AN EXPLANATION OF SELF-POTENTIAL ANOMALIES DUE TO SUBSURFACE FLUID FLOW AT ACTIVE FAULTS

*Shin'ya Sakanaka[1]
Faculty of Engineering and Resource Science, Akita University[1]

Self-potential surveys were carried out at Owens Valley, California and at Ota fault in Northeast of Japan in 2000 and 2001. A self-potential anomaly up to +100mV was observed around active fault in Owens Valley. On the other hand, large amount of self-potential anomalies could not recognized in at Ota fault. We attempt to explain these anomalies by upwelling or down-flowing underground water through numerical simulation. The explanation is somewhat different from those introduced in previous studies.

The electrokinetic effect is one of the most important mechanisms that produce SP (Self-Potential) anomalies in crustal activities. The electrokinetic effect relates the motion of ions with fluid through porous media. Consequently the SP anomaly corresponds the upwelling or downflowing of water in geothermal areas or fracture zones in the fault structure. Large SP anomalies more than 100mV have been sometimes observed at volcanic and geothermal areas due to the electrokinetic effect. On the other hand, the SP anomalies at active faults are generally small around a few tens of millivolts. Fitterman (1979) and Murakami et al. (1984) produced theoretical distributions of SP around a fault system based on analytical methods. Here we attempt to use a numerical simulation method in order to explain observed SP anomalies due to subsurface fluid flow around fault areas. Recently we obtained the SP data at a fault in Owens Valley, California and at the Ota fault in Northeast of Japan. A positive anomaly up to 100mV was recognized on the basin side in Owens Valley. On the other hand, a positive anomaly about a few mV was observed on the mountainside at the Ota fault.