In this work we show that induced earthquakes in the Rangely oil field, Colorado, were triggered by small anthropogenic pore pressure changes but largely driven by natural tectonic stresses acting on the fault prior to reservoir operations. We also find that extensive depletion of the Rangely oil field moved parts of the fault in the field away from failure, in agreement with the Mohr-Coulomb theory. However, most earthquakes recorded in this area occurred in regions hydraulic disconnected from the reservoir. Thus our results supports the hypothesis that small anthropogenic pore pressure perturbations close to natural faults can advance the occurrence of earthquakes if faults are critically stable.