Electromagnetic Abnormity Recorded by Borehole TOA Installment during the Mojiang MS 5. 9 Earthquake on September 8,20181

As an achievement of the cooperation with Japan, TOA electromagnetic observation station was established with an 800m borehole antenna and put into service in 1992 in Dali, Yunnan province,China. Li Wuxian et al. (2003) summarized main anomalous variation characters by analyzing 23 strong earthquakes with magnitudes more than 5. 0 recorded in the first ten years. This work mainly presents the electromagnetic changes prior to the last Mojiang MS5. 9 earthquake on September 8, 2018. First of all, the initial weak signals appeared in two ULF channels out of three observing channels ( CH10. 01-0. 10Hz, CH20. 1-1. 0Hz and CH31-9kHz) on May 30, 2018 at Dali TOA electromagnetic station. The information recorded was characterized by wave-like changes with magnitudes of ACH1≤0. 26mV in CH1 and pulse-like impulses of ACH2≤0. 6mV in CH2, respectively. Then, abnormal information gradually enhanced either in magnitudes or in occurrence 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 the magnitudes being greater than or equal to 10mV. At the same time, the clear wave-like signals also appeared in CH1 with a maximum magnitude of ～0. 6mV on June 28 and reached its climax. From then on, the information started to decrease from the end of July and only weak signals occasionally occurred till the end of August 2018, when obvious anomaly was recorded again in two ULF channels with maximum magnitudes of ACH1～0. 2mV and ACH2～0. 3mV respectively. Generally, these signals did not appear continuously but group by group and accumulated intensively only in ULF band instead of VLF band during the total period. 10 days later, the Mojiang MS5. 9 earthquake occurred on September 8, 2018, 300km away from Dali TOA station, and a coseismic response was also recorded at this time. Thus, these ULF electromagnetic abnormities could be probably attributed to the Mojiang event.