Laboratory experiments highlight a systematic b value decrease during the stress increase period before failure, and some large natural events are known to show a precursory decrease in the b value. However, short‐term forecast models currently consider only the generic probability that an event can trigger subsequent seismicity in the near field. While the probability increase over a stationary Poissonian background is substantial, selected case studies have shown through cost‐benefit analysis that the absolute main shock probability remains too low to warrant significant mitigation actions. We analyze the probabilities considering both changes in the seismicity rates and temporal changes in the b value. The precursory b value decrease in the 2009 L'Aquila case results in an additional fiftyfold probability increase for a M6.3 event. Translated into time‐varying hazard and risk, these changes surpass the cost‐benefit threshold for short‐term evacuation.

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Earthquake catalogs describe the distribution of earthquakes in space, time, and magnitude, which is essential information for earthquake forecasting and the assessment of seismic hazard and risk. Available high-resolution (HR) catalogs raise the expectation that their abundance of small earthquakes will help better characterize the fundamental scaling laws of statistical seismology. Here, we investigate whether the ubiquitous exponential-like scaling relation for magnitudes (Gutenberg–Richter [GR], or its tapered version) can be straightforwardly extrapolated to the magnitude–frequency distribution (MFD) of HR catalogs. For several HR catalogs such as of the 2019 Ridgecrest sequence, the 2009 L’Aquila sequence, the 1992 Landers sequence, and entire southern California, we determine if the MFD agrees with an exponential-like distribution using a statistical goodness-of-fit test. We find that HR catalogs usually do not preserve the exponential-like MFD toward low magnitudes and depart from it. Surprisingly, HR catalogs that are based on advanced detection methods depart from an exponential-like MFD at a similar magnitude level as network-based HR catalogs. These departures are mostly due to an improper mixing of different magnitude types, spatiotemporal inhomogeneous completeness, or biased data recording or processing. Remarkably, common-practice methods to find the completeness magnitude do not recognize these departures and lead to severe bias in the b-value estimation. We conclude that extrapolating the exponential-like GR relation to lower magnitudes cannot be taken for granted, and that HR catalogs pose subtle new challenges and lurking pitfalls that may hamper their proper use. The simplest solution to preserve the exponential-like distribution toward low magnitudes may be to estimate a moment magnitude for each earthquake.

Seismic monitoring of the Basel Enhanced Geothermal System has been running for more than a decade. Yet the details of the long‐term behavior of its induced seismicity remained unexplored because a seismic event catalog with consistent detection sensitivity and magnitudes did not exist. This knowledge is essential for developing guidelines and mitigation procedures on how to safely operate and terminate injection activities. Only few observational data exist that cover all phases of such projects in a consistent manner. Here we describe a method that overcomes these deficiencies based on sensitive matched filter detection and a machine learning approach to remove false detections. With an emphasis on consistency, we create a catalog that contains more than 280,000 events down to Mw−1.5. The much higher temporal resolution allows us to analyze induced microearthquakes in great detail and to gain new insights. We resolved temporal variations of seismicity parameters and, in the post‐operational phase, a preferential temporal clustering of events. We find a breakdown in the Gutenberg‐Richter scaling during reservoir stimulation, which may have physical reasons or could be caused by a method‐independent detection limit during high event rates. The scaling breakdown has implications for the design of Adaptive Traffic Light Systems and may limit the potential of real‐time mitigation strategies in future Enhanced Geothermal System projects. Nevertheless, our catalog gives the opportunity to study the temporal evolution of the sequence in unprecedented detail, which will help to better understand the physical processes in a geothermal reservoir, potentially not only in the Basel case.

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