Piezomagnetic fields associated with a dislocation source in a layered elastic medium

The piezomagnetic effect is defined as a change in magnetization with applied stress. Changes in the geomagnetic field caused by the piezomagnetic effect, referred to as the piezomagnetic field, have been theoretically estimated and compared by previous studies to interpret observed variations in the geomagnetic field. However, the piezomagnetic field estimated in previous studies may not provide an accurate estimation because they ignored spatial variations in elasticity, leading to only a rough approximation of the properties of Earth's crust. In this paper, a semi-analytical procedure for calculating the piezomagnetic field arising from a point dislocation source embedded in a layered elastic medium is derived. Following a well-established method of the vector surface harmonic expansion, all of the governing equations written in partial differential equations in a real domain, together with the linear constitutive law of the piezomagnetic effect, are converted to a set of ordinary differential equations in a wavenumber domain. Equations in the wavenumber domain are solved analytically, and each component of the piezomagnetic field in the real domain is obtained after applying the Hankel transform. By using the derived procedure, the piezomagnetic and displacement fields due to a finite fault with strike-slip, dip-slip, and tensile-opening mechanisms are estimated for media with layered elasticity structures. Results for a finite fault are obtained by integrating the point source solution over the fault plane. The results of the numerical analysis allow the effect of heterogeneities in rigidity on the piezomagnetic effect to be examined and implications for the findings of previous investigations to be drawn. In cases where the moment-release at the dislocation source is fixed, the effect of the rigidity differences between upper and lower layers on the piezomagnetic field is minor even in the case where the Curie point depth is near the source of dislocation. This result is in contrast to a previous study that assumed the Mogi model and suggested that heterogeneities in the horizontal direction may be of importance when combined with layered rigidity structures. A contrast is seen between the piezomagnetic and displacement fields corresponding to models with layered rigidity structures: the piezomagnetic field is roughly proportional to the moment-release on a source fault, whereas the displacement field is proportional to slip or opening of the fault. Provided that the rigidity of the crust increases with increasing depth, the calculated piezomagnetic field is likely to have been underestimated in many earlier studies, which assumed uniform rigidity and a geodetically inverted size of slip

Comment on “Piezoelectricity as a mechanism on generation of electromagnetic precursors before earthquakes” by J.H. Wang

Wang (2021, hereafter JHW) recently investigated elastic–electric coupling (EEC) in terms of the piezoelectric effect to assess both the plausibility of and necessary conditions for generating pre-earthquake electromagnetic phenomena, including variations in the total electron content (TEC). The study considered a 1-D model to simulate the piezoelectric effect, and derived a quantitative relationship between the dislocation and intensity of the electric field. One of JHW's conclusions was that the piezoelectric effect is a potential mechanism for generating previously reported pre-earthquake TEC anomalies. However, the quantitative discussion in JHW contains a serious error. JHW had chosen an incorrect mode between the two solutions during the derivation of a quantitative relationship between the displacement and the electric field, which subsequently led to an incorrect estimation of the ratio of the generated electric field to the displacement. The opposite conclusion to that drawn by JHW is attained when a correct mode is used

A procedure for stable electrical measurements on a rock sample against high contact resistance as a prerequisite for electrical tomography

令和4年度 Conductivity Anomaly研究会日時：令和4年12月26日（月）09:25-18:30, 12月27日（火）09:00-16:30場所：京都大学防災研究所連携研究棟3階301号室およびZoo

Magnetotelluric and temperature monitoring after the 2011 sub-Plinian eruptions of Shinmoe-dake volcano

Three sub-Plinian eruptions took place on 26–27 January 2011 at Shinmoe-dake volcano in the Kirishima volcanic group, Japan. During this event, GPS and tiltmeters detected syn-eruptive ground subsidence approximately 7 km to the WNW of the volcano. Starting in March 2011, we conducted broad-band magnetotelluric (MT) measurements at a site located 5 km NNW of the volcano, beneath which the Shinmoe-dake magma plumbing system may exist. In addition, temperature monitoring of fumaroles and hot-springs near the MT site was initiated in July 2011. Our MT data record changes in apparent resistivity of approximately ±5%, along with a ±1◦ phase change in the off-diagonal component of the impedance tensor (Zxy and Zyx ). Using 1-D inversion, we infer that these slight changes in resistivity took place at relatively shallow depths of only a few hundred meters, at the transition between a near-surface resistive layer and an underlying conductive layer. Resistivity changes observed since March 2012 are correlated with the observed temperature increases around the MT monitoring site. These observations suggest the existence beneath the MT site of pathways which enable volatile escape

Seismicity controlled by resistivity structure : the 2016 Kumamoto earthquakes, Kyushu Island, Japan

The M JMA 7.3 Kumamoto earthquake that occurred at 1:25 JST on April 16, 2016, not only triggered aftershocks in the vicinity of the epicenter, but also triggered earthquakes that were 50–100 km away from the epicenter of the main shock. The active seismicity can be divided into three regions: (1) the vicinity of the main faults, (2) the northern region of Aso volcano (50 km northeast of the mainshock epicenter), and (3) the regions around three volcanoes, Yufu, Tsurumi, and Garan (100 km northeast of the mainshock epicenter). Notably, the zones between these regions are distinctively seismically inactive. The electric resistivity structure estimated from one-dimensional analysis of the 247 broadband (0.005–3000 s) magnetotelluric and telluric observation sites clearly shows that the earthquakes occurred in resistive regions adjacent to conductive zones or resistive-conductive transition zones. In contrast, seismicity is quite low in electrically conductive zones, which are interpreted as regions of connected fluids. We suggest that the series of the earthquakes was induced by a local accumulated stress and/or fluid supply from conductive zones. Because the relationship between the earthquakes and the resistivity structure is consistent with previous studies, seismic hazard assessment generally can be improved by taking into account the resistivity structure. Following on from the 2016 Kumamoto earthquake series, we suggest that there are two zones that have a relatively high potential of earthquake generation along the western extension of the MTL

Observation results by the TAMA300 detector on gravitational wave bursts from stellar-core collapses

We present data-analysis schemes and results of observations with the TAMA300 gravitational-wave detector, targeting burst signals from stellar-core collapse events. In analyses for burst gravitational waves, the detection and fake-reduction schemes are different from well-investigated ones for a chirp-wave analysis, because precise waveform templates are not available. We used an excess-power filter for the extraction of gravitational-wave candidates, and developed two methods for the reduction of fake events caused by non-stationary noises of the detector. These analysis schemes were applied to real data from the TAMA300 interferometric gravitational wave detector. As a result, fake events were reduced by a factor of about 1000 in the best cases. The resultant event candidates were interpreted from an astronomical viewpoint. We set an upper limit of 2.2x10^3 events/sec on the burst gravitational-wave event rate in our Galaxy with a confidence level of 90%. This work sets a milestone and prospects on the search for burst gravitational waves, by establishing an analysis scheme for the observation data from an interferometric gravitational wave detector

An attempt to correct strain data measured with vault-housed extensometers under variations in temperature

Strain data obtained by vault-housed extensometers have precisions on the order of nanostrains, but they are distorted by variations in temperature, which cause two types of noise: “actual variations” due to the thermo-elastic effect of the Earth's crust, and “false variations” due to the thermal expansion of extensometer, which occurs when the extensometers themselves are subjected to variations in temperature. Here, I explore a method of removing false variations, which are severe when the vault is located at shallow depths. If variations in temperature at arbitrary points inside a vault are estimated, false variations can be removed from the recorded variations in strain. I derive formulae that enable variations in temperature to be estimated at various points in a vault, based on measured variations at reference points. The formulation is valid if some simplification is allowed. I examined whether variations in temperature inside a vault can be estimated in terms of the derived formulae, and obtained the following results. When the reference temperature data are obtained from adequate points in the vault, variations in temperature at another point can be estimated with an accuracy of 0.1 °C. However, when the reference temperature data are obtained from outside the vault, estimated variations in temperature are rather inaccurate, which means that the false variations in strain cannot be removed accurately. Moreover, the data indicate that the thermal diffusivity of the ground is temporally variable, and this introduces another difficulty in correcting false variations in strain data. These results indicate that correcting the distortions in strain data due to variations in temperature is much more difficult than anticipated

Temporal variations in magnetic signals generated by the piezomagnetic effect for dislocation sources in a uniform medium

Fault ruptures in the Earth's crust generate both elastic and electromagnetic (EM) waves. If the corresponding EM signals can be observed, then earthquakes could be detected before the first seismic waves arrive. In this study, I consider the piezomagnetic effect as a mechanism that converts elastic waves to EM energy, and I derive analytical formulas for the conversion process. The situation considered in this study is a whole-space model, in which elastic and EM properties are uniform and isotropic. In this situation, the governing equations of the elastic and EM fields, combined with the piezomagnetic constitutive law, can be solved analytically in the time domain by ignoring the displacement current term. Using the derived formulas, numerical examples are investigated, and the corresponding characteristics of the expected magnetic signals are resolved. I show that temporal variations in the magnetic field depend strongly on the electrical conductivity of the medium, meaning that precise detection of signals generated by the piezomagnetic effect is generally difficult. Expected amplitudes of piezomagnetic signals are estimated to be no larger than 0.3 nT for earthquakes with a moment magnitude of ≥7.0 at a source distance of 25 km; however, this conclusion may not extend to the detection of real earthquakes, because piezomagnetic stress sensitivity is currently poorly constrained

Improved models of the piezomagnetic field for the 2011 Mw 9.0 Tohoku-oki earthquake

To assess the feasibility of observing changes in the magnetic field produced by the piezomagnetic effect, an improved model of the piezomagnetic field corresponding to the Mw 9.0 Tohoku-oki earthquake is presented. In contrast to an earlier study, the proposed model explicitly considers the spatial distribution of slip on the seismic fault, and the results from this new model differ significantly from those of the previous model where slip distributions were ignored. Quantitative aspects of the piezomagnetic effect are discussed through comparisons of data and models. One feature clarified is that, because the fault rupture is so far offshore, the expected amplitudes are quite small at onshore existing observation sites; consequently, there would have been little chance of observing sizable piezomagnetic signals at inland sites during the Tohoku-oki earthquake. Nevertheless, piezomagnetic signals were reportedly detected at a few sites, possibly indicating that the stress sensitivity or the initial magnetization was larger (by several factors) than assumed. On the other hand, relatively large variations in the magnetic field of up to 10 nT may have occurred offshore. This means that if ocean-bottom sensors had been installed, larger piezomagnetic signals would have been detected. Moreover, the piezomagnetic field in offshore areas is sensitive to the detailed slip distribution, suggesting that observations of the magnetic field at ocean-bottom sites might provide important constraints on determination of slip models