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Water injection and Enhanced Geothermal System (EGS) technologies have been used to exploit heat resources from geothermal reservoirs. Detecting spatial and temporal changes in reservoir physical properties is important for monitoring reservoir condition changes due to water injection and EGS. Here, we determine high‐resolution models of the temporal changes in the three‐dimensional P wave velocity and attenuation (Vp and Qp) structures between the years 2005 and 2011 in the northwestern part of The Geysers geothermal field, California, using double‐difference seismic velocity and attenuation tomography. The northwest Geysers has a shallow normal temperature reservoir (NTR) underlain by a high temperature reservoir (HTR) that has substantial underutilized heat resources but may be more fully utilized in the future through EGS. In the southeastern part of the northwest Geysers, however, EGS has been successfully but unintentionally applied for at least 50 years because the waters injected into the NTR have been flowing into the HTR. Our models are well resolved in this area and show that the NTR and HTR have different seismic responses (seismicity, Vp, and Qp) to water injection, which can be explained by the injection‐induced differences in fracturing and saturation that are likely related to their geological properties. Our results indicate that the joint analysis of changes in seismicity, velocity, and attenuation is valuable for characterizing changes in reservoir fracturing and saturation conditions. Our results suggest that high‐permeability zones and/or pre‐existing permeable fault zones are important for the success of EGS at The Geysers and potentially other geothermal systems. Plain Language Summary: To deal with the declining steam pressure and fluid content, water injection is a commonly employed technology to promote the production of geothermal resources from the naturally permeable geothermal reservoirs. To exploit heat energy from the hot, dry but impermeable rocks, water injection technology can be used to create permeability and increase fluid saturation. Such a man‐made reservoir is called Enhanced Geothermal System (EGS). Monitoring changes in material properties of reservoirs due to water injection and EGS is important for evaluating and adjusting the production and injection strategies. Here, we use seismic tomography to image temporal changes in the three‐dimensional subsurface seismic velocity and attenuation structures of the northwestern part of The Geysers geothermal field, California. The northwest Geysers has a normal temperature reservoir (NTR) at ∼1–2‐km depths underlain by a high temperature reservoir (HTR). The southeastern part of the northwest Geysers has been exploited for >50 years with unintentionally created but successful EGS. Our joint analysis of changes in seismicity and seismic velocity and attenuation clearly reveal different injection‐induced changes in fracturing and saturation conditions in the NTR and HTR in this area that are likely related to their geological properties. Key Points: High‐resolution imaging of temporal changes in Vp and Qp reveals detailed injection‐induced changes in fracturing and saturation conditionsDecreased Vp and Qp and diffuse seismicity suggest diffuse fluid flow, increased fracturing, and increased but still partial saturationDecreased Vp, increased Qp, and concentrated seismicity suggest confined fluid flow, increased fracturing, and full saturation [ABSTRACT FROM AUTHOR]