Application of polarization method to geomagnetic data from the station Chengdu

Based on the 1-second sampling data derived from FHDZ-M15 digital recording geometry of the station Chengdu, we applied polarization method to analyzing the possible ultra-low frequency electromagnetic phenomena associated with M_L≥5.0 earthquakes surrounding the station. It was shown that the long-term curve of the polarization value of the station Chengdu exhibits annual variations. By applying the Fourier fitting, the periodic annual variations were eliminated, and the analysis results on residuals between polarization values and fitting ones indicate that the increasing of the polarization value has a close relationship with M_L≥5.0 earthquakes which would happen in two months, and the appearance duration of the high value was one day to five days. Meanwhile, there was no correlation between the ultra-low frequency electromagnetic phenomena and the space current activity; the abnormal amplitude of polarization value before the thrust earthquake is higher than that of the strike-slip type.