Towards an Improved Seismic Source Catalog for Switzerland

We present an updated and extended seismotectonic earthquake catalog of Switzerland and surrounding regions. This SECOS25 catalog serves as input for the seismic-source component of Switzerland’s next generation seismic hazard model to be developed by the Swiss Seismological Service (SED) over the coming years. The SECOS25 catalog includes 51 years of instrumental seismicity detected and located by the SED between 1975 and 2025. For the digital era of the SED catalog (phase picks and seismograms available in digital form) starting in 1984, hypocenters were consistently relocated in absolute terms using a recent 3D P and S-wave crustal velocity model. Starting from these improved hypocenters, double-difference relative relocations were performed at different scales (local clusters as well as regional scales), combining differential times from manual picks and waveform cross correlations. A merging procedure was developed that selects the preferred location method (bulletin location, absolute relocation, relative relocation) based on location-quality criteria for each hypocenter. The proposed procedure provides various hypocenter uncertainty measures and ensures the maximum possible hypocenter-location accuracy and precision for each event. Local magnitudes were revised using a new set of consistent amplitude measurements (doi.org/10.1785/0120250032) and a new magnitude model. For each earthquake, we provide moment magnitudes from native methods if available (either from moment tensors or spectral fitting) or revised scaling relationships. Finally, we link the hypocenters with catalogs of moment tensors (containing about 80 solutions) as well as first-motion focal mechanisms (containing about 600 solutions).The SECOS25 catalog serves as a base for further down-stream seismotectonic components of the seismic-source model. This includes heat maps of seismic activity and moment release across Switzerland as well as maps of deformation regimes and stress orientations derived from the analysis and inversion of focal mechanisms. Finally, we apply an enhanced version of the HyFi method (doi.org/10.1029/2023JB026352) to the SECOS25 catalog to systematically identify previously unknown seismically active fault segments and their orientations. The derived information will contribute to a refined definition of seismic zonation as well as faulting styles and preferred rupture orientations required for hazard computations. The SECOS25 catalog and derived products therefore also contribute to an improved understanding of present-day seismotectonic processes in the Central Alpine region.