The estimation of the stress‐field orientation from focal mechanisms of earthquakes is a relevant tool to understand crustal mechanics and the physics of earthquakes. In global seismology, Formal Stress Inversion (FSI) is a well‐established technique to study tectonic processes associated with the occurrence of large earthquakes (e.g., Hardebeck and Michael, 2004; Yoshida et al., 2012). Information on the stress‐field orientation is also relevant for the exploitation of hydrocarbon and geothermal reservoirs. Knowledge of the orientation of the maximum horizontal stress (σ_Hmax) is crucial for reservoir development, such as drilling and leakage risk assessment (...

We develop an improved methodology for reliable high‐resolution inversions of focal mechanisms to background stress field orientation and stress ratio R in two or three dimensions. The earthquake catalog is declustered to remove events likely affected strongly by local stress interactions rather than reflecting the large‐scale background stress field. The declustered data are discretized with the k‐means algorithm into groups containing a number of focal mechanisms between a minimum number N_min and 2N_min. Synthetic tests indicate that N_min ≈ 30 provides stable inversion results under different stress regimes and noise conditions when R ≈ 0.5, while N_min ≈ 45 is needed for R near 0 or 1. Additional synthetic tests compare the performance of selecting the fault plane of each focal mechanism using (a) the plane with lowest misfit angle between the slip vector from the focal mechanism and shear traction from the stress tensor and (b) the plane with highest instability coefficient representing proximity to the optimally oriented fault for given stress field and friction coefficient. The instability criterion is found to provide more accurate inversion results under all tested stress regimes, stress ratios, and noise conditions. The refined inversion methodology combines selecting fault planes using the instability criterion iteratively with a damped simultaneous inversion of different focal mechanism groups. Results characterizing neighborhoods of discretized domains merged during the damped inversion provide high‐resolution information independent of the discretization. Some aspects of the methodology are illustrated with focal mechanism data from the San Jacinto Fault Zone in Southern California.

We investigate source processes of fluid‐induced seismicity from The Geysers geothermal reservoir in California to determine their relation with hydraulic operations and improve the corresponding seismic hazard estimates. Analysis of 869 well‐constrained full moment tensors (M_w 0.8–3.5) reveals significant non‐double‐couple components (>25%) for about 65% of the events. Volumetric deformation is governed by cumulative injection rates with larger non‐double‐couple components observed near the wells and during high injection periods. Source mechanisms are magnitude dependent and vary significantly between faulting regimes. Normal faulting events (M_w < 2) reveal substantial volumetric components indicating dilatancy in contrast to strike‐slip events that have a dominant double‐couple source. Volumetric components indicating closure of cracks in the source region are mostly found for reverse faulting events with M_w > 2.5. Our results imply that source processes and magnitudes of fluid‐induced seismic events are strongly affected by the hydraulic operations, the reservoir stress state, and the faulting regime.

Analysis of past and present stimulation projects reveals that the temporal evolution and growth of maximum observed moment magnitudes may be linked directly to the injected fluid volume and hydraulic energy. Overall evolution of seismic moment seems independent of the tectonic stress regime and is most likely governed by reservoir specific parameters, such as the preexisting structural inventory. Data suggest that magnitudes can grow either in a stable way, indicating the constant propagation of self‐arrested ruptures, or unbound, for which the maximum magnitude is only limited by the size of tectonic faults and fault connectivity. Transition between the two states may occur at any time during injection or not at all. Monitoring and traffic light systems used during stimulations need to account for the possibility of unstable rupture propagation from the very beginning of injection by observing the entire seismicity evolution in near‐real time and at high resolution for an immediate reaction in injection strategy.

We investigate the source characteristics of picoseismicity (M_w < −2) recorded during a hydraulic fracturing in situ experiment performed in the underground Äspö Hard Rock Laboratory, Sweden. The experiment consisted of six stimulations driven by three different water injection schemes and was performed inside a 28‐m‐long, horizontal borehole located at 410‐m depth. The fracturing processes were monitored with a variety of seismic networks including broadband seismometers, geophones, high‐frequency accelerometers, and acoustic emission sensors thereby covering a wide frequency band between 0.01 and 100,000 Hz. Here we study the high‐frequency signals with dominant frequencies exceeding 1000 Hz. The combined seismic network allowed for detection and detailed analysis of 196 small‐scale seismic events with moment magnitudes M_W < −3.5 (source sizes of decimeter scale) that occurred solely during the stimulations and shortly after. The double‐difference relocated hypocenter catalog as well as source parameters were used to study the physical characteristics of the induced seismicity and then compared to the stimulation parameters. We observe a spatiotemporal migration of the picoseismic events away and toward the injection intervals in direct correlation with changes in the hydraulic energy (product of fluid injection pressure and injection rate). We find that the total radiated seismic energy is extremely low with respect to the product of injected fluid volume and pressure (hydraulic energy). The radiated seismic energy correlates well with the hydraulic energy rate. The obtained fault plane solutions for particularly well‐characterized events signify the reactivation of preexisting rock defects under influence of increased pore fluid pressure on fault plane orientations in good correspondence with the local stress field orientation.

Earthquake precursory processes have been central to scientific inquiry for nearly a century. Recent advancements in earthquake monitoring, geodesy, and data analysis including artificial intelligence, have substantially improved our understanding of how earthquake sequences unfold leading to the mainshock. We examine the available seismological and geodetic evidence describing preparatory processes in 33 earthquake sequences with M_W [3.2–9.0] across different tectonic and stress conditions. Our analysis reveals common patterns, and sheds light on the interplay of structural, tectonic and other boundary conditions that influence the dynamics of earthquake sequences, and hence, in the seismo-geodetic observables prior to the mainshock. We place particular emphasis on connecting observed phenomena to the underlying physical processes driving the sequences. From our findings, we propose a conceptual framework viewing earthquake preparation as a process involving several juxtaposed driving physical mechanisms on different temporal and spatial scales, jointly leading to the stress increase in the future epicenter.