Abstract: We analyzed repeated magnetotelluric (MT) observation data during the 10 years from 2004 to 2013 in 5 points in Lanzhou and nearby areas using the normalized variation rate method (for short, NVRM). The result shows, (1) significant apparent resistivity anomalies synchronizingl appeared at continuous frequency for all observation points from the penetration depth of a few kilometers to more than 20 kilometers before the Minxian—Zhangxian M_S6.6 earthquake. Among 3 measuring points occurred abnormal changes in NS direction within almost the same depth, and in the NS direction of each measuring point, abnormal frequency range is large and good continuity compared to EW direction. (2) Continuous frequency anomalous changes in the NS and EW directions show synchronous rise in the normalized variation rate curves, due to changes of in situ geo-electrical resistivity of the measurement point, reflecting the increase of geo-electrical resistivity in the range of corresponding exploration depth. Continuous frequency point decline in the NS direction and upward changes in the EW direction in the normalized variation rate vurves reflect anisotropic geo-electrical resistivity change of the measuring point in situ underground medium or the change of transversal heterogeneity nearby. It is found that these results further reveal the earthquake monitoring capabilities of the repeated MT observation.

Key words: Magnetotelluric observation, Apparent resistivity change, The 2013 Minxian—Zhangxian earthquake