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A new time series interferometric synthetic aperture radar method is presented based on a strain model. Traditionally, the interferometric phases are optimized based only on a constant set of statistically homogeneous points throughout the observing period, and thus the decorrelated phase cannot be well recovered and possible incoherent (or coherent) phases are (are not) used to optimize the target phase. In the proposed method, a strain model, representing the geophysical relationship of spatially adjacent points' deformations, is employed to optimize the interferometric phases interferogram‐by‐interferogram. The unwrapped phase time series can be estimated from the wrapped interferograms based on the linear iterative weighted least squares method, which is simple and can effectively suppress the decorrelation noise. Both simulations and analysis of data over The Geysers geothermal field (TG), California are conducted, validating that the proposed method can reduce the decorrelations and simultaneously preserve the deformation signals. The displacement results of TG from 2007–2011 L‐band ascending ALOS‐1 data and 2015–2020 C‐band ascending/descending Sentinel‐1 data reveal that (a) the displacement rate of TG is decreasing from 2007–2011 to 2015–2020, and (b) temporally variable displacement signals in addition to steady displacement can be observed from 2015 to 2020 for both the east‐west and the vertical components. Plain Language Summary: The interferometric synthetic aperture radar (InSAR) technique is capable of accurately measuring ground surface displacements at high spatial resolution and large coverage. This paper proposes a strain‐model based InSAR time series method, here referred to as the SIGMA method. The SIGMA method aims to enhance the capability of InSAR to measure displacements and can achieve higher accuracy and density of InSAR displacement measurements. Both simulations and real‐data experiments are conducted to illuminate the benefits of the SIGMA method in the presence of substantial decorrelation noise. Particularly, the SIGMA method is used to investigate the surface displacements of The Geysers geothermal field, California, during 2007–2011 and 2015–2020. These time periods are complementary to those considered in previous studies, providing an important data set to investigate the long‐term evolution of ground surface deformation, as well as the effects of recent fluid injection and production practices in the geothermal field. The new method should be of value for improved geo‐hazards monitoring. Key Points: A new time series interferometric synthetic aperture radar (InSAR) method based on a strain model and iterative weighted least squares is proposedThe method can effectively suppress decorrelation noise and improves the InSAR accuracy and densityThe surface deformation of The Geysers geothermal field, California is investigated with 2007–2011 ALOS‐1 and 2015–2020 Sentinel‐1 data [ABSTRACT FROM AUTHOR]

The generation of strong earthquakes is a long‐debated problem in seismology, and its importance is increased by the possible implications for earthquake forecasting. It is hypothesized that the earthquake generation processes are anticipated by several phenomena occurring within a nucleation region. These phenomena, also defined as preparatory processes, load stress on the fault leading it to reach a critical state. In this paper, we investigate the seismicity preceding 19 moderate (Mw ≥ 3.5) earthquakes at The Geysers, Northern California, aiming to verify the existence of a preparatory phase before their occurrence. We apply an unsupervised K‐means clustering technique to analyze time series of physics‐related features extracted from catalog information and estimated for events occurred before the mainshocks. Specifically, we study the temporal evolution of the b‐value from the Gutenberg‐Richter (b), the magnitude of completeness (Mc), the fractal dimension (Dc), the inter‐event time (dt), and the moment rate (M˙0 ${\dot{M}}_{0}$). Our analysis shows that out of 19 moderate magnitude events considered, a common preparatory phase for 11 events is clearly identified, plus other five events for which we can guess a preparatory phase but with different characteristics from the previous ones. The latter result confirms that even within the same tectonic context different possible activation behaviors may exist. The duration of the preparatory process ranges between about 16 hr and 4 days. We observe that also for the retrieved preparatory process a decrease in b, Mc, and Dc, and an increase of M˙0 ${\dot{M}}_{0}$. Finally, we show a clear correlation between events with a preparation phase and the location of injection's wells, suggesting an important role of fluids in the preparatory process. Plain Language Summary: We investigate the preparatory phase of moderate magnitude‐induced earthquakes at The Geysers geothermal field in California by studying the spatiotemporal evolution and dynamic properties of small magnitude events and using an unsupervised machine learning approach. To this aim, we rely on features extracted from seismic catalog information that are used for a K‐means clustering analysis. Our results highlight changes in the seismicity characteristics before the moderate‐induced earthquakes. We find that most of the analyzed target earthquakes present a preparatory phase, and that most of the cases the latter presents common characteristics in their key features. We show that the seismicity clusters in space and time before moderate events also becoming more energetic. We estimate a duration for the detected preparatory phases that ranges from 16 hr to 4 days. Finally, we show that the presence of a preparatory phase is correlated to the proximity of injection's wells, suggesting a significant role of fluids in the earthquake's nucleation process. Key Points: Moderate‐induced earthquakes at The Geysers can be preceded by a preparatory phase detectable using K‐means clustering on catalog featuresThe detected preparatory phase has common characteristics presenting negative trends of b‐value, Mc, and Dc and a positive trend of M˙0 ${\dot{M}}_{0}$The earthquakes with a preparatory phase are located nearer to the injection's wells in the area than the events without it [ABSTRACT FROM AUTHOR]

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]