Perturbation of the electric current by a resistivity anomaly and its application to earthquake prediction.

Honkura, Yoshimori

doi:10.5636/jgg.28.47

Cited by 13 publications

(6 citation statements)

References 10 publications

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“…Electric field distortion due to a hemisphere Consider the electrostatic effect of a conducting hemisphere in a uniformly conducting half-space ( Figure 1 ( 1)The solution is extracted by symmetry from the similar electrostatic problem for a uniformly conductinR s(Jhere, in a homogeneous medium, excited by a constant static field (Ward, 1967). In a spherical coordinate system (r, e, <\» with the origin at the center of the sphere, the potential is given by (Honkura, 1975) f;(r, e, <\» = (-Eo + ~)r sin e cos <\>, rsR 2within the inhomogeneity and fe(r, e, <\» = ( -Eor + §) sin e cos <\>, r 2 R (3) outside the hemisphere but within the conducting half-space (z 2 0).…”

Section: Galvanic Scattering Due To a Conducting Hemispherementioning

confidence: 99%

Analytic investigations of the effects of near‐surface three‐dimensional galvanic scatterers on MT tensor decompositions

1991

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An outcropping hemispherical inhomogeneity embedded in a two‐dimensional (2-D) earth is used to model the effects of three‐dimensional (3-D) near‐surface electromagnetic (EM) “static” distortion. Analytical solutions are first derived for the galvanic electric and magnetic scattering operators of the heterogeneity. To represent the local distortion by 3-D structures of fields which were produced by a large‐scale 2-D structure, these 3-D scattering operators are applied to 2-D electric and magnetic fields derived by numerical modeling to synthesize an MT data set. Synthetic noise is also included in the data. These synthetic data are used to study the parameters recovered by several published methods for decomposing or parameterizing the measured MT impedance tensor. The stability of these parameters in the presence of noise is also examined. The parameterizations studied include the conventional 2-D parameterization (Swift, 1967), Eggers’s (1982) and Spitz’s (1985) eigenstate formulations, LaTorraca et al.’s (1986) SVD decomposition, and the Groom and Bailey (1989) method designed specifically for 3-D galvanic electric scattering. The relationships between the impedance or eigenvalue estimates of each method and the true regional impedances are examined, as are the azimuthal (e.g., regional 2-D strike, eigenvector orientation and local strike) and ellipticity parameters. The 3-D structure causes the conventional 2-D estimates of impedances to be site‐dependent mixtures of the regional impedance responses, with the strike estimate being strongly determined by the orientation of the local current. For strong 3-D electric scattering, the local current polarization azimuth is mainly determined by the local 3-D scattering rather than the regional currents. There are strong similarities among the 2-D rotation estimates of impedance and the eigenvalue estimates of impedance both by Eggers’s and Spitz’s first parameterization as well as the characteristic values of LaTorraca et al. There are striking similarities among the conventional estimate of strike, the orientations given by the Eggers’s, Spitz’s (Q), and LaTorraca et al.’s decompositions, as well as the estimate of local current polarization azimuth given by Groom and Bailey. It was found that one of the ellipticities of Eggers, LaTorraca et al., and Spitz is identically zero for all sites and all periods, indicating that one eigenvalue or characteristic value is linearly polarized. There is strong evidence that this eigenvalue is related to the local current. For these three methods, the other ellipticity differs from zero only when there are significant differences in the phases of the regional 2-D impedances (i.e., strong 2-D inductive effects), implying the second ellipticity indicates a multidimensional inductive response. Spitz’s second parameterization (U), and the Groom and Bailey decomposition, were able to recover information regarding the actual regional 2-D strike and the separate character of the 2-D regional impedances. Unconstrained, both methods can suffer from noise in their ability to resolve structural information especially when the current distortion causes the impedance tensor to be approximately singular. The method of Groom and Bailey, designed specifically for quantifying the fit of the measured tensors to the physics of the parameterization, constraining a model, and resolving parameters, can recover much of the information in the two regional impedances and some information about the local structure.

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Section: Galvanic Scattering Due To a Conducting Hemispherementioning

confidence: 99%

Analytic investigations of the effects of near‐surface three‐dimensional galvanic scatterers on MT tensor decompositions

1991

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“…Analytical solutions for 3D DC anomalies have usually been applied using uniform primary fields ( Honkura 1976; Ward and Hohmann 1987). In our solution a dipole field is used instead of a homogeneous and uniform primary field.…”

Section: Introductionmentioning

confidence: 99%

An approximate analytical approach to compute geoelectric dipole–dipole responses due to a small buried cube

Szalai

Szarka

2000

Geophysical Prospecting

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A simple analytical solution is presented for computing direct current (DC) electric field distortion due to a small cube in a homogeneous half‐space, measured with a dipole–dipole array on the surface. Both the transmitter and the receiver may have any orientation; furthermore their position on the horizontal surface and the depth of the cube can be freely selected. It is shown that a simple approximate analytical method may replace more complicated 3D numerical modelling algorithms. The approximation lies in the linearization of the problem: the secondary source (i.e. the cube) is considered as a system of three perpendicular electric dipoles. In spite of this first‐order approximation, in the case of realistic depths z (zR≈0.1–0.5, where R is the transmitter–receiver distance), this approximate solution fits very well with true 3D numerical modelling results, and with analogue modelling results if aR≤0.1, where a is the length of the side of the cube. Due to its simplicity, this method could be used for computing DC field distortion effects, estimating parameter‐sensitivities, or even determining some initial models for further inversions.

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“…Changes in magnetotelluric parameters were estimated by Honkura (1976)on the assumption that the induced electric field is approximately uniform in the crust and the effect of secondary induction is negligibly small. This assumption is valid only for periods which are sufficiently long for the penetration depth to be deeper than the crustal depth.…”

Section: Discussionmentioning

confidence: 99%

Time-dependence of electromagnetic transfer functions and their association with tectonic activity

Niblett¹,

Honkura

1980

Geophysical Surveys

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Abstract. Laboratory experiments and theoretical considerations have suggested that anomalous dilatant regions can develop in the earth's crust during the period of strain accumulation prior to an earthquake. For moderate and major earthquakes such anomalous regions could be tens or even hundreds of kilometers in extent and should be detectable at the surface with appropriate survey or sounding techniques. Since electrical resistivity is one of the rock properties likely to be strongly modified in a dilatant zone, magnetoteUuric impedance and geomagnetic transfer functions might be expected to show time-dependent precursory effects if monitored over a period of time above the focal region of an impending earthquake. Such experiments have been conducted in Japan and in other parts of the world and several examples ofresitivity changes in the crust prior to earthquake occurrence have been reported. These results and their association with local seismicity are reviewed in this paper. The available evidence indicates that transfer functions and impedance can display significant time-dependent response to changing crustal conditions in some regions. However, the correspondence between these effects and earthquake occurrence is usually not very clear.

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Perturbation of the electric current by a resistivity anomaly and its application to earthquake prediction.

Cited by 13 publications

References 10 publications

Analytic investigations of the effects of near‐surface three‐dimensional galvanic scatterers on MT tensor decompositions

Analytic investigations of the effects of near‐surface three‐dimensional galvanic scatterers on MT tensor decompositions

An approximate analytical approach to compute geoelectric dipole–dipole responses due to a small buried cube

Time-dependence of electromagnetic transfer functions and their association with tectonic activity

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