Case study: Romania Within the Statistical Threshold for a Richter Magnitude 8+ Earthquake?

The Southeastern European country of Romania is among the most seismically active throughout the continent and currently faces a substantial seismic risk, particularly in the Vrancea region, which accounts for approximately 90% of the earthquakes. The present study evaluates the value of the statistical probability that a Richter Magnitude 8+ earthquake will occur in the area utilising the Poisson method, and introduces a novel statistical approach for approximating intervals of recurrence. The Gutenberg–Richter law, covered by the log⁡ 10 N = a − b * M formula, describes an exponential decline in earthquake frequency with increasing magnitude, typically characterized by b ≈ 1.0 in active tectonic regions. Inverting this frequency (1 / N) highlights the relative prevalence of lower-magnitude events. For the Vrancea seismic zone, provisional inverted recurrence frequencies are estimated at 0.10 for M6.0, 0.08 for M6.5, 0.03 for M7.0, 0.01 for M7.5, 0.005 for M8.0, 0.0014 for M8.5, 0.0004 for M9.0, 0.00008 for M9.5, and 0.00001 for M10.0. These figures suggest a roughly tenfold decrease in earthquake occurrence with each unit increase in magnitude, consistent with regional b-values between 0.94 and 1.0 reported in Vrancea seismicity studies. Additionally, the N [M(x+0.5)] = 2 * N (Mx) + 14 * (n0) heuristic formula may apply for the M7 - M7.8 magnitude range in Vrancea, in which N is equal to the number of years and 6.5 < x < 7.3. Recent megathrust earthquakes, such as the 2015 M7.8 Nepal, as well as the 2023 M7.8, M7.5 and M6.7 Türkiye events - which were followed by a M5.7 earthquake ten days afterward, as well as numerous aftershocks, in the Southwestern Carpathians - have possibly displayed a potential for long-range dynamic and static stress perturbations exceeding 1,500 km, as any seismic event of scales from M7.5 above are typically capable of sending waves of tectonic stress above 1,500 kilometres afar. These may influence seismic behavior across distant fault systems, including the Carpathian–Alpine region, through indirect stress transfer mechanisms. Notably, geodetic measurements around Mount Everest suggest an uplift of approximately 4 meters by 1995, followed by a reported drop in elevation post-2015, potentially illustrating the complex tectonic adjustments following major seismic events. An analogy into geology may apply from biological systems, in which tectonic stresses can be transferred throughout lengthy distances, just as muscular contractions can significantly affect distant parts of the human body, and particularly the ones with a lower extent of stability. In the same manner, areas of significant mountain ring curvature that likely include the Vrancea region, may be particularly affected by any transmitted wave of seismic stress, regardless of the distance from the emitting source of such stress. Furthermore, the continuous movement of the African plate toward the Eurasian plate with an average speed of 2.15 cm per year could also have been gradually amplifying existing transfers of tectonic stress via the African, Asian and European plates, potentially having brought additional, indirect implications for both Asia Minor and Southeastern Europe, and the consideration of long-term tectonic changes around the Eastern Mediterranean would be particularly important. For the purpose of International Health and Safety guidelines, local, national and international authorities should not rule out the occurrence of an earthquake with a magnitude of up to M9, given the lack of historic data regarding the occurrence of an M8.5+ earthquake within the past 800 years [N (M8.5) ~ 700 years, threshold level of significant probability at 60%, maximum probability of ~63% resulted from seismic stress transfers], let alone since records first began around 800 years ago. Provisional statistical modelling estimated an overall capped probability of 75-80% that an M8+ event will occur in Vrancea, Romania, within the next several years - visibly exceeding the 60% threshold of statistical concern. Any seismic event reaching and exceeding M8 in Vrancea would likely be felt throughout widespread areas of Europe, and the medium-high average depth of earthquakes in Vrancea (i.e. generally around 65-190 km underneath the sea level) would be a major factor to such a phenomenon. Such overall findings underscore the importance of integrating regional recurrence modeling with broader geodynamic interactions in future seismic hazard assessments.