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Induced earthquakes pose a substantial challenge to many geo-energy applications, and in particular to Enhanced Geothermal Systems. We demonstrate that the key factor controlling the seismic hazard is the relative size distribution of earthquakes, the b -value, because it is closely coupled to the stress conditions in the underground. By comparing high resolution observations from an Enhanced Geothermal System project in Basel with a loosely coupled hydro-mechanical-stochastic model, we establish a highly systematic behaviour of the b -value and resulting hazard through the injection cycle. This time evolution is controlled not only by the specific site conditions and the proximity of nearby faults but also by the injection strategy followed. Our results open up new approaches to assess and mitigate seismic hazard and risk through careful site selection and adequate injection strategy, coupled to real-time monitoring and modelling during reservoir stimulation.

Induced seismicity is one of the main factors that reduces societal acceptance of deep geothermal energy exploitation activities, and felt earthquakes are the main reason for closure of geothermal projects. Implementing innovative tools for real-time monitoring and forecasting of induced seismicity was one of the aims of the recently completed COSEISMIQ project. Within this project, a temporary seismic network was deployed in the Hengill geothermal region in Iceland, the location of the nation’s two largest geothermal power plants. In this paper, we release raw continuous seismic waveforms and seismicity catalogues collected and prepared during this project. This dataset is particularly valuable since a very dense network was deployed in a seismically active region where thousand of earthquakes occur every year. For this reason, the collected dataset can be used across a broad range of research topics in seismology ranging from the development and testing of new data analysis methods to induced seismicity and seismotectonics studies.Measurement(s)Seismic waveforms (seismograms) • Seismicity (Origin time, location and magnitude of earthquakes)Technology Type(s)Seismic stations (velocity sensors) • SeisComP data acquisition and processing system

The world’s energy supply depends critically on hydraulic fracturing (HF) to access otherwise uneconomical resources. Unfortunately, HF also has the potential to induce larger earthquakes – with some projects being prematurely terminated because of perceived earthquake risks. To de-risk HF, we use a suite of statistical tests to discern if some physical process has restricted the growth of earthquake magnitudes. We show that all stage stimulations at both UK PNR-1z and Helsinki St1 indicate bound fracture growth, implying a more controllable operation. Contrastingly, stimulations at Utah FORGE and UK PNR-2 sequentially transitioned into unbound fault reactivation. The problematic stages (that ultimately led to the termination of PNR-2) are clearly distinguishable. We postulate that our research can discriminate fracture stimulation from fault reactivation, contributing to the de-risking of HF operations worldwide. Our statistical tests provide a framework for model falsification, which can guide physical insights into the bounding processes.