Geothermal investigations in an area of induced seismic activity, northern São Paulo State, Brazil

Abstract

Geothermal investigations were carried out in Nuporanga, state of São Paulo (Brazil), where occurrence of seismic activity has been found to be closely related to opening of groundwater wells. Results of macroseismic studies show that seismic activity had its beginning in May 1977, soon after completion of drilling of the COLABA well and most of the initial seismic events are located close to it. Geothermal investigations were initiated in September 1977 on the assumption that fluid movements associated with seismic activities are capable of producing short-term time-dependent changes in the local thermal regime and in the hope that identification of such time-dependent changes would contribute to a better understanding of the nature of local seismicity. Results of thermal logs in the COLABA well reveal the existence of an unusual thermal regime with a constant temperature zone (CTZ) down to 175 m followed by a zone in which temperature rises rapidly (TGZ), in the interval of 180-204 m. Repeated thermal logs carried out over a period of three months reveal temperature drops of up to 0.8°C taking place in the TGZ immediately after periods of intense microseismic activity. Temperature measurements of pumped water also show changes of lesser magnitude, occurring in the CTZ, closely related to the frequency of seismic events. Substantial temperature changes related to periods of seismic activity were also observed in two nearby wells in Nuporanga. The available geothermal and macroseismic data have been used in the development of a simple model of the process that have triggered seismicity in Nuporanga. According to this model the COLABA well acts as a natural siphon drawing water from a perched aquifer at depths of less than 40 m and injecting it to a fault zone at about 175 m. The model allows a coherent explanation for the observed correlation between seismicity and absence of pumping. During periods in which pumping is suspended the static level of water in the well is high and the pressure exerted by the water column induces a reduction in frictional resistance at the fault zone. During pumping the water level falls to its dynamic level and consequently the pressure at the fault zone is also lower. Such pressure changes are apparently sufficient to trigger microseismic activity in Nuporanga. The observed temperature drops immediately after tremors could be attributed to the cooling effect associated with the penetration of relatively cold water from upper levels into newly opened fractures. Also the small but significant rise in temperatures during seismically quiescent periods can be considered as a result of warming up of stationary fluid bodies within fracture zones. Measures taken on the basis of this model has been successful in switching off seismicity on at least two occasions. In view of this success we conclude that geothermal investigations can be of considerable use in understanding the nature of fluid movements associated with near-surface earthquakes.