)(ポスターセッション,日本火山学会2007年秋季大会)

Piezomagnetic field associated with a numerical solution of the Mogi model in a non-uniform elastic medium

SUMMARY In this paper we evaluate changes in the geomagnetic field due to the piezomagnetic effect produced by a stress field of dilatation sources represented by numerical solutions in a non-uniform elastic medium. In a previous work on piezomagnetic modelling a uniform elastic medium was assumed, but in this study we deal with a model of the Earth's crust which consists of a stratified structure. We used the mathematical expressions of the Thomson–Haskell propagation matrix technique and estimated the displacement and strain components for the source type mentioned above. The aim of this study is to evaluate how the piezomagnetic changes are affected by a non-uniform elastic medium. We carried out some case studies to evaluate the piezomagnetic effect for a simple two-layer model and investigated the influence of two related variables, the depth of the lower boundary of the layer with elastic properties, z1, and the depth of the Curie point isotherm, H0. From this evaluation it turns out that the piezomagnetic effect depends strongly on the depth of the elastic boundary.

A Green's function for tectonomagnetic problems in an elastic half-space.

A variety of tectonic models can be constructed by superposing the displacement field solution of a single force operative at a point in an elastic half-space. Piezomagnetic changes associated with the same models are given as well by the linear combination of fundamental piezomagnetic potentials, which arise from stress-induced magnetization produced by a unit single force at a point. The earth is assumed a homogeneous and isotropic elastic semi-infinite medium having a uniformly magnetized top layer. The method is adaptable to (a) surface load, (b) strain nuclei, (c) dislocation and (d) volume source problems. The prescribed potentials are obtained with the aid of a surface integral representation of the tectonomagnetic field (SASAI, 1983). As an application example, piezomagnetic field is calculated for a uniform circular load.