Electrical potential anomalies are often measured associated with geothermal areas and volcanoes. In these systems, fluid flow is usually mostly restricted to faults and fracture networks. An equation describing electrical potential anomalies of electrokinetic nature associated with fluid upflow induced by a thermal source along faults is derived. The electrical potential anomaly is related to the depth of the thermal reservoir, the temperature difference between the surface and the reservoir of the geothermal area, the thermal expansion of water, and the streaming electrical potential coupling coefficient in the fault zone. A quantitative calculation of the electrokinetic anomaly is provided by comparing this model to a self-potential survey of the Cerro-Prieto geothermal field [Fitterman and Corwin, 1982>. The predictions of the model agree well with the field measurements. ¿ 1998 American Geophysical Union