Unexpected changes in resistivity monitoring for earthquakes of the Longmen Shan in Sichuan, China, with a fixed Schlumberger sounding array

An electrical resistivity monitoring experiment using a fixed Schlumberger sounding array was conducted at Pixian in Sichuan Province, China, from January 1984 to June 1989, for the purposes of better understanding some geoelectrical phenomena and detecting earthquake-associated changes. Ancillary data of well water fluctuations and rainfall changes, along with seismicity pattern, fault map, atmospheric humidity, and temperature, have been collected. A very peculiar resistivity behavior was observed: the direction of the change in apparent resistivity for large electrode expanders was opposite to that of the resistivity change for the overburden. The geoelectrical structure beneath the monitoring array could be inferred from resistivity sounding data and available geological information. A method of data analysis applying singular value decomposition to the first-order sensitivity matrix yields the estimated change in true resistivity for the constituent medium in the sense of generalized least-squares. The sensitivity analysis shows that for the Pixian station, the abnormal annual variations in resistivity monitoring with large electrode spacings are related to the multilayered resistivity section with conductive substratum and are caused by the resistivity changes of overlying layer because the sensitivity coefficient for the topmost layer for such a section is negative. The resistivity changes of the overlying layer could be attributed mainly to seasonal rainfall changes. During the experiment, no significant resistivity changes are correlated with the earthquakes of ML 5.4 or less in the Longmen Shan and adjacent regions. However, a detailed interpretation of the observed phenomenon is helpful to improve our understanding of possible resistivity precursors to earthquakes.