Analysis on precursory abnormal characteristics of geoelectric field at Changli station

We analyzed the geoelectric field data recorded at Changli station from 2002 to 2010, and identified seven sets of abnormal signals after eliminating a lot of noises based on the multi-dipole observation system. The abnormal signals correspond to six M_L>4.0 earthquakes that occurred within 400 km from Changli station, and appeared generally within 32 days before the earthquakes, indicating that the abnormal signals are most likely seismic electric signals (SESs). The SESs have the following characteristics: ① each set of the SESs has the same polarity for both long and short dipoles; ② a good uniformity of the signals is found in each direction, e.g., 76% of the ratios of signal amplitudes (ΔV) of long dipoles to those of short dipoles in the same direction are within the range of 1—2. Five of the seven sets of SESs have their largest signal amplitude in NE direction, and two sets have their largest signal amplitude in NS direction. The signal amplitudes in EW direction are smaller than in the other two. The seven sets of SESs recorded at Changli station show that the signal amplitude ΔV is related to earthquake magnitude M_L and epicentral distance r as follows: M=lg(ΔV·r)+1.5.