SECOS24: New insights into seismicity, deformation and crustal stresses in the Central Alps Region from a baseline seismotectonic earthquake catalog

Abstract

The base data for any seismotectonic study consist of accurate and precise hypocenter information, consistent magnitude estimates, and focal mechanisms derived either from the analysis of first-motion (FM) polarities or moment-tensor (MT) inversions. In this study, we present a new baseline seismotectonic earthquake catalog of Switzerland and surrounding regions (SECOS24), which covers the Central Alps (CA) region between 45.4°N/5.6°E and 48.4°N/11.1°E. The SECOS24 catalog includes instrumental seismicity routinely detected and located by the Swiss Seismological Service (SED) between 1975 and 2024 (about 49 years). For the digital era of the SED bulletin (phase picks and seismograms available in digital form) starting in 1984, hypocenters were consistently relocated in absolute terms using a recent Pg and Sg 3-D velocity model. Starting from these improved hypocenters, double-difference relative relocations were performed at different scales (single clusters as well as at regional scales), combining differential times from manual picks and waveform cross correlations. Based on available solutions and resulting location quality, a preferred solution was selected for each hypocenter of the SECOS24 catalog, in order to provide the maximum possible hypocenter accuracy and precision for each event. The SECOS24 catalog contains about 36,000 earthquakes with magnitudes ranging between ML -0.7 to 5.3. In addition to ML, the catalog reports complementary magnitudes for a subset of events. For 71 events, an MW magnitude was derived from a revised MT inversion for events starting in 1999. For events since 2009, a spectral MW was calculated if possible. This magnitude compilation allows for the assessment and improvement of existing ML-MW scaling relations. Finally, we linked each hypocenter with the revised MT catalog as well as solutions of an augmented FM catalog, which contains 492 high-quality, manually reviewed mechanisms based on P-wave first-motion polarities. The SECOS24 catalog is used for down-stream seismotectonic analysis of the CA region. In this presentation, we show updated maps of seismicity and moment release in the CA and their foreland. In addition, we provide updated maps of deformation regimes and stress orientations derived from the analysis and inversion of the FM data. Besides previously known features, the SECOS24 catalog reveals several new features in the CA and their foreland like newly imaged seismogenic fault zones, lateral changes in the deformation regime along the Alpine Front of the CA, and ongoing shortening at shallow crustal levels in the Jura fold-and-thrust belt. In addition, the updated stress inversion provides more stable results and, in several places, higher spatial resolution in comparison to previous studies. The SECOS24 catalog therefore contributes to an improved understanding of present-day tectonic processes in the CA region and is crucial input for next-generation seismic hazard models of the region.