Development of Broadband Interferometer System for Pre-Earthquake Electromagnetic Radiation in LF Band :Design and Performance of Antenna Elements

Various preseismic electromagnetic variations have been reported so far. Oike et al. reported an increase in the number of electromagnetic pulses in the LF band for the 1995 Kobe Earthquake. However, there is a problem that the electromagnetic pulse due to lightning activity, which is a strong electromagnetic radiation source in the LF band, cannot be sufficiently distinguished from the electromagnetic pulse associated with earthquakes. At that time, it was difficult to observe waveforms with the observation equipment (especially digital measurement units), but with the development of today's ICT equipment and Internet technology, it is possible to realize an LF band broadband interferometer that can estimate the spatio-temporal sources of electromagnetic radiation. If it is an electromagnetic pulse due to lightning activity, the electromagnetic radiation source will move with the front or cloud, and if it is associated with an earthquake, the electromagnetic radiation source will be concentrated near the focal region. In this presentation, we will report the progress of the development of the LF band broadband interferometer, and the waveform analysis and pulse number variation of the nearby earthquake that occurred during the test of the interferometer element.

The developed system is a capacitive circular flat plate fast antenna, consisting of a 500 kHz low-pass filter, a 16bit AD converter, and a PC for data recording, and records 100 ms before and after the pulse waveform that exceeds the trigger level with 4 MHz sampling. The system is installed on the roof of the Faculty of Science Building No.5, Chiba University, and is conducting test observations.

First, we counted the total number of pulses recorded by the system, created an amplitude histogram, and targeted the top 15% of the pulses to investigate hourly fluctuations in the number of pulses. We calculated the average value m and standard deviation σ for the entire analysis period, and defined the anomaly in the number of pulses as m + 2σ. Next, using pulse waveforms and the mine location network blitzortung.org, waveforms (near and distant mines) caused by mine discharges were identified. In addition, we analyzed the earthquakes that occurred within 100 km of the epicenter distance and satisfied log(Es)>8 during the observation period, and investigated the relationship with the earthquakes. where Es=10^1.5M+4.8/r² (M: magnitude, r: focal distance). As a result, 4 days before the M5.0 earthquake on November 27, 2018, an abnormal increase in the number of pulses greater than m+2σ was observed, unrelated to the anti-mine. Although similar pulse waveforms did not exceed the m+2σ threshold, they were also observed prior to four other log(Es) > 8 earthquakes during the observation period, and these pulses were associated with preseismic electromagnetic waves. Possible pulse due to radiation. On the other hand, it is also possible that the pulse waveform is caused by cloud discharge, and in order to discriminate between electromagnetic radiation caused by cloud discharge and earthquake precursor electromagnetic radiation, electromagnetic radiation position determination using an interferometer and comparison with satellite images and meteorological data are required. also found to be essential.