TITLE:
On the Tempo-Spatial Evolution of the Lower Ionospheric Perturbation for the 2016 Kumamoto Earthquakes from Comparisons of VLF Propagation Data Observed at Multiple Stations with Wave-Hop Theoretical Computations
AUTHORS:
Tomokazu Asano, Masashi Hayakawa
KEYWORDS:
VLF/LF Propagation Anomaly, Ionospheric Perturbations, Earthquake Precursor, Earthquake Prediction, the 2016 Kumamoto Earthquakes, Japanese VLF/LF Network, Wave-Hop Computation, Nighttime Fluctuation Method, Atmospheric Oscillation Hypothesis
JOURNAL NAME:
Open Journal of Earthquake Research,
Vol.7 No.3,
July
23,
2018
ABSTRACT:
There have been published many papers on VLF (very low frequency) characteristics
to study seismo-ionospheric perturbations. Usually VLF records (amplitude
and/or phase) are used to investigate mainly the temporal evolution of
VLF propagation anomalies with special attention to one particular propagation
path. The most important advantage of this paper is the simultaneous use
of several propagation paths. A succession of earthquakes (EQs) happened in
the Kumamoto area in Kyusyu Island; two strong foreshocks with magnitude
of 6.5 and 6.4 on 14 April (UT) and the main shock with magnitude 7.3 on 15
April (UT). Because the EQ epicenters are not far from the VLF transmitter
(with the call sign of JJI in Miyazaki prefecture), we can utilize simultaneously
8 observing stations of our network all over Japan. Together with the use of
theoretical computations based on wave-hop theory, we try to trace both the
temporal and spatial evolutions of the ionospheric perturbation associated
with this succession of EQs. It is found that the ionospheric perturbation begins
to appear about two weeks before the EQs, and this perturbation becomes
most developed 5 - 3 days before the main shock. When the perturbation is
most disturbed, the maximum change in vertical direction is depletion in the
VLF effective ionospheric height of the order of 10 km, and its horizontal
scale (or its radius) is about 1000 km. These spatio-temporal changes of the
seismo-ionospheric perturbation will be investigated in details in the discus-sion, a comparison has made with the VLF characteristics of the 1995 Kobe
with the same magnitude and of the same fault-type, and a brief discussion on
the generation mechanism of seismo-ionospheric perturbation is finally made.