Abstract
By using network observation of subionospheric VLF (very low frequency)/LF (low frequency) signals in Japan and in Russia, we have found a significant ionospheric perturbation prior to the recent 2011 March 11 Japan earthquake (EQ) which occurred at sea proximate to the Tohoku area on the main island (Honshu) of Japan was an exceptionally huge plate-type EQ. A remarkable anomaly (with a decrease in the nighttime amplitude and also with enhancement in dispersion) was detected on March 5 and 6 along the propagation path from the NLK (Seattle, USA) transmitter to Chofu (together with Kochi and Kasugai). We also have observed the corresponding VLF anomaly during a prolonged period of March 1 - 6, with minima in the nighttime amplitude on March 3 and 4 along the path from JJI (Miyazaki, Kyushu) to Kamchatka, Russia. This ionospheric perturbation has been discussed extensively with respect to its reliability. (1) How abnormal is this VLF/LF propagation anomaly? (2) What was the temporal evolution of terminator times? (3) Were there any solar-terrestrial effects (especially the effect from geomagnetic storms) on the VLF/LF propagation anomaly? (4) The effect of any other EQs and foreshock activities on the VLF/LF anomaly? (5) Were there any correlations with other related phenomena? Finally, (6) are there any other examples of a VLF/LF propagation anomaly for oceanic EQs? We then compared the temporal properties of ionospheric perturbations for this EQ with those of a huge number of inland EQs and compared the corresponding spatial scale with the former result of the same oceanic 2004 Sumatra EQ with nearly the same magnitude. Finally, the generation mechanism of those seismo-ionospheric perturbations is briefly discussed.
Highlights
It has been recently shown that there exist electromagnetic precursors to earthquakes (EQs)
(1) How abnormal is this VLF/LF propagation anomaly? (2) What was the temporal evolution of terminator times? (3) Were there any solar-terrestrial effects on the VLF/LF propagation anomaly? (4) The effect of any other EQs and foreshock activities on the VLF/LF anomaly? (5) Were there any correlations with other related phenomena? (6) are there any other examples of a VLF/LF propagation anomaly for oceanic EQs? We compared the temporal properties of ionospheric perturbations for this EQ with those of a huge number of inland EQs and compared the corresponding spatial scale with the former result of the same oceanic 2004 Sumatra EQ with nearly the same magnitude
These papers were based on an abundant number of land EQs over seven years which lend a further support to our previous similar statistical studies based upon a fewer number of events and over smaller time periods (Rozhnoi et al 2004; Maekawa et al 2006; Kasahara et al 2008; Chakrabarti 2010)
Summary
It has been recently shown that there exist electromagnetic precursors to earthquakes (EQs) The lower ionospheric perturbation as detected by subionospheric VLF/LF propagation is shown to be statistically significantly correlated with EQs which take place within the wave sensitive area of the great-circle path and with magnitude tentatively greater than 6.0 and shallower depth (< 40 km) (Hayakawa et al 2010a, b). These papers were based on an abundant number of land EQs over seven years which lend a further support to our previous similar statistical studies based upon a fewer number of events and over smaller time periods (Rozhnoi et al 2004; Maekawa et al 2006; Kasahara et al 2008; Chakrabarti 2010). The 2011 Japan EQ was extremely huge with a magnitude of 9.0 and is characterized by an oceanic EQ taking place in the Pacific Ocean due to plate movement leading to a premise for us to show whether the ionospheric perturbation is generated prior to this oceanic EQ and, if so, to compare the characteristics of seismo-ionospheric perturbation for this sea EQ with the former properties of many land EQs
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