Abstract: We use a relatively simple model of an underground current source co-located with the earthquake hypocenter to estimate the magnitude of the equivalent dipole required to produce observable ground signatures. The Wenchuan M_S8.0 earthquake on May 12, 2008, is used as an example of a large typical inland event. Results show that for an observed 1.3 mV/m pulse at 1 Hz at Hebei Gaobeidian observing station, 1440 km away from the epicenter, during the abnormity climax, and a typical conductivity of 7.0×10^-4 S/m, the expected equivalent dipole order is 10¹³ A·m, and that of the seismotelluric current is 10⁸ A if the main fracture length 150 km is considered. The electromagnetic field 2D distribution on the ground of this dipole is of the characteristics of orientation or selectivity. There is near relationship between the orientation of the electric element and the electromagnetic field amplitude. At the same time, the electromagnetic fields attenuate rapidly as the observing distance increases. All these can give part evidence of the fact that electromagnetic abnormities observed related with earthquakes are of orientation or selectivity. The instalment may not observe a signal if it locates in weak field regions even though it is well within the detectable range. This result put some requests on how to select locations and layout when an earthquake observing network is in preparation.

Key words: Underground dipole model, The 2008 Wenchuan M_S8.0 earthquake, Equivalent dipole, 2D distribution of electromagnetic field