We show that near–real-time seismic monitoring of fluid injection allowed control of induced earthquakes during the stimulation of a 6.1-km-deep geothermal well near Helsinki, Finland. A total of 18,160 m3of fresh water was pumped into crystalline rocks over 49 days in June to July 2018. Seismic monitoring was performed with a 24-station borehole seismometer network. Using near–real-time information on induced-earthquake rates, locations, magnitudes, and evolution of seismic and hydraulic energy, pumping was either stopped or varied—in the latter case, between well-head pressures of 60 and 90 MPa and flow rates of 400 and 800 liters/min. This procedure avoided the nucleation of a project-stopping magnitudeMW2.0 induced earthquake, a limit set by local authorities. Our results suggest a possible physics-based approach to controlling stimulation-induced seismicity in geothermal projects.

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The long‐term temporal and spatial changes in statistical, source, and stress characteristics of one cluster of induced seismicity recorded at The Geysers geothermal field (U.S.) are analyzed in relation to the field operations, fluid migration, and constraints on the maximum likely magnitude. Two injection wells, Prati‐9 and Prati‐29, located in the northwestern part of the field and their associated seismicity composed of 1776 events recorded throughout a 7 year period were analyzed. The seismicity catalog was relocated, and the source characteristics including focal mechanisms and static source parameters were refined using first‐motion polarity, spectral fitting, and mesh spectral ratio analysis techniques. The source characteristics together with statistical parameters (b value) and cluster dynamics were used to investigate and understand the details of fluid migration scheme in the vicinity of injection wells. The observed temporal, spatial, and source characteristics were clearly attributed to fluid injection and fluid migration toward greater depths, involving increasing pore pressure in the reservoir. The seasonal changes of injection rates were found to directly impact the shape and spatial extent of the seismic cloud. A tendency of larger seismic events to occur closer to injection wells and a correlation between the spatial extent of the seismic cloud and source sizes of the largest events was observed suggesting geometrical constraints on the maximum likely magnitude and its correlation to the average injection rate and volume of fluids present in the reservoir.

The spatiotemporal, kinematic, and source characteristics of induced seismicity occurring at different fluid injection rates are investigated to determine the predominant physical mechanisms responsible for induced seismicity at the northwestern part of The Geysers geothermal field, California. We analyze a relocated hypocenter catalog from a seismicity cluster where significant variations of the stress tensor orientation were previously observed to correlate with injection rates. We find that these stress tensor orientation changes may be related to increased pore pressure and the corresponding changes in poroelastic stresses at reservoir depth. Seismic events during peak injections tend to occur at greater distances from the injection well, preferentially trending parallel to the maximum horizontal stress direction. In contrast, at lower injection rates the seismicity tends to align in a different direction which suggests the presence of a local fault. During peak injection intervals, the relative contribution of strike-slip faulting mechanisms increases. Furthermore, increases in fluid injection rates also coincide with a decrease in b values. Our observations suggest that regardless of the injection stage, most of the induced seismicity results from thermal fracturing of the reservoir rock. However, during peak injection intervals, the increase in pore pressure may likewise be responsible for the induced seismicity. By estimating the thermal and hydraulic diffusivities of the reservoir, we confirm that the characteristic diffusion length for pore pressure is much greater than the corresponding length scale for temperature and also more consistent with the spatial extent of seismicity observed during different injection rates.

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Estimating the expected size of the largest earthquake on a given fault is complicated by dynamic rupture interactions in addition to geometric and stress heterogeneity. However, a statistical assessment of the potential of seismic events to grow to larger sizes may be possible based on variations in magnitude distributions. Such variations can be described by the b-value, which quantifies the proportion of small-to large-magnitude events. The values of b vary significantly if stress changes are large, but additional factors such as geometric heterogeneity may affect the growth potential of seismic ruptures. Here, we examine the influence of fault roughness on b-values, focal mechanisms, and spatial localization of laboratory acoustic emission (AE) events during stick-slip experiments. We create three types of roughness on Westerly granite surfaces and study AE event statistics during triaxial loading of the lab faults. Because both roughness and stress variations are expected to influence b, we isolate roughness contributions by analyzing AEs at elevated stresses close to stickslip failure. Our results suggest three characteristics of seismicity on increasingly rough faults: (1) seismicity becomes spatially more distributed, (2) b-values increase, and (3) focal mechanisms become more heterogeneous, likely caused by underlying stress field heterogeneity within the fault zones. Localized deformation on smooth faults, on the other hand, promotes larger rupture sizes within the associated homogeneous stress field, which is aligned with the macroscopic stress orientation. The statistics of earthquake magnitude distributions may help quantify these fault states and expected rupture sizes in nature.

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A slip tendency analysis is used to assess the reactivation potential of shear and dilational fractures in a deep geothermal reservoir in the Northeast German Basin, based on the notion that slip on faults is controlled by the ratio of shear to normal stress acting on the plane of weakness in the in situ stress field. The reservoir rocks, composed of Lower Permian sandstones and volcanics, were stimulated by hydraulic fracturing. A surprisingly low microseismic activity was recorded with moment magnitudes M W ranging from -1.0 to -1.8.The slip tendency analysis suggests a critically stressed reservoir exists in the sandstones, whereas the volcanic rocks are less stressed. Rock failure first occurs with an additional pore pressure of 20 MPa. Presumed failure planes form a conjugate set and strike NW and NE. Slip failure is more likely than tensional failure in the volcanic rocks because high normal stresses prevent tensional failure. These results from slip tendency analysis are supported by the spatial distribution of recorded microseismicity. Source characteristics indicate slip rather than extension along presumed NE striking failure planes. This suggests that slip tendency analysis is an appropriate method that can be used to understand reservoir behavior under modified stress conditions.