Variation of magnetotelluric apparent resistivity during the major aftershock period of Lushan earthquake in epicentral area

The present paper analyzes the relationship between the apparent resistivity change and major aftershock activities based on magnetotelluric measurements data from May 3 to 24， 2013． A group of self-power and cross-power spectra are obtained with the robust method and coherence limit based on characteristics of magnetotelluric method. Then the spectra causing apparent resistivity and phase curve unsmooth are removed according to their shape， and the high-quality data are obtained after removing disturbed data． The results show that the apparent resistivity tended to increase gradually with maximum change by 27% during the measurement period but not in a monotonous manner． The yx polarized apparent resistivity ρ_ayx reached its maximum after nearly every main aftershock cluster, but among each clusters the minimum resistivity appeared. Finally the analyses on earthquake generating mechanisms indicate that the earthquake resistivity effect is due to the change in pore fluid in micro fractures， therefore it is speculated that the increase trend of the resistivity during the period of Lushan aftershocks is caused by stress recovery and porosity recovery after the Lushan earthquake.