Multi-Scale On-Fault Seismic Monitoring at the Bedretto Underground Laboratory: An Operational Framework

The Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG, ETH Zurich) operates a dense, multi-scale seismic monitoring framework for on-fault observation of induced seismicity during underground stimulation experiments. The system integrates permanent seismic stations (500 to 10k samples per second, sps), high-rate experimental deployments (up to 200k sps), and distributed acoustic sensing (DAS; up to 4000 sps), providing high spatial and temporal data coverage at the reservoir and local scale. This contribution documents real-time seismic monitoring during underground experiments and the generation of associated post-processed earthquake catalogues.

The combined instrumentation is used for monitoring of hydraulic stimulation and geothermal project experiments, including the Fault Activation and Earthquake Rupture (FEAR) ERC project and Geothermal Test-Bed (GTB) activities. Permanent monitoring provides manually reviewed earthquake locations and magnitudes in a continuous reference earthquake catalogue, continuously updated since 2021. Experimental deployments enable near real-time event detection, characterization, and Traffic Light System (TLS) alerting, including borehole DAS observations since 2024.

For the FEAR and GTB experiments, additional post-processed catalogues provide optimal yet conventional event parameters based on the final instrumentation configuration, optimized phase picking and hypocenter locations. The resulting catalogues provide consistent absolute locations and high-resolution double-difference relocations based on fully repicked waveform data.

Event magnitudes are estimated using multiple complementary approaches. The backbone national network provides a Swiss-specific local magnitude (Mlhc) using station corrections, while the permanent BedrettoLab network applies conventional local magnitude (MLc) from Wood–Anderson amplitudes. Experimental catalogues include relative moment magnitudes derived from acoustic emission amplitude ratios (MwA) calibrated against collocated accelerometers. Ongoing developments include spectral moment magnitudes and DAS strain-rate-based magnitude estimates to improve characterization of small-magnitude events.

The BedrettoLab monitoring framework, implemented in SeisComP, addresses challenges such as electromagnetic noise, tunnel operations, and heterogeneous sensor coverage while ensuring continuous acquisition, archiving, and FDSN-compliant data access. Beyond Bedretto, this on-fault, densely instrumented approach demonstrates how integrated DAS, permanent, and experimental monitoring can substantially improve event detection, location accuracy, and magnitude estimation, providing a transferable framework for induced seismicity monitoring, operational decision-making, and risk mitigation in geothermal and underground engineering applications.