Towards a harmonized operational earthquake forecasting model for Europe

We develop a harmonized earthquake forecasting model for Europe based on the epidemic-type aftershock sequence (ETAS) model to describe the spatiotemporal evolution of seismicity. We propose a method modification that integrates information from the 2020 European Seismic Hazard Model (ESHM20) about the spatial variation in background seismicity during ETAS parameter inversion based on the expectation-maximization (EM) algorithm. Other modifications to the basic ETAS model are explored, namely fixing the productivity term to a higher value to balance the more productive triggering by high-magnitude events with their much rarer occurrence and replacing the b-value estimate with one relying on the b-positive method to observe the possible effect of short-term incompleteness on model parameters. Retrospective and pseudo-prospective tests demonstrate that ETAS-based models outperform the time-independent benchmark model as well as an ETAS model calibrated on global data. The background-informed ETAS variants using the b-positive estimate achieve the best scores overall, passing the consistency tests and having a good score in the pseudo-prospective experiment. Our test results also highlight promising areas for future exploration, such as avoiding the simplification of using a single b value for the entire region, reevaluating the completeness of the used seismic catalogs, and applying more sophisticated aftershock spatial kernels.