Abstract: In this paper, five repeated magnetotelluric sounding data at the Laohushan Fault Zone, Gansu province from Oct of 2004 to Sep of 2006 have been analyzed. In this period, a set of M>3.0 earthquakes occurred in the fault zone, with the largest M_L4.6 about 10 km from the fault. The results indicate that the method of magnetotelluric sounding can observe the premonitory change of deep electric structure caused by seismic activity. The earthquake preparation process causes the resistivity of medium to drop, rise in the epicentral area and then earthquake occurs in the corresponding depth.

Key words: Magnetotelluric sounding, Laohushan fault zone, Response function, Earthquake monitoring