Electrical resistivity variations before and after the pingtung earthquake in the wushanting mud volcano area in Southwestern Taiwan

The extensive eruption of fluid from the mud volcanoes in the Yanchao area of southwestern Taiwan reveals the activities of the active Chishan fault. A series of time-lapse resistivity imaging measurements were initially conducted in the Wushanting Natural Landscape Preservation Area in Yanchao to evaluate the relationship between resistivity change and fault activity. Resistivity measurements were conducted first along seven 30- to 60-m survey lines to build up the regional model for mud volcanoes. We then conducted consecutive hourly and daily measurements to evaluate the short-term resistivity variations. Monthly observation was initiated along two 60-m lines in July of 2006. On December 26, 2006, two successive earthquakes with magnitudes of 6.96 and 6.99 hit the town of Hengchun in Pingtung County, about 120 km southwest of the monitoring site. Before the Pingtung earthquake, the resistivity at the research site was less than 25 ohm-m. Two regions with relatively low resistivity were found at a depth greater than 4-m at the positions of the two mud volcano craters. The two low-resistivity regions indicate the locations of conduits for the mud fluid. The major changes of resistivity are located in the vadose zone between the surface and a depth of 3-m. After the Pingtung earthquake, the maximum resistivity increased in the vadose zone by 7 ohm-m and 20 ohm-m on survey lines D and E, respectively. In the zone, the estimated water content of D and E decreased by 7% and 10%, respectively, after the earthquake. We suggest that the decrease of resistivity in the vadose zone most likely reflects the decrease of water content, itself caused by the earthquake tremors' increased emission of gas. Currently, we are continuing the resistivity-monitoring surveys and hope to provide more data to clarify the seasonalvariation patterns and to compare them with the previous findings.