Episodes of increased seismic moment release around the Cerro Prieto Geothermal Field (CPGF) show a significant correlation with three large increases of sustained fluid extraction, with delays of about 1 yr. Increased seismic activity involves three large strike-slip earthquakes: Imperial Valley (ML = 6.6, 15 October 1979) and Victoria (ML = 6.1, 9 June 1980), which occurred after a production increase in 1979, and Cerro Prieto (ML = 5.4, 7 February 1987), which occurred after another production increase in 1986.
The probabilities of the observed correlation between production increase and increased seismic activity occurring by chance (for binomial or Poisson seismicity distributions) are rather small, although their significance is not very large due to the small number of data. High probability of triggering by fluid extraction at times of high probability of earthquake occurrence from tectonic loading suggests a possible connection between production and occurrence of ML ≧ 5.4 earthquakes near CPGF.
The strong earthquakes occurred at distances from CPGF within ranges over which induced seismicity has been observed for other engineering activities worldwide. Seismic diffusivity, calculated from their hypocentral distances and delay times from production increases, are in good agreement with values estimated worldwide for reservoirs, and estimated pressure changes at the earthquake sites, induced by production in CPGF, can be large enough for triggering.
The observed time correlation, plus supporting statistical and spatio-temporal observations, although not conclusive, suggest the possibility of large tectonic earthquake triggering by extraction activity at the Cerro Prieto Geothermal Field. This possibility should be considered for estimation of seismic hazard and earthquake prediction in the Mexicali-Imperial Valley region.