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[MIS04-P10] Pre-earthquake phenomena in MF radio band propagation around sunset time
Keywords:earthquake prediction, Middle frequency band propagation, Molchan’s error diagram
Some papers which have reported earthquakes and VLF (Very Low Frequency) band propagations show existence of relationship between the both and possibility of earthquake predictions. Anomaly at the bottom of ionosphere seems to be cause of anomalous propagation on VLF band.
We have been monitored broadcasting waves on MF (Middle Frequency) band over decade at Kiryu, Gunma prefecture in Japan. Propagations on MF band depend on activity of ionosphere day time and night time. In normally MF band waves become stronger propagation around sunset time, because D-layer in the ionosphere disappears with time gone from daytime to nighttime. However, we have sometimes experienced anomalous propagations, which have abnormal fluctuations in received waves around sunset time. Moreover, some days later of the anomalous propagation, earthquakes have sometimes observed. Therefore, we have investigated the relationship between occurrence of earthquakes and anomalous propagations in MF band around sunset time.
Four broadcasting waves in MF band are targeted for this investigation, NHK Radio 2 774kHz from Akita, STV Radio 1440kHz from Sapporo, NHK Radio 1 666kHz from Osaka and RCC Chugoku Radio 1350kHz from Hiroshima. They all are transmitting in Japan and are propagating to observation point at Kiryu in Japan. Propagation distance range of four waves is from 395 km to 767 km. In normal propagation the received signal strength shows monotonous increase around sunset time. But in the case of propagation with anomaly the received signal strength has fluctuation and consequently has peaks before the sunset time. We have focused on the time interval between the peak time of received signal strength and sunset time. When the time interval the two exceeds an artificially determined threshold time, it is defined as “occurrence of anomalous propagation.” The artificially determined threshold is varied for statistical evaluation. Then, we have investigated the relationship between earthquakes and the anomalous propagations with various threshold.
In order to evaluate the relationship between the both, statistical estimation was adopted. Statistical items for evaluation are Hit rate, Prediction rate, Probability gain and Molchan’s error diagram for earthquake prediction. In Molchan’s error diagram the prediction rate is evaluated with varying the determined threshold time to be regard as anomaly.
As the result of the investigation, the maximum prediction rate was 0.429 in NHK Ration 2 from Akita and probability gain 3.18 in NHK Radio 1 from Osaka, respectively. Earthquakes whose epicenters were within the area with 400km from the ionospheric reflection points had frequently anomalies in MF band propagation around sunset time. On the other hand, Earthquakes whose epicenters were ocean area had less relation between the both. Moreover, in case of two hops propagation the lead time, which is time interval from occurrence of anomalous propagation and occurrence of earthquake, was longer than in case of one hop propagation. And, the results by Molchan’s error diagrams for all four broadcasting waves show the relationship between earthquakes and anomalous propagations in MF band, in which the prediction rate exceeds beyond the 95% confidence intervals (p-value is 5%).
This report indicates only statistical relationship between earthquakes and anomalous propagations in MF band broadcasting waves, no physical evidence. Therefore, elucidation of the physical phenomenon at bottom of ionosphere and MF band propagation is needed. That is the future work.
We have been monitored broadcasting waves on MF (Middle Frequency) band over decade at Kiryu, Gunma prefecture in Japan. Propagations on MF band depend on activity of ionosphere day time and night time. In normally MF band waves become stronger propagation around sunset time, because D-layer in the ionosphere disappears with time gone from daytime to nighttime. However, we have sometimes experienced anomalous propagations, which have abnormal fluctuations in received waves around sunset time. Moreover, some days later of the anomalous propagation, earthquakes have sometimes observed. Therefore, we have investigated the relationship between occurrence of earthquakes and anomalous propagations in MF band around sunset time.
Four broadcasting waves in MF band are targeted for this investigation, NHK Radio 2 774kHz from Akita, STV Radio 1440kHz from Sapporo, NHK Radio 1 666kHz from Osaka and RCC Chugoku Radio 1350kHz from Hiroshima. They all are transmitting in Japan and are propagating to observation point at Kiryu in Japan. Propagation distance range of four waves is from 395 km to 767 km. In normal propagation the received signal strength shows monotonous increase around sunset time. But in the case of propagation with anomaly the received signal strength has fluctuation and consequently has peaks before the sunset time. We have focused on the time interval between the peak time of received signal strength and sunset time. When the time interval the two exceeds an artificially determined threshold time, it is defined as “occurrence of anomalous propagation.” The artificially determined threshold is varied for statistical evaluation. Then, we have investigated the relationship between earthquakes and the anomalous propagations with various threshold.
In order to evaluate the relationship between the both, statistical estimation was adopted. Statistical items for evaluation are Hit rate, Prediction rate, Probability gain and Molchan’s error diagram for earthquake prediction. In Molchan’s error diagram the prediction rate is evaluated with varying the determined threshold time to be regard as anomaly.
As the result of the investigation, the maximum prediction rate was 0.429 in NHK Ration 2 from Akita and probability gain 3.18 in NHK Radio 1 from Osaka, respectively. Earthquakes whose epicenters were within the area with 400km from the ionospheric reflection points had frequently anomalies in MF band propagation around sunset time. On the other hand, Earthquakes whose epicenters were ocean area had less relation between the both. Moreover, in case of two hops propagation the lead time, which is time interval from occurrence of anomalous propagation and occurrence of earthquake, was longer than in case of one hop propagation. And, the results by Molchan’s error diagrams for all four broadcasting waves show the relationship between earthquakes and anomalous propagations in MF band, in which the prediction rate exceeds beyond the 95% confidence intervals (p-value is 5%).
This report indicates only statistical relationship between earthquakes and anomalous propagations in MF band broadcasting waves, no physical evidence. Therefore, elucidation of the physical phenomenon at bottom of ionosphere and MF band propagation is needed. That is the future work.