Aftershock analysis and forecasting for the crustal seismicity in Romania

Abstract Romania is known for its persistent seismicity at intermediate-depths in the Vrancea region, however crustal areas are also a significant source of seismic hazard, although large shallow events are less common. This study is a first attempt to characterize statistically and propose a forecasting model for two recent aftershock sequences occurred at crustal depths in 2014 and 2023, following mainshocks of moderate magnitudes (Mw5.4 for both mainshocks). We apply a robust approach based on a state-of-the-art procedure developed and tested previously for Japan, which is able to determine in quasi real-time the parameters of the Gutemberg-Richter law and Omori-Utsu law for aftershocks and provide probability estimates of larger events, which can be updated in real time. For both the 2014 Vrancea-Marasesti and 2023 Gorj sequences we test several relatively short (hours to day) learning periods and subsequent forecasting periods. Both sequences are characterized by relatively high b-values (~ 1.2), obtained for all tested learning periods, which may point out to the release of stress following the mainshocks or the presence of crustal fluids in the studied regions. The aftershock decay is characterized by a parameter p of around 1.0, commonly observed for crustal aftershock sequences. The c-value, which indicates the onset time of the power-law decay of aftershocks, is on the order of minutes to hours. Although the two studied sequences follow mainshocks having the same magnitude, the probability of larger aftershocks for the 2023 Gorj sequence are larger. The results obtained in this study are encouraging for the development of a real-time monitoring and forecasting system for the Romanian crustal seismicity.