A Consistent High-Resolution Catalog of Induced Seismicity in Basel Based on Matched Filter Detection and Tailored Post-Processing

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 M-w-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. Plain Language Summary Fluid injections into the deep underground, such as performed in geothermal projects, may cause earthquakes. These induced earthquakes provide important information about the involved physical processes but can sometimes be stronger than acceptable and hinder a project to continue. Avoiding unacceptable earthquakes requires a better understanding of the immediate and long-term seismic response of the underground to such operations. However, inconsistencies of existing earthquake catalogs and their generally low resolution restrict our ability to understand these processes. Such catalog restrictions also affected the case of the Basel deep geothermal project. As a result, specifically, the long-term behavior of its induced seismicity remained unexplored. To overcome these deficiencies, a consistent catalog with high resolution is needed. We reinvestigate the induced seismicity in Basel in detail over its whole life span (12 years). Using seismograms of known earthquakes, we search for similar earthquakes and detect an abundance of smaller ones that were previously unknown. To ensure catalog consistency, we further develop advanced techniques that provide robust magnitude estimates and maintain a high detection sensitivity. Like increasing the resolution of an image with a spyglass, the new catalog reveals previously unseen details of this particular sequence. In the injection period, for instance, we find deviations from the expected behavior of earthquakes and their magnitude distribution. These findings make it necessary to rethink earthquake mitigation strategies in geotechnical projects.