Abstract

Recordings of a search-coil magnetometer located in Yongsheng in the Yunnan province of China have been used to find atmospheric ULF/ELF precursors to the 2011 Tohoku earthquake (EQ) with M=9 which occurred on 11 March 2011 with a large epicentral distance of about 4100 km. The combined characteristics of the horizontal magnetic fields, the ratio of tangential and radial field components in the numerator and root mean square of ellipticity in the denominator, were used to detect seismo-related ULF/ELF signals. This value is found to increase before the EQ, and then to decrease during a few weeks, with a reliable maximum on 8 March (3 days before the EQ). We also analyzed azimuth distributions of pulsed radiation in the frequency range 2-5 Hz during the previous week before the main shock. This radiation reached a maximal value on 10 March - one day before the EQ date, and the azimuth of its source is estimated to coincide with the position of the main shock. The results are further compared with the ULF/ELF precursors of the same EQ observed at 3 stations in Japan by Ohta et al. (2013).

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