TITLE:
Characteristic Variations of VLF/LF Signals during a High Seismic Activity in Japan in November 2016
AUTHORS:
Tomokazu Asano, Alexander Rozhnoi, Maria Solovieva, Masashi Hayakawa
KEYWORDS:
VLF/LF Propagation Anomalies, ?onospheric Perturbations, Earthquake Precursor, Short- and Long-Range Monitoring
JOURNAL NAME:
Open Journal of Earthquake Research,
Vol.6 No.4,
November
3,
2017
ABSTRACT:
The measurements of very low frequency/low frequency (VLF/LF) signals at
the Petropavlovsk-Kamchatsky (PTK) and Yuzhno-Sakhalinsk (YSH) stations
in Russia and several observing stations in Japan were used for the analysis of
lower ionospheric perturbations in possible association with two earthquakes
(EQs) which occurred offshore the Pacific Ocean of Japan in November 2016.
The first EQ with M (magnitude) = 6.1 (depth 42 km) happened close to the
coast line on 11 November (UT). The second EQ was recorded in the sea on
21 November (UT) with M = 6.9 (depth 11 km) and they had a series of aftershocks
with M up to 5.6. As for the long-range monitoring, the significant
negative nighttime amplitude decreases as propagation anomalies were found
for two subionospheric paths: NWC (Australia)-PTK and JJY (Fukushima,
Japan)-YSH during about a week, mainly before the first EQ. The anomalies
of signal in the path JJY-PTK were observed 4 - 5 days before the second EQ
and 3 days after it. Extensive analyses have been performed as well for these
two EQs by using the short-range monitoring of VLF data observed at all of
the seven VLF/LF stations in Japan in relation to the JJY signal. As related
with the 1st EQ, there were observed anomalies on the two paths of JJY-STU
(Suttsu) and JJY-NSB (Nakashibetsu) (both stations in Hokkaido) on 2 and 3
November. While, for the 2nd EQ clear anomalies have been observed on 14
and 15, and on 21 November at Ito station in Izu peninsula, Kamakura, Togane
and Katsuura in Chiba. Taking into account the possible influence of
other factors which can produce perturbations in VLF/LF signals and also using
control paths, we may conclude that observed anomalies were very likely
to be signatures of lower ionospheric perturbations caused by impending EQs.
Finally, we try to estimate the possible perturbation scale for both EQs.