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Abstract

The Epidemic-Type Aftershock Sequence (ETAS) model is the most effective mathematical model to describe the short-term space-time clustering. The setup of the ETAS model is complicated by the high number of unknown and correlated parameters. The most recent ETAS parameterizations introduce the space-time variability of some parameters that make the model calibration even more arduous. Here, we investigate the ETAS model in an opposite perspective; we look for the simplest ETAS parametrization that can satisfactorily describe the short-term space-time earthquake clustering in crustal tectonic regions; we name this model simplETAS. Owing to its simplicity, simplETAS is easy applicable in almost all crustal tectonic regions. This ease of use leads implicitly to some important features that are worth being remarked; simplETAS provides i) important physical indications on which clustering parameter is mostly region and time-independent; ii) a reference model that can be used as benchmark to measure the relative forecasting skill of other models in earthquake forecasting experiments; iii) a simple but effective approach to generate synthetic earthquake catalogs of different length that can be used in seismic hazard and risk analysis, overcoming all problems related to the declustering and the Poisson assumption. Finally, we show that the simplETAS model calibrated with the Italian seismicity of the last decades is able to describe satisfactorily the space-time occurrence of the out-of-sample largest earthquakes in historical catalog.