Anomaly Analysis of Earth Resistivity Observations before the Wenchuan Earthquake

Based on the data of earth resistivity observations in Sichuan province and surrounding areas,anomalous phenomena of earth resistivity were analyzed.The results showed that,prior to the Wenchuan 8.0 Earthquake,the apparent resistivity amplitude at one observing direction decreased about 7.2% at Pixian station nearest to the epicenter,with duration time more than 2 years,and turned to rise 5 months before the quake,while the earthquake took place during the recovering stage.In addition,there were a few stations along the North-South Seismic Belt exhibiting step-wise variations 2 months prior to the quake,with maximal amplitude change exceeding 10%.The directional feature of earth resistivity anomalies and spatial distribution of anomalous stations reflected that the anomalies in earth resistivity observation was closely related to the strike of the tectonic belt and the earthquake mechanism.