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.
Highlights
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
Taking into account the possible influence of other factors which can produce perturbations in VLF/LF signals and using control paths, we may conclude that observed anomalies were very likely to be signatures of lower ionospheric perturbations caused by impending EQs
There has been an enormous progress in the field of seismo-electromagnetics (e.g., [1] [2]), and it is a general consensus that electromagnetic effects in the lithosphere, and in the atmosphere and ionosphere do appear prior to an EQ
Summary
There has been an enormous progress in the field of seismo-electromagnetics (e.g., [1] [2]), and it is a general consensus that electromagnetic effects in the lithosphere, and in the atmosphere and ionosphere do appear prior to an EQ. Statistical studies on VLF propagation anomalies have been done [3] [9] [10] [18] [19] [20], which have indicated a close correlation between VLF propagation anomalies and EQs. Though recognizing the importance of statistical studies, case studies still remain valuable, because we can obtain the detailed information on the ionospheric perturbation and its association with other phenomena for different EQs. Huge EQ events have been treated, including the 1995 Kobe EQ [8], the 2007 Niigata-chuetsu EQ [21] [22], the 2014 Sumatra EQ [23] [24], the 2011 Tohoku EQ [25] [26]. A combination of those data seen from far away from Russia (long-range monitoring) and observed at several stations in Japan (shortrange monitoring) must be of great importance in finding the structure of the ionospheric perturbation
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