Background: The intermediate-depth seismicity in the Vrancea region (Romania) is characterized by localized and persistent earthquake activity that culminates about two or three times in a century with the occurrence of a large event (M ≥ 6.5). Here we have revisited some important seismicity characteristics, using earthquake catalog data spanning two different time periods: 1960–1999 and 2005–2013. Methods: we have determined the b-value of the frequency-magnitude distribution of earthquakes, using a maximum likelihood procedure, and estimated the parameter β to quantify anomalous seismicity rate decreases and increases. Results: by using data from the first period, we have confirmed the existence of a decreased b-value in the deepest part of the seismogenic zone; by using data from the second period, we have statistically confirmed the seismic quiescence that preceded the occurrence of the 1977 M7.4 Vrancea earthquake. Conclusions: the decreased b-value has been interpreted either in terms of an increased lithostatic stress with depth or as an indicator of the depth range where the next major Vrancea earthquake may occur. The time variation of the seismicity parameter β may reveal anomalous seismic quiescence and increased earthquake rates that may precede the occurrence of large earthquakes.

The source mechanism of the seismic events during the sequence of the moderate-size crustal earthquake of November 22, 2014 of Vrancea region (Romania)

The purpose of this paper is to analyze the use of the fixed-pointed algorithm FastICA on a mix of four voices with the aim of separating each of the voices. Two experiments where performed, one with real data with artificial mixing and the other one with real data with real mixing. Following these experiments, discussions were held and some conclusions were drawn.

(1) Background: The intermediate-depth seismicity in the Vrancea region (Romania) is characterized by localized and persistent earthquake activity that culminates about two or three times in a century with the occurrence of a large event (M 6.5). Here we have revisited some important seismicity characteristics, using earthquake catalog data spanning two different time periods: 2005 – 2013 and 1960 – 2000. (2) Methods: we have determined the b-value of the frequency-magnitude distribution of earthquakes, using a maximum likelihood procedure, and estimated the parameter to quantify anomalous seismicity rate decreases and increases. (3) Results: by using data from the first period, we have confirmed the existence of a decreased b-value in the deepest part of the seismogenic zone; by using data from the second period, we have statistically confirmed the seismic quiescence that preceded the occurrence of the 1977 M7.4 Vrancea earthquake. (4) Conclusions: the decreased b-value has been interpreted either in terms of an increased lithostatic stress with depth or as an indicator of the depth range where the next major Vrancea earthquake may occur. The time variation of the seismicity parameter may reveal anomalous seismic quiescence and increased earthquake rates that may precede the occurrence of large earthquakes.

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 (M_w5.4 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 Gutenberg–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-Oltenia sequences we test several relatively short (hours to one day) learning periods and subsequent forecasting periods. Both sequences are characterized by a Gutenberg–Richter b-value around 0.8–0.9, which is only slightly lower than the average of 1.0 for worldwide seismicity. 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 moment magnitude, the probability of larger aftershocks for the 2023 Gorj-Oltenia 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. Graphical Abstract