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关键词:  墨江5.9级地震  深井电磁异常  同震响应
Electromagnetic abnormity recorded by borehole TOA installment during the Mojiang M 5.9 earthquake on September 8, 2018
Mei Li
China Earthquake Networks Center
As an achievement of the cooperation with Japan, TOA electromagnetic observing station was established with an 800 m borehole antenna and put into service in 1992 in Dali, Yunnan province, China. Li et al. (2003) had made a summary of main anomalous variation characters by analyzing 23 strong earthquakes with magnitudes more than 5.0 recorded during the first ten year’s observation after its service. This work mainly presents the electromagnetic changes prior to the last Mojiang M 5.9 earthquake on September 8, 2018. First of all, initial weak signals appeared in two ULF channels out of three observing channels (CH1 0.01–0.1Hz, CH2 0.1–1 Hz and CH3 1–9 kHz) on 30 May 2018 at Dali TOA electromagnetic station. The information recorded is characterized by pulse–like impulses with a magnitude of ACH1 ≤ 0.26 mV in CH1 and wave–like changes of ACH2 ≤ 0.6 mV in CH2 respectively. Second, abnormal information gradually enhances either in magnitudes or in occurring frequency. Pulse–like signals were full of lattices of recording paper for CH2 during June 24–25 and slopped over the recording paper during June 28–29, with magnitudes equaling to or more than 1 mV. At the same time, obvious wave–like signals also happened in CH1 with a maximum magnitude of ~6 mV on June 28 and it reached its climax. From then on, The information started to decrease from the end July and only weak signals occasionally occurred till the end August, when obvious anomaly was recorded again in two ULF channels, with maximum magnitudes of ACH1 ~2 mV and ACH2 ~0.3 mV respectively. These signals summarily appeared not continuously but group by group and accumulated intensively only in ULF band instead of VLF band during the total period. Ten days later, the Mojiang M 5.9 earthquake happened on September 8, 2018, 300 km away from Dali TOA station, and a coseismic response had also been recorded in this time. So these ULF electromagnetic abnormities probably are attributed to this Mojiang event.
Key words:  Mojiang M 5.9 earthquake  Borehole electromagnetic abnormity  Coseismic response