The Kumamoto area of Kyusyu Island was attacked by a series of large earthquakes (EQs) in April, 2016. The first two foreshocks had the magnitudes of 6.5 and 6.4, and about 1 day later there was the main shock on 15 April (UT) with magnitude 7.3. These are fault-type EQs, and so we would expect a variety of electromagnetic precursors to these EQs because we had detected different phenomena for the 1995 Kobe EQ, same fault-type EQ. As for the lithospheric effect, the ULF data at Kanoya observatory (about 150 km from the EQ epicenters) are used, but the simple statistical analysis could not provide us with any clear evidence of ULF radiation from the lithosphere. However, our conventional analyses indicated clear signatures in the atmosphere as ULF/ELF impulsive emissions and also in the ionosphere as observed by means of VLF propagation anomalies and ULF depression. ULF/ELF radiation appeared on 8-11 April (in UT) (maximum on 10 and 11 April (UT)), while ULF depression took place on 8 and 10 April (in UT), so that both atmospheric radiation and ionospheric perturbation took place nearly during the same time period.
Even after the disastrous 2011 Tohoku earthquake (EQ) [
In this paper, we pay attention to the electromagnetic phenomena in possible association with this Kumamoto EQ series. It is needless to say that statistical studies are highly required on the correlation of any seismogenic phenomenon with EQs on the basis of long-term observation (e.g., [
There happened three successive EQs in the Kumamoto area with an extremely low probability of medium-term forecast: 1) M = 6.5 (depth~9 km) (geographic coordinates: 32.788˚N, 130.704˚E) at 12:26:41.1 UT on 14 April, 2) M = 6.4 (depth~8 km) (32.697˚N, 130.720˚E) at 15:03:50.6 UT on 14 April, and 3) M = 7.3 (depth~10 km) (32.791˚N, 130.754˚E) at 16:25:15.7 UT on 15 April (or 0.25:15.7 JST on 16 April). The last, main shock, happened about 1 day after the foreshocks, and the report by JMA has indicated that two former EQs were associated with the Hinaku fault, while the latter, with the nearly Futagawa fault. The epicenters of these EQs are shown in
There may be a few interesting electromagnetic phenomena in possible association
with the Kumamoto EQ event [
There is a ULF observatory at KNY belonging to JMA whose geographic coordinates are 31˚25'27''N, 130˚52'48''E (
only in the frequency band of 10 - 20 mHz (0.01 - 0.02 Hz) during the period of 15 January to the end of May, 2016 as observed at KNY, because the most conspicuous results are obtained at this frequency. The top panel of
First, we pay attention to the lithospheric radiation (direct radiation from the lithosphere) as seen from the 2nd to 4th panels in
On the other hand, even the conventional statistical analysis on Dh and δDep has yielded very clear peaks about one week before the EQ. Especially a peak on 8 April is very notable, with δDep exceeding greatly the value of 10. Nobody can oppose to this presence. The lead time of about one week is found to be very consistent with our previous statistical results [
The observatories of ULF/ELF electromagnetic radiation are indicated in
Schekotov et al. (2007) [
Though the details of signal processing method have been summarized in [
1) Direction finding (arrival direction)
The source direction of seismo-atmospheric ULF/ELF radiation is determined as being perpendicular to the main axis of polarization ellipse. The angle between the main axis of the polarization axis and the d (EW)-component axis is denoted by θ, and its tangent is given by the following equation.
tan ( 2 θ ) = 2 A h A d A d 2 − A h 2 cos ( φ h − φ d ) (1)
Here A h , A d and φ h , φ d are instantaneous amplitudes and phases of the field component signals. h corresponds to the NS component of magnetic field, while d, the EW component. They are computed from appropriate complex signals which are obtained using from the real signals (Uh and Ud) by means of the Hilbert transform. The last ones are extracted from the recorded signals with narrow-band filtration.
For the required accuracy of direction finding, we have chosen the signal whose amplitude is 5 times the average amplitude in order to have sufficient signal to noise ratio.
2) Detection of the radiation
In [
Δ S = P h h P d d − 1 rms ( tan β ) (2)
The numerator refers to the ratio of two horizontal spectral components Phh (NS component of magnetic field) and Pdd (EW component). And the denominator is the root mean square (rms) of the deviation of the signal ellipticity. The expression of β is given then by the following equation.
β = 1 2 arcsin { Im ( P d h − P h d ) [ ( P h h − P d d ) 2 + 4 P h h P d d ] 1 / 2 } (3)
Here Im means imaginary part. Because Shekotov et al. (2007) [
We have calculated the field component power spectral densities, Phh, Pdd and their cross-power spectral densities Phd, Pdh were calculated by using Fourier transforms with frequency resolution of about 0.1 Hz. Spectral components in a frequency range from 0.1 to 24 Hz were used, and they were averaged over one-Hz intervals such as 0.1 - 1, 1.1 - 2, ∙∙∙∙∙∙, 23.1 - 24 Hz, so that we have 24 spectral components.
so that the results only from SHI are plotted there. The azimuthal directions of ULF/ELF radiation on the active periods of 8 to 11 April are found to be approximately directed to the Kyushu area, being consistent with the speculation that those ULF/ELF emissions are likely to be generated close to the EQ epicenters.
1) VLF monitoring
Based on our network observation of subionospheric VLF/LF signals, Hayakawa and Asano (2016) [
Asano and Hayakawa (2017) [
2) ULF depression
The horizontal component of ULF waves observed at KNY has been investigated to investigate the ULF depression [
We here summarize the observational results on electromagnetic precursors to the 2016 April Kumamoto EQ event.
1) Lithospheric effect
As for the direct lithospheric ULF radiation, we cannot come to any definite conclusion on the presence of seismogenic ULF emissions simply on the basis of the statistical analysis.
2) Atmospheric effect
Next, how about the indirect effect of seismogenic atmospheric phenomena? The characteristic parameter ∆S is clearly increased on April 8 to 11 (in UT), which shows the presence of seismogenic ULF/ELF (f = 0.1 - 24 Hz) impulsive emissions. Then, the direction finding result for those ELF emissions indicates that the azimuths are approximately directed to Kyushu Island, but it is not clear that the azimuth is not always coincident with the EQ epicenter.
3) Ionospheric effect
The lower ionosphere is definitely perturbed with the use of our subionospheric VLF/LF network. It is found that the ionosphere is extremely perturbed on 10 - 12 April (in UT). In exact consistence with this result, the ULF depression (as a signature of lower ionospheric perturbation) at KNY indicated very clear effects on 8 and 10 April (in UT).
The conventional statistical analyses used in this paper to explore electromag- netic precursors to the 2016 April Kumamoto EQ event, have indicated clear precursory signatures both in the atmosphere and ionosphere (as ULF/ELF atmospheric radiation and ionospheric perturbation, respectively), but lithospheric ULF radiation is not so evident. The importance of these two phenomena has already been confirmed for the recent 2011 Tohoku EQ, so that the occurrence of these two precursory phenomena among the many, 1) atmospheric ULF/ELF radiation and 2) ionospheric perturbation are likely to be universal for either inland (fault-type) or sea (subduction) EQ, which would be of the first priority for short-term EQ prediction [
It seems interesting to make a comparison between the present results for the Kumamoto EQ event and those for the 1995 Kobe EQ [
Finally it seems to the authors that inland, fault-type EQs are rich in electromagnetic precursors in contrast to sea (subduction) EQs. There are only few publications paying attention to this difference in electromagnetic characteristics between inland and sea EQs. Parrot (2011) [
Schekotov, A., Izutsu, J., Asano, T., Potirakis, S.M. and Hayakawa, M. (2017) Electromagnetic Precursors to the 2016 Kumamoto Earthquakes. Open Journal of Earthquake Research, 6, 168-179. https://doi.org/10.4236/ojer.2017.64010