M. Hayakawa

Results of ultra-low-frequency magnetic field measurements during the Guam Earthquake of 8 August 1993

We report the results of measurements of ultra-low-frequency magnetic noise during a large earthquake (Ms=7.1) at Guam of 8 August, 1993 (depth ∼60 km). The ULF observing system is located in the Guam Island, about 65 km from the epicenter. Several distinct features of this analysis are summarized. (1) We have proposed rather sophisticated statistical analyses (monthly mean, standard deviation) in order to estimate the wave intensity and polarization (i.e. ratio Z/H). (2) A comparison between the ULF wave activity and ΣKp, is useful in distinguishing between the space geomagnetic pulsations and non-space emissions. (3) Then, the use of the ratio (Z/H) is found to be of essential importance in discrimating the emissions presumably of seismic origin from space plasma waves. (4) The statistical analysis of the temporal evolution of this ratio, has yielded that it shows a broad maximum only about one month before the earthquake, and this suggests that the emissions during this period are very likely to be magnetic precursors. (5) The temporal variation of Z component is similar to that for the Loma Prieta earthquake such that it shows a broad maximum ten days ∼ two weeks before the earthquake and another increase a few days before the earthquake. (6) The emissions presumably associated with the earthquake are of noise-like nature, and their main frequency is 0.02 ∼ 0.05 Hz (with maximum intensity ∼0.1 nT).

Fractal analysis of ULF geomagnetic data associated with the Guam Earthquake on August 8, 1993

This paper presents a fractal analysis of the ULF electromagnetic data obtained at Guam observatory during a strong (Ms=8.0) earthquake occurred on August 8, 1993 near the Guam island. We have already reported the presence of seismogenic ULF emissions for this quake (Hayakawa et al., GRL, 1996). The chosen period covered four months before and two months after the earthquake. It was found that the spectrum of emissions at noon exhibited a power law behavior f−β which is typical for self-organized critical dynamics. The slope of the spectrum in the noon sector fluctuates in a range from 2.5 to 0.7 with a tendency to gradually decrease during the process of the earthquake preparation. We consider this to be a candidate for a precursory signature. The similar effect was found for the night sector. We suggest that the focal zone was at the critical stage of self-organized criticality process before the quake, related to active microfracturing processes followed by the generation of ULF emissions as we detected.

Generation of ULF electromagnetic emissions by microfracturing

Microfracturing electrification is discussed as a possible explanation of ULF electromagnetic emissions observed before and after earthquakes. Using a space-time simple model of microfracture progression, it is possible to compare theoretical results and observed results with special reference to the intensity, spectrum, and temporal development of ULF magnetic field variations.

Penetration characteristics of electromagnetic emissions from an underground seismic source into the atmosphere, ionosphere, and magnetosphere

Theoretical calculations are made on electromagnetic fields in the frequency range 10−2 to 10−2; Hz on the ground surface and above the ionosphere induced by stochastic microcurrent activity inside the future seismic sources on the assumption of cylindrical symmetry of the effective current and three types of polarization. The inhomogeneity of the ground and atmosphere conductivity and anisotropy of the ionosphere are taken into consideration. The intensity of ULF magnetic and electric precursors observed on the ground, and their spatial distribution can be explained by using the results of the present computations. It is found that only the fields from a magnetic type source can penetrate into the magnetosphere and generate propagating Alfven waves. The expected values of magnetospheric electric and magnetic field are 1-10 μV m−1 Hz−1/2 and 1–10 pT Hz−1/2 respectively, and the horizontal scale of their distribution is about 100–200 km. Finally, these theoretical predictions are compared with the corresponding results of satellite observations.

Precursory effects in the subionospheric VLF signals for the Kobe earthquake

Abstract The subionospheric VLF Omega signal transmitted from Tsushima, Japan (geographic coordinates: 34°37′N, 129°27′E) was continuously received at Inubo (35°42′N, 140°52′E). This data was analyzed during an 8-month period centered on the great Hyogo-ken Nambu (Kobe) earthquake, M g =7.2 on 17 January 1995, the epicenter located inside of Fresnel zone of the VLF path. To clarify the possible effect, we developed the special TT (terminator time) method of data processing, which was useful for our short VLF path (distance ∼1000 km). We discovered a statistically significant change of TT characteristics, which began a few days before the main shock and probably continued a few weeks after it as a transient oscillation with period ∼10 days. By simple modelling, it was shown that TT changes could be caused by the decrease of the VLF reflection height by ∼2 km. The possible underlying mechanisms of the effect are not defined; however, an increase of the regular electric field due to radon exhalation before the earthquake or an intensification of planetary waves by seismically influenced atmospheric turbulence might be considered.

Electromagnetic anomalies associated with 1995 Kobe earthquake

Abstract Occurrences of anomalous electro-magnetic phenomena at varied frequency ranges, covering ELF to VHF, have been reported in relation to the 17 January 1995 Kobe earthquake (M7.2), by several independent research groups. Prominent pre-seismic peaks, which could have been emitted from the focal area, were observed on 9-10 January in ELF, VLF, LF and HF ranges. Whether these changes were truly related to the earthquake is not certain, because atmospheric (thunderbolt discharge) activities also peaked on 9-10 January. The nomalous changes were markedly enhanced toward the catastrophe in agreement with many reports on unusual radio/TV noise. Anomalous transmission of man-made electromagnetic waves in VLF and VHF ranges was also detected from a few days before the earthquake, indicating the possibility that the ionosphere above the focal zone was disturbed at the final stage of the earthquake preparation process.

Interrelation between ELF transients and ionospheric disturbances in association with sprites and elves

Red sprites and elves were observed in Japan during the winter of 1998/99 in Hokuriku region by the group of Tohoku Univ. [Fukunishi et al., EOS, 80(46), F217, 1999]. We analyze quantitatively the results from coordinated measurement consisting of ELF transients, VLF subionopheric disturbances and lightning discharges associated with the optical events. We find the clear straightforward relationship between charge transfer of the parent discharge calculated from ELF (f < 15 Hz) and the ionospheric disturbances regardless of the types of optical events indicating significant atmosphere-mesosphere-ionosphere coupling. Sprites tend to associate with a large ionospheric disturbance (−13 ∼ + 4.6 dB) with a large charge transfer (52 ∼ 175 C), whereas a large lightning peak current (+223 ∼ + 470 kA) (or slow-tail amplitude) leading to the strong EMP is necessary to initiate elves, but with rather small ionospheric disturbances.

On the generation mechanism of ULF seismogenic electromagnetic emissions

Abstract Microfracturing electrification is suggested as a possible mechanism for explaining ULF electromagnetic emissions observed before and after the earthquakes. This effect appears as fast fluctuation of microcracks and leads to the origination of wideband electromagnetic noise. This noise dissipates outside the source region and produces ULF emissions on the ground surface with an upper cutoff frequency ∼1 Hz due to the skin depth attenuation. Each microcurrent results from charge relaxation during microcrack opening and depends on the time of opening and conductivity of the rock medium. The normal size distribution of microcracks, their fast opening and healing (intermittence), and average size progression due to stress corrosion are assumed. Using this model, it is possible to compare these theoretical explanations with the observational results with reference to the intensity, frequency spectrum and temporal development of ULF magnetic field variations.

Current status of seismo-electromagnetics for short-term earthquake prediction

Short-term (timescale of hours, days and weeks) earthquake (EQ) prediction is of essential importance to mitigate EQ disasters. Short-term EQ prediction has so far been based on seismic measurements (i.e. mechanical observation of crustal movements), but it was concluded in Japan about 10 years ago that EQ prediction is impossible by means of the mechanical method. Hence, there has been an increased interest and a lot of progress in non-seismic measurement during the last decade. A new approach was developed where electromagnetic measurements provide microscopic information on the lithosphere. The present paper is intended to give a history of short-term EQ prediction, and also we hope that this paper reviews the current status of a new science field, ‘seismo-electromagnetics’. We make a general review of different phenomena taking place in the lithosphere, atmosphere and the ionosphere, but we pay more attention to the subjects of our preference including lithospheric ultra low frequency (ULF) electromagnetic emissions, and seismo-ionospheric perturbations.

Wave characteristics of tweek atmospherics deduced from the direction‐finding measurement and theoretical interpretation

Detailed wave characteristics (incident and azimuthal angles, wave polarization, and their frequency dependences) of tweek atmospherics have been elucidated on the basis of the application of our field-analysis direction finding to the VLF data observed in southern China. It is then found that the wave polarization of the first-order mode at the frequency above its cutoff frequency (ƒ1c = 1.7 ∼ 1.8 kHz) is always left handed and it becomes exactly left-handed circular when the wave frequency decreases down to the ƒ1c, together with the fact the incident angle becomes zero (vertical) when the frequency approaches ƒ1c. Whereas the zeroth-order mode at frequency below ƒ1c is found to be linearly polarized. These wave properties are first interpreted qualitatively, and then we try to explain them in terms of the full wave theory in the Earth-ionosphere waveguide in which the realistic electron density profiles of the lower ionosphere are assumed. Finally, it is suggested that the measurement of wave polarization, as given in this paper, would be useful in studying the formation mechanism of tweek tails, the coupling into whistler mode waves of lightning discharges, the lower ionospheric density profile, etc.