On the Anomalies in ULF Magnetic Field Variations Prior to the 2008 Sichuan Earthquake

Qi Li¹, Alexander Schekotov², Tomokazu Asano³, Masashi Hayakawa^3,4,5

¹Institute of Geophysics, China Earthquake Administration, Beijing, China.
²Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia.
³Earthquake Analysis Laboratory, Information Systems Inc., Minato-ku, Japan.
⁴Hayakawa Institute of Seismo Electromagnetics, Co. Ltd., University of Electro-Communications (UEC), Chofu, Japan.
⁵UEC, Advanced Wireless Communications Research Center, Chofu, Japan.

DOI: 10.4236/ojer.2015.42005                

Abstract

There have been reported several papers on the ionospheric F region perturbations prior to the 2008 Sichuan earthquake (EQ) (magnitude 8.0), but it seems that very few reports have been published on the characteristics of ground-based ULF (ultra low frequency) magnetic field variations for this EQ. This paper deals with two different aspects of ground-based ULF magnetic field variations: 1) ULF radiation from the lithosphere, and 2) depression of ULF horizontal magnetic field as a signature of lower ionospheric perturbations. ULF data from two Chinese stations [Chengdu (epicentral distance, 80 km) and Xichang (about 300 km away from the EQ epicenter)] are analyzed, with paying attention to the local nighttime period (LT = 22 h to 02 h, UTC = 14 h to 18 h) in order to avoid man-made noise. We have analyzed powers of the horizontal component (H²), vertical component (Z²), polarization as their ratio (Z²/H²), depression of the horizontal component (as an inverse of horizontal magnetic field component power, 1/H²) and δDep as a variation of depression at a particular frequency of 0.01 - 0.02 Hz (10 - 20 mHz). It is then found that there seems no clear signature of lithospheric ULF radiation. Whilst, the most evident fact is the finding of depression of ULF horizontal magnetic field at Chengdu a few days before the Sichuan EQ, which suggests that the lower ionosphere was perturbed before the EQ. The characteristics of the lower ionospheric perturbations are compared with those of upper ionospheric perturbations reported before.

Keywords

Earthquake, Earthquake Prediction, ULF Magnetic Variation, Lithospheric ULF Radiation, ULF Depression

Conflicts of Interest

The authors declare no conflicts of interest.

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