Tetrafluoromethane (CF₄) concentrations were measured in 14 groundwater samples from the Cuyama Valley, Mil Potrero and Cuddy Valley aquifers along the Big Bend section of the San Andreas Fault System (SAFS) in California to assess whether tectonic activity in this region is a significant source of crustal CF₄ to the atmosphere. Dissolved CF₄ concentrations in all groundwater samples but one were elevated with respect to estimated recharge concentrations including entrainment of excess air during recharge (C_re; ∼30 fmol kg^−1 H₂O), indicating subsurface addition of CF₄ to these groundwaters. Groundwaters in the Cuyama Valley contain small CF₄ excesses (0.1–9 times C_re), which may be attributed to an in situ release from weathering and a minor addition of deep crustal CF₄ introduced to the shallow groundwater through nearby faults. CF₄ excesses in groundwaters within 200 m of the SAFS are larger (10–980 times C_re) and indicate the presence of a deep crustal flux of CF₄ that is likely associated with the physical alteration of silicate minerals in the shear zone of the SAFS. Extrapolating CF₄ flux rates observed in this study to the full extent of the SAFS (1300 km × 20–100 km) suggests that the SAFS potentially emits (0.3–1)×10^-1kg CF₄ yr^−1 to the Earth's surface. For comparison, the chemical weathering of ∼7.5×10⁴km² of granitic rock in California is estimated to release (0.019–3.2)×10^-1kg CF₄ yr^−1. Tectonic activity is likely an important, and potentially the dominant, driver of natural emissions of CF₄ to the atmosphere. Variations in preindustrial atmospheric CF₄ as observed in paleo-archives such as ice cores may therefore represent changes in both continental weathering and tectonic activity, including changes driven by variations in continental ice cover during glacial–interglacial transitions.