Towards New Magnitude Models for Switzerland

We present new local magnitude models for Switzerland, derived using a top-quality input dataset of ground-motion intensity measures, obtained through an automated and consistent waveform processing workflow (https://doi.org/10.1785/0120250032). The input dataset includes Wood-Anderson displacement response amplitudes calculated from 150,000 waveforms generated by about 15,000 earthquakes in the region of interest. This dataset is substantially larger than those used in previous magnitude studies in Switzerland and contains a large number of recordings at short distances for smaller events (about 5,000 earthquakes with magnitudes lower than 1, and about 1500 records at hypocentral distances shorter than 5 km), which were sparse in earlier studies. The parametrization of the local magnitude model is based on a detailed investigation of attenuation characteristics of the Wood-Anderson response amplitudes, comprising: (i) linear and logarithmic distance terms to represent different physical mechanisms of seismic wave propagation; (ii) hinge distances to allow modelling the effects of Moho reflections and the associated changes in attenuation rate; (iii) regional adjustments (Swiss Alps vs Swiss northern Foreland) based on the length and location of the surface projection of the source-to-site ray paths. Model coefficients are determined through mixed-effects regressions, thus allowing the derivation of station-magnitude correction terms consistent with the reference rock-like ground type used for mapping seismic hazard in Switzerland. We assess and validate the model’s performance via uncertainty analyses, validation against recent data not used in model calibration, validation against recordings of major Swiss events, and comparisons with results obtained from alternative, fully independent modelling approaches (including non-parametric and 2D cell-based methods). The new model yields systematically lower magnitudes, with an average difference of 0.1-0.2 magnitude units for smaller events with magnitudes below 2.5 and for earthquakes located in the Swiss northern Foreland, compared to the currently authoritative catalogue magnitudes. Based on the new candidate magnitude model, we present and discuss an updated empirical scaling relationship between local and moment magnitudes.