Comparing foreshock characteristics and foreshock forecasting in observed and simulated earthquake catalogs

In this paper, we compare the empirical results regarding foreshocks obtained from the Japan data with results for synthetic catalogs in order to clarify whether or not the corresponding results are consistent with the description of the seismicity by a superposition of background activity and epidemic‐type aftershock sequence (ETAS) models. This question is important, because it is still controversially discussed whether the nucleation process of large earthquakes is driven by seismically cascading (ETAS type) or by aseismic accelerating processes. To explore the foreshock characteristics, we first applied the same clustering algorithms to real and synthetic catalogs and analyzed the temporal, spatial, and magnitude distributions of the selected foreshocks. Most properties are qualitatively the same in the real data and in synthetic catalogs. However, we find some quantitative differences particularly in the temporal acceleration, spatial convergence, and magnitude dependence, which also depend on the assumed synthetic catalogs. Furthermore, we calculated forecast scores based on a single‐link cluster algorithm which could be appropriate for real‐time applications. We find that the Japan Meteorological Agency catalog yields higher scores than all synthetic catalogs and that the ETAS models having the same magnitude sequence as the original catalog performs better (more close to the reality) than ETAS models with randomly picked magnitudes. We also find that the ETAS model that takes account of the triggering effect by small earthquakes below threshold magnitude performs more closely to the reality.