We studied the background seismic activity in the Cerro Prieto geothermal field (CPGF). Earthquakes were located below the exploitation area of the CPGF, between 3 and 12 km deep. This activity is located on a horstlike structure below the geothermal field and coincides with the zone of maximum subsidence in the CPGF. Two earthquake swarms occurred, one in November 2000 and the second in February 2001, along the southeast–northwest strike of the Cerro Prieto fault and in the neighborhood of the Cerro Prieto volcano. Magnitudes of the located microevents range from –0.3 to 3.5. A V p/ V s ratio of 1.91 for the region below the volcano suggests a water-saturated medium and/or a partial-melt medium. Seventy-six focal mechanisms of individual microearthquakes reveal significant variation in the orientation of the P and T axes. Focal mechanism inversions using the Gephart (1990) code also reveal significant lateral stress variation of the best-fit stresses. Below the CPGF, the orientation of the best-fit maximum, intermediate, and minimum stresses are (plunge, azimuth): σ 1 = (66°, 72°), σ 2 = (6°, 328°), and σ 3 = (24°, 235°). For the November 2000 earthquake swarm, the best-fit principal stress orientations are σ 1 = (12°, 117°), σ 2 = (11°, 25°), and σ 3 = (74°, 253°), whereas for the February 2001 earthquake swarm they are σ 1 = (36°, 235°), σ 2 = (24°, 127°), and σ 3 = (45°, 11°). The σ 1 orientations are considerably different from the orientation of the maximum compressive regional stress that lies approximately north–south. On 1 June and 10 September 1999, two earthquakes of M w 5.2 and 5.3 occurred in the basement at depths of 7.4 and 3.8 km below the CPGF. Waveform modeling resulted in fault-plane geometries given by strike = 236°, dip = 60°, slip = –58° (normal) and strike = 10°, dip = 90°, slip = 159° (right-lateral strike slip), respectively. An observed triangular source time function of 0.7 sec and a double source with a total duration of 1.9 sec were used to calculate the synthetics seismograms for these two events. Static stress drops and seismic moments for the June and September events are Δ σ = 82.5 MPa, M = 7.65 × 1016 N m and Δ σ = 31.3 MPa and M = 1.27 × 1017 N m. These stress drops are typical of continental events rather than of events originating in spreading centers. We concluded from the focal mechanism inversions of the background seismicity and the June and September 1999 events that complex stress heterogeneity exits in the CPGF as a result of the continual thinning of the crust in the Cerro Prieto basin.