5:15 PM - 6:30 PM
[MIS08-P04] Development of Broadband Antenna System for Observation of Pre-Earthquake Electromagnetic Phenomena in LF range and Its Initial Observations
Keywords:Earthquake, Lightning Discharge
Increases in the electromagnetic pulse in the LF band have been reported as one of the electromagnetic phenomena preceding large earthquakes. Before the 1995 Hyogo-ken Nanbu Earthquake (M7.2), Oike et al. reported an increase in the number of pulses, and this phenomenon is considered to be the precursor for the earthquake. On the other hand, it has been suggested that some of these pulse increases are strongly affected by lightning. The issue is to distinguish between electromagnetic pulses caused by lightning and electromagnetic pulses that are precursors to earthquakes. Therefore, we developed the observation system for LF band electromagnetic waves, which can record detailed pulse waveforms, and investigated the relationship between the number of pulses and earthquakes, and analyzed their waveforms to extract the characteristics of the earthquake precursor pulse.
The observation system consists of a capacitive fast antenna with a circular flat plate, a 500 kHz low-pass filter, a 16-bit AD converter, and a PC for data recording. 100 ms data before and after the pulse waveform exceeding the trigger level (V_pp = 6000 [du]) is registered at 4 MHz sampling. The system was installed on the top of the roof of Science Building No. 5, Chiba University. The analyzed period is from May 1, 2018, to May 31, 2019.
First, the total number of pulses recorded in the system was counted and the amplitude histogram was created. We decided to investigate the temporal variation of the top 15% of pulses (V_pp ≧ 28880 [d.u.]). The mean value m and standard deviation σ for the entire analyzed period was calculated, and the anomaly in the number of pulses was defined as m + 2σ. Next, we identify well-known pulses related to the lightning with waveforms and products of lightning discharge (cloud to ground discharge only) location estimated by blitzortung.org. Then, we investigate the relationship between the temporal change of the number of the pulse and earthquakes. We selected seven earthquakes that occurred during the observation period within an epicenter distance of 100 km and satisfy log (Es )>8, where Es=101.5M+4.8/r2, M and r are the magnitude and hypocenter distance, respectively. As a result, an abnormal increase in the number of pulses of m + 2σ threshold and without lightning determined by blitzortung was observed four days before the M5.0 earthquake on November 27, 2018. Similar pulse waveforms did not exceed the m + 2σ threshold but were also observed before the other 4 earthquakes with log (Es)>8 that occurred during the observation period. These pulses may be due to seismic precursor electromagnetic radiation. On the other hand, it is possible that the pulse waveform is due to the discharge in the clouds. It is found that the identification of the location of discharge should be completed by an interferometer system to distinguish between the cloud-cloud discharges and earthquake-related discharge.
The observation system consists of a capacitive fast antenna with a circular flat plate, a 500 kHz low-pass filter, a 16-bit AD converter, and a PC for data recording. 100 ms data before and after the pulse waveform exceeding the trigger level (V_pp = 6000 [du]) is registered at 4 MHz sampling. The system was installed on the top of the roof of Science Building No. 5, Chiba University. The analyzed period is from May 1, 2018, to May 31, 2019.
First, the total number of pulses recorded in the system was counted and the amplitude histogram was created. We decided to investigate the temporal variation of the top 15% of pulses (V_pp ≧ 28880 [d.u.]). The mean value m and standard deviation σ for the entire analyzed period was calculated, and the anomaly in the number of pulses was defined as m + 2σ. Next, we identify well-known pulses related to the lightning with waveforms and products of lightning discharge (cloud to ground discharge only) location estimated by blitzortung.org. Then, we investigate the relationship between the temporal change of the number of the pulse and earthquakes. We selected seven earthquakes that occurred during the observation period within an epicenter distance of 100 km and satisfy log (Es )>8, where Es=101.5M+4.8/r2, M and r are the magnitude and hypocenter distance, respectively. As a result, an abnormal increase in the number of pulses of m + 2σ threshold and without lightning determined by blitzortung was observed four days before the M5.0 earthquake on November 27, 2018. Similar pulse waveforms did not exceed the m + 2σ threshold but were also observed before the other 4 earthquakes with log (Es)>8 that occurred during the observation period. These pulses may be due to seismic precursor electromagnetic radiation. On the other hand, it is possible that the pulse waveform is due to the discharge in the clouds. It is found that the identification of the location of discharge should be completed by an interferometer system to distinguish between the cloud-cloud discharges and earthquake-related discharge.