We investigate the source mechanisms of 80 earthquakes at the Salton Sea geothermal field in southern California with local magnitudes ranging from 2.2 to 3.8, combining data from a local network of shallow short‐period borehole geophones and regional broadband seismometers. The performance of two moment tensor inversion techniques based on P‐wave first‐motion amplitudes and full waveforms is compared. The inversion using full waveforms is unable to reproduce the observed seismograms to a detail sufficient for successful moment tensor estimation, most likely due to small‐scale heterogeneities in the crust beneath the geothermal field. Only a combination of P‐wave amplitude inversion and moment tensor refinement techniques enables a robust determination of the source types. We find that most shallow earthquakes display statistically significant non‐double‐couple components, indicating that the ruptures involve processes other than simple shear. The largest volumetric components are inferred for events located near active injection and production wells and primarily correspond to a normal‐faulting regime. In contrast, strike‐slip events are dominated by shear motion, and their locations extend well beyond the reservoir. Our observations suggest that both short‐term effective stress reduction and long‐term reservoir depletion influence the source characteristics of the occurring seismicity. Furthermore, we highlight the necessity for a careful evaluation of the optimum moment tensor inversion technique as a prerequisite for reservoir characterization based on source properties of seismicity related to fluid injection.