Abstract
Preseismic VLF electromagnetic pulses occasionally increase a few days before large earthquakes, especially inland earthquakes. More than two decades ago, the Tokai University group developed a digital recording system for collecting the preseismic electromagnetic pulse data and showed remarkable results. However, due to the limitations of personal computers’ data storage and CPU power during that time, they discontinued the observation. We relaunched this research using current technology. This paper shows the development of the new observation system and presents preliminary results. In addition, we introduce an electromagnetic-wave arrival discrimination algorithm that combines the autoregressive model and the Akaike information criterion, which are commonly used for automatic waveform reading in seismology, to obtain accurate data on the time of arrival (TOA) of electromagnetic waves. Then, source positioning was performed using TOA of electromagnetic waves. Seven electromagnetic pulses near the epicenter were observed 2 days before the largest inland earthquake (M = 5.6) that occurred near the observation network during the observation period (2016–2020). These VLF pulses may be a seismic precursory phenomenon because they were not electromagnetic pulses originating from lightning. These results encourage future observations.
Highlights
Many Japanese seismologists, having experienced the Tohoku Earthquake in 2011 and the Kobe Earthquake in 1995, are extremely pessimistic about realizing the earthquake prediction
We found that the electromagnetic wave of elliptical polarization was observed much more than the electromagnetic wave of linear polarization
The electromagnetic field was possibly distorted because other antennas and outdoor units of air conditioners were installed on the roof
Summary
Many Japanese seismologists, having experienced the Tohoku Earthquake in 2011 and the Kobe Earthquake in 1995, are extremely pessimistic about realizing the earthquake prediction. The observation was conducted as a part of the national earthquake prediction research project by using the most advanced digital technology in the 21st century. The research should be relaunched using coil sensors with the same frequency characteristics, given that the electromagnetic waves preceding an earthquake is unknown. We used a 12-bit, six-channel, 100 MHz sampling A/D converter This A/D converter was originally developed for the measurement of gamma rays, X-rays, muons, etc. Asada and his group analyzed only the linearly polarized magnetic components of electromagnetic waves to conduct direction findings via the Goniometer method. Note that the trigger time was recorded when the signal intensity exceeded a certain threshold
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