Further study on the anomalies in apparent resistivity in the NE configuration at Chengdu station associated with Wenchuan M_S8.0 earthquake

Chengdu geoelectric station is located merely 35 km from the epicenter of the 12 May 2008 Wenchuan M_S8.0 earthquake. This paper deals with the remarkable descending tendency of apparent resistivity with strong distortion of seasonal variations at the station associated with the Wenchuan earthquake on a new point of view. Firstly， the long-term observation data since 1978 were applied to the investigation in order to determine the stage features in the whole course of the apparent resistivity observation; secondly， more attention was paid to the close relationship between apparent resistivity and water-table changes to recognize the possible influence of the water-table variations on the determination of tendency anomalies in apparent resistivity at Chengdu station; and thirdly， the previous results of in-situ experiments at the station were utilized to give a nearly quantitative analysis of the tendency anomaly in the third stage of the whole courses of observation. The analysis has identified two segments of anomalous behaviors in apparent resistivity which could be associated with the Wenchuan M_S8.0 earthquake after checking the influence of water-table variations. The first segment showed a slight ascending of 1.9% from Feb. 2004 to Feb. 2006; the second one had a very huge descending of -6.8% from Feb. 2006 to Feb. 2008. The deduced variations of -22.3% in true resistivity in the deep media， which had never been seen in the previous time of the whole course of observation at the station， could be taken as one of the constraints for researches on the preparation processes of the earthquake. The results have shown important complements to the case studies of apparent resistivity anomalies in China， especially to those with strong distortion of seasonal variations in the anomaly development without synchronous observation of water-table variations.