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Abstract
We carried out a simultaneous study of ground-based magnetic field and lower ionospheric anomalies during major earthquakes occurring around Japan in 2010 and 2012. Ultra Low Frequency (ULF) geomagnetic field waveforms of Esashi station and Very Low Frequency (VLF) Japanese transmitter (JJY) electric signal amplitude received in Moshiri station Hokkaido during nighttime (22:00-02:00 LT) were used to minimize the local interference. Twenty earthquakes having magnitude greater than 5.5 were considered for the data analysis for two years. Nighttime amplitude fluctuations and polarization from the received VLF transmitter signal amplitude and ULF magnetic field respectively were calculated to identify anomalous signatures in relation to every earthquake. We found most earthquakes analyzed indicating VLF amplitude anomalies simultaneously occurred with ULF magnetic field anomalies within a week prior to the earthquakes. Stronger anomalies were observed for larger magnitude and shallower earthquakes. Focal mechanism of earthquakes was also examined to identify the effectiveness of generating anomaly. Both VLF and ULF anomalies were observed for reverse fault type earthquakes occurring under the strong pressure in the crust. Obtained results may indicate the common anomaly source both for VLF and ULF in the lithosphere and are consistent with currently proposed Lithosphere-Atmosphere-Ionosphere (LAI) coupling scenarios during the earthquake preparation period.
Highlights
In this study we carry out the simultaneous study of anomalies observed by Ultra Low Frequency (ULF) magnetic field and Very Low Frequency (VLF) transmitter signals in relation to major earthquakes occurring in Japan in the years of 2010 and 2012
We have presented the simultaneously analyzed results of ULF magnetic field data and VLF subionospheric propagation data for the years of 2010 and 2012 in relation to major earthquakes around Japan
We found that most earthquakes occurring within the first Fresnel zone show a decrease in VLF amplitude within a week before
Summary
Among many seismo-electromagnetic parameters, anomalous signatures of ULF magnetic field and VLF transmitter amplitude during the seismic activities have been found much promising for earthquake prediction purpose. Studies of ULF magnetic field anomalies during the seismic activities have been reported by using different methods such as amplitude anomaly method, polarization parameter method, fractal analysis method etc. In this study we carry out the simultaneous study of anomalies observed by ULF magnetic field and VLF transmitter signals in relation to major earthquakes occurring in Japan in the years of 2010 and 2012. We try to study the physical mechanism of anomalies for analyzed earthquakes inferred from types of earthquakes determined by their focal mechanism
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