Katsumi Hattori

ULF electromagnetic precursors for an earthquake at Biak, Indonesia on February 17, 1996

ULF electromagnetic emissions associated with a large earthquake occurred at Biak Island, Indonesia at 5h 59m UT on February 17, 1996 (magnitude (Mw) = 8.2 and depth = 20 km from USGS catalog), have been investigated on the basis of ULF magnetic observations at two stations, Biak and Darwin in Australia (about 1,200 km apart). Though a simple analysis of magnetic field intensity (horizontal and vertical magnetic field components) at both stations was found to be closely correlated with geomagnetic ΣKp activity, a detailed analysis of the difference of H and Z components at the two stations, the polarization analysis (Z/H) and fractal analysis (frequency spectrum slope) at these two stations has yielded that the ULF emissions (in the frequency range from 5 mHz to 30 mHz) about 1.5–1.0 months before the quake are likely to be a precursory signature of the quake with its intensity on the order of 0.2–0.3 nT.

Co‐seismic geoelectric potential changes observed in Japan

Co-seismic signals of geoelectric potential have been observed for several earthquakes (EQs) in Japan. It has been argued that if there are precursory changes, stronger signals should be at the time of EQ. Our results prove that co-seismic signals do exist, but they start with the arrival of seismic waves and not at the origin time of EQs. Their amplitude does not scale with the dipole length. Both facts indicate that the changes are local effects of passing seismic waves and not due to electric signals emitted from the seismic source. Observed changes contain two components; oscillatory and offset/decay ones. The former is synchronized with seismic waves, whereas the latter, with a rise time of a few seconds and a decay time of 0.1–1 hour, is suspected to be related to EQ induced subsurface hydrological change. In both cases, the origin of electric potential variations may be electrokinetic.

Direction finding of chorus emissions in the outer magnetosphere and their generation and propagation

Abstract The GEOS l satellite wave data have been used to determine the wave normal directions of chorus emissions in the off-equatorial region (geomagnetic latitude ∼ 17) of the outer magnetosphere based on the direction finding of the wave distribution function method. The local time and L shell of the data analysed are L.T. ≅ 10 h and L ∼ 7.60, respectively, and only one event is extensively studied. There are two frequency bands of emissions; one above and the other below one half the electron gyrofrequency, but the latter is the dominant emission and is investigated here. Two types of chorus are dealt with ; rising tones and impulsive ones. About 90% of chorus emissions analysed are found to be composed of a single plane wave, which is in contrast to plasmasphcric hiss. There is no conspicuous relationship between θ (the angle of wave normal direction with respect to the Earths magnetic field) and frequency for rising tones, and their θs take larger values in a range of 30°–55, whereas a tendency seems to appear, for an impulsive charus. for waves at higher frequencies to travel at a larger angle to the field. The wave normal directions observed have been discussed as functions of wave frequency normalized by the local electron gyrofrequency and the slope of an emission, d ƒ d t . A comparison of the present results on the off-equatorial wave normal directions with the previous equatorial direction finding results have made it possible to discuss the generation and propagation mechanism of chorus emissions with different structures including rising tones, falling tones, constant frequency tones and impulsive (burstlike) ones, and finally we have pointed out future work for investigation.

Further evidence of triggering chorus emissions from wavelets in the hiss band

Abstract The results of detailed spectral analyses and direction finding for ELF data containing simultaneous hiss and chorus have been reported on the basis of the data observed on-board GEOS 1 satellite in the outer magnetosphere at a high geomagnetic latitude (Λ ∼- 25°). A combination of the present results with those from our previous equatorial studies has yielded the following findings : (1) each chorus element has a tendency to originate from the hiss band and is asymptotic to the hiss band ; (2) the intensity and occurrence of the chorus are closely correlated with the intensity of the underlying hiss ; (3) the hiss band exhibits structures or wavelets (i.e. monochromatic wave component with significant duration) and we notice the existence of the causative wavelet at the foot of each chorus element ; (4) when we expect triggering chorus from a wavelet near the upper edge of the hiss band, the duration of the wavelet increases with decreasing intensity ; (5) very similar φ (azimuthai angle) values are noticed for both the hiss and chorus, suggesting that both phenomena come from the same source region. These observational facts might lead us to conclude that a wavelet existing near the upper edge of the hiss band is able to generate a chorus emission through coherent wave-particle interaction in the outer magnetosphere, as in the case of active VLF wave injection experiments.

Statistical analysis of ULF seismomagnetic phenomena at Kakioka, Japan, during 2001–2010

To clarify and verify the ultralow frequency (ULF) seismomagnetic phenomena, we have performed statistical studies on the geomagnetic data observed at the Kakioka (KAK) station, Japan, during 2001–2010. We investigated the energy of ULF geomagnetic signals of the frequency around 0.01 Hz using wavelet transform analysis. To minimize the influences of artificial noises and global geomagnetic perturbations, we used only the geomagnetic data observed at nighttime (LT 2:30 A.M. to 4:00 A.M.) and utilized observations from a remote station, Kanoya, as a reference. Statistical results of superposed epoch analysis have indicated that ULF magnetic anomalies are more likely to appear before sizable earthquake events (Es > 108) rather than after them, especially 6–15 days before the events. Further statistical investigations show clearly that the ULF geomagnetic anomalies at KAK station are more sensitive to larger and closer events. Finally, we have evaluated the precursory information of ULF geomagnetic signals for local sizable earthquakes using Molchans error diagram. The probability gain is around 1.6 against a Poisson model. The above results have indicated that the ULF seismomagnetic phenomena at KAK clearly contain precursory information and have a possibility of improving the forecasting of large earthquakes.

Seismo-ionospheric anomalies of the GPS-TEC appear before the 12 May 2008 magnitude 8.0 Wenchuan Earthquake

In this paper, the total electron content (TEC) of Global Ionosphere Maps (GIMs) is normalized and employed to study the seismo-ionospheric anomalies at the time of the 12 May 2008 M 8.0 Wenchuan Earthquake. The space weather conditions are taken into account. It is found that remarkable reductions appear locally around the epicentre and their conjugate points during the daytime of 29 April and 6–10 May 2008. A global study and a strict criterion are applied to detect anomalies. Results show that the anomalies on 29 April and 6 and 7 May 2008, which are respectively days 13, 6 and 5 before, are possibly related to the earthquake. The conjugate signature implies the seismo-generated electric field is essential.

Ray-tracing study of the plasmapause effect on non-ducted whistler-mode wave propagation

Abstract The effect of the plasmapause on whistler-mode wave propagation and generation is investigated by means of the two-dimensional ray tracings started at the equator over a wide frequency range, 0.1 0.5) waves. These theoretical findings are discussed with their relevance to the experimental facts of different magnetospheric whistler-mode emissions.

GCV-aided linear reconstruction of the wave distribution function for the ground-based direction finding of magnetospheric VLF/ELF waves

For the purpose of the direction finding of magnetospheric VLF/ELF radio waves, the reconstruction of the wave distribution function has been studied on the linear regularization of Phillips-Tikhonov type and on the associated optimization with the generalized cross-validation (GCV) in numerically analyzing spectral matrix data. On the ground-based wave observation, one finds by numerical simulation that the GCV well behaves in spite of the small number of data and that the linear algebra approach has the advantages of automatically selecting the best image and of computational facility. From the results of simulation the feasibility of direction finding is discussed.

ULF geomagnetic changes possibly associated with the 2008 Iwate-Miyagi Nairiku earthquake

Anomalous ULF geomagnetic field changes associated with earthquake is one of the most convincing and promising phenomena due to deeper skin depth. Since ULF signals associated with large earthquakes are weak, effective signal discrimination methods should be required. In this study, we investigate ULF geomagnetic changes possibly associated with the 2008 Iwate-Miyagi Nairiku earthquake based on spectral density ratio analysis, geomagnetic transfer function analysis, fractal analysis, and direction finding analysis. Geomagnetic data observed at Esashi, where the epicentral distance is about 47 km and Kakioka, the distance is about 317 km, and as a reference station have been analyzed. Wavelet transform have been performed for the spectral density analysis instead of FFT method. Before the earthquake, the variation of spectral density ratio, Sz/Sx and Sz/Sy, at the nearest station of Esashi exhibits an apparent increase from the trend. On the contrary, there are no corresponding significant changes at a remote station of Kakioka. The level of peak is beyond the 3 σ and its duration is 3 days. The lead time is about 3–4 weeks before the earthquake. At the periods from 3 to 105 sec, similar anomalous changes occurred. Results of transfer function analysis are similar. Fractal analysis and direction finding analysis have been also applied. The obtained fractal dimension and direction of arrival of ULF waves show the possible relation to the earthquake. These facts suggest the anomalous change is a possible candidate of earthquake-related ULF magnetic change.

Gradients and phase velocities of ULF geomagnetic disturbances used to determine the source of an impending strong earthquake

Results of studying the behavior of the vectors of gradients and phase velocities of ULF geomagnetic disturbances (F < 1 Hz) in the Japan seismic zone are presented. The gradient and phase velocity vectors along the Earth’s surface were determined using data of the group of three high-sensitivity three-component magnetovariation stations (MVC-3DS) located at triangle vertices at a small (∼5 km) distance from one another (magnetic gradiometer). Two such groups of stations were installed in 1999 southwest and southeast of Tokyo at a distance of ∼150 km from each other. It has been indicated that, several months before strong earthquakes (M > 5), the values of gradients and phase velocities start anomalously changing, and directions toward sources of impending strong earthquakes appear in the distribution of gradient vector directions. Directions from sources of impending earthquakes appear in the distribution of phase velocity vector directions. It is proposed to use gradients and phase velocities of ULF and ELF geomagnetic disturbances as one of the factors in a short-term prediction of strong earthquakes.