Andrei, Dumitru N. (2026), The Vrancea Seismic Cycle Revisited: A Double-Percolation Framework and Vertical Stress Resetting at Intermediate Depths, Cunoașterea Științifică, 5:2, 21-27, DOI: 10.58679/CS64085, https://www.cunoasterea.ro/en/the-vrancea-seismic-cycle-revisited/

 

Abstract

The Vrancea seismic zone represents a unique case of concentrated intermediate-depth seismicity in Europe, characterized by recurrent large earthquakes occurring within a narrow subcrustal volume between approximately 70 and 180 km depth. Observational studies indicate that seismic activity is confined to a small, nearly vertical seismogenic body, commonly interpreted as a remnant subducted slab or a delaminated lithospheric fragment. Seismic catalog analyses further reveal the presence of two depth-separated active segments, centered around 90 km and 140 km, which appear to participate alternately in the seismic cycle. In addition, independent investigations document a depth-dependent stress regime, with evidence for vertical extension in the 100-130 km depth range, as well as a correlation between intermediate-depth seismicity and dehydration reactions in hydrous minerals. This study introduces a (ANDREI, 2025) revised conceptual model for the Vrancea seismic cycle, utilizing a double-percolation framework. Within this approach, each active depth segment in the Vrancea zone is defined by a distinct threshold of internal connectivity. These thresholds are determined by the presence and evolution of microfractures, the movement and accumulation of fluids, and the development of mechanically weakened zones throughout the subcrustal volume. By recognizing the unique percolation characteristics of each segment, the model aims to explain how seismic energy is released in discrete intervals and why alternating depth segments participate in the seismic cycle. This framework emphasizes the role of internal structural changes and the connectivity of stress-affected regions in driving the patterns of intermediate-depth seismicity observed in Vrancea. Earthquake occurrence is interpreted as the manifestation of rapid volumetric stress reorganization triggered when these percolation thresholds are exceeded. Stress release is dominated by a mechanism of vertical stress resetting, reflecting both the geometry of the seismogenic volume and the depth-dependent stress regime, rather than repeated brittle rupture along a fixed fault plane. This framework provides a physically consistent explanation for the depth, recurrence, and structural persistence of Vrancea earthquakes and suggests testable hypotheses for future multidisciplinary investigations.

Keywords: Vrancea seismic zone, intermediate-depth earthquakes, seismic cycle, percolation threshold, vertical stress resetting, slab dynamics

Ciclul seismic Vrancea revizuit: Un cadru de dublă percolare și resetarea tensiunii verticale la adâncimi intermediare

Rezumat

Zona seismică Vrancea reprezintă un caz unic de seismicitate concentrată la adâncimi intermediare în Europa, caracterizată prin cutremure recurente de mari dimensiuni care au loc într-un volum subcrustal îngust, între aproximativ 70 și 180 km adâncime. Studiile observaționale indică faptul că activitatea seismică este limitată la un corp seismogenic mic, aproape vertical, interpretat în mod obișnuit ca o placă subductă remanentă sau un fragment litosferic delaminat. Analizele catalogului seismic relevă în plus prezența a două segmente active separate în adâncime, centrate în jurul a 90 km și 140 km, care par să participe alternativ la ciclul seismic. În plus, investigațiile independente documentează un regim de solicitări dependent de adâncime, cu dovezi de extensie verticală în intervalul de adâncime de 100-130 km, precum și o corelație între seismicitatea la adâncimi intermediare și reacțiile de deshidratare în mineralele hidratate. Acest studiu introduce un model conceptual revizuit (ANDREI, 2025) pentru ciclul seismic Vrancea, utilizând un cadru de dublă percolare. În cadrul acestei abordări, fiecare segment activ de adâncime din zona Vrancea este definit de un prag distinct de conectivitate internă. Aceste praguri sunt determinate de prezența și evoluția microfracturilor, de mișcarea și acumularea fluidelor și de dezvoltarea zonelor slăbite mecanic în întregul volum subcrustal. Recunoscând caracteristicile unice de percolare ale fiecărui segment, modelul își propune să explice modul în care energia seismică este eliberată în intervale discrete și de ce segmentele de adâncime alternante participă la ciclul seismic. Acest cadru subliniază rolul modificărilor structurale interne și al conectivității regiunilor afectate de solicitări în determinarea modelelor de seismicitate la adâncimi intermediare observate în Vrancea. Apariția cutremurelor este interpretată ca manifestarea unei reorganizări volumetrice rapide a tensiunilor declanșate atunci când aceste praguri de percolare sunt depășite. Eliberarea tensiunilor este dominată de un mecanism de resetare verticală a tensiunilor, reflectând atât geometria volumului seismogen, cât și regimul de solicitări dependent de adâncime, mai degrabă decât ruptura fragilă repetată de-a lungul unui plan de falie fix. Acest cadru oferă o explicație fizic consistentă pentru adâncimea, recurența și persistența structurală a cutremurelor din Vrancea și sugerează ipoteze testabile pentru viitoare investigații multidisciplinare.

Cuvinte cheie: zona seismică Vrancea, cutremure de adâncime intermediară, ciclu seismic, prag de percolare, reașezare a tensiunii verticale, dinamica plăcii

 

CUNOAȘTEREA ȘTIINȚIFICĂ, Volumul 5, Numărul 2, Iunie 2026, pp. 21-27
ISSN 2821 – 8086, ISSN – L 2821 – 8086, DOI: 10.58679/CS64085
URL: https://www.cunoasterea.ro/en/the-vrancea-seismic-cycle-revisited/
© 2026 Dumitru N. ANDREI. Responsabilitatea conținutului, interpretărilor și opiniilor exprimate revine exclusiv autorilor.

 

The Vrancea Seismic Cycle Revisited: A Double-Percolation Framework and Vertical Stress Resetting at Intermediate Depths

Dumitru N. ANDREI[1]
dumitru.andrei@vivascience.org.ro

[1] Independent researcher

 

Introduction

Intermediate seismicity is one of the most difficult problems of modern seismology, as classical brittle rupture mechanisms become insufficient at great depths. The Vrancea seismic zone, located under the south-eastern curvature of the Carpathians, is a special case due to the intensity and persistence of the seismic activity concentrated in a small subcrustal volume. Classical models based on elastic rebound, friction on fault planes or weak pull forces only partially explain the recurrence of major earthquakes and the depth segmentation of seismic sources (Maria Rogozea, 2009), (Laura Petrescu, 2025). These observations justify the need for a conceptual framework that includes volumetric processes, depth-dependent properties, and stress reorganization mechanisms.

Observational constraints

Spatial confinement and geometry

The intermediate seismicity in the Vrancea area is limited to a subcrustal volume with low lateral extension and a vertical development of over 100 km (Andreea Craiu T. P., 2022),  (Andreea Craiu M. C., 2023). Seismic tomography indicates the presence of a high-velocity body extended to depths of 200-350 km, interpreted as descending lithosphere or delaminated material (Ivan Koulakov, 2010), (Goetz Bokelmann, 2014).

Depth segmentation of seismic sources

The seismic catalogs highlight two main intervals capable of generating major earthquakes: an upper segment (80-110 km) and a lower segment (120-170 km) (Maria Rogozea, 2009). The temporal distribution suggests an alternation of energy release between these segments.

Triggering at intermediate depths

At these depths, classic brittle rupture is unlikely. However, seismicity correlates with dehydration reactions of hydrated minerals in a weak subduct, which can induce transient weakening of the environment (Manea, 2021),  (Andreea Craiu T. P., 2022),  (Andreea Craiu M. C., 2023).

Depth-dependent stress regime

Studies of focal mechanisms indicate a transition from horizontal compression to vertical extension between about 100 and 130 km deep  (Andreea Craiu T. P., 2022) suggesting a fundamental change in the way stress is accommodated.

Conceptual model: double percolation

In this study, percolation is defined as reaching a critical threshold of internal connectivity in the seismogenic volume, involving microcracks, fluids, and mechanically weakened areas. A percolation-type behavior was previously suggested in the analysis of the Vrancea seismic cycles (Maria Rogozea, 2009).

The upper segment reaches a first threshold (Percolation A) generating earthquakes around 90 km, while the lower segment, controlled by different pressure-temperature conditions, reaches a second threshold (Percolation B), associated with events at about 140 km (Manea, 2021). The 100-130 km interval functions as a transition zone between the two regimes.

Vertical stress resetting mechanism

The accumulation of stress occurs volumetrically in an almost vertical seismogenic body. Exceeding the percolation thresholds leads to a rapid reorganization of the stress field. Due to lateral geometric constraints, the stress release is dominated by vertical components, in accordance with the observed vertical extension (Andreea Craiu T. P., 2022). Vertical stress reset includes a fast phase – the main earthquake and a slow post-seismic relaxation phase (Laura Petrescu, 2025), (Maria Rogozea, 2009), allowing cycles to be repeated without mechanical exhaustion of the environment (Laura Petrescu, 2025).

Discussion

The double percolation model complements classical approaches to the seismic cycle with its emphasis on internal connectivity and volumetric processes. It explains the alternation of active segments and the persistence of seismicity without assuming the existence of a single and recurrent fault surface (Maria Rogozea, 2009)

Limitations and future perspectives

The conceptual nature of the proposed model necessitates additional validation. Current constraints on parameters governed by fluid distribution and microcracks are limited. Future research may benefit from integrating mechanical-hydro-chemical modeling, as well as conducting comparative analyses with other regions characterized by intermediate seismicity (Laura Petrescu, 2025). Furthermore, phenomena related to percolation should be examined in conjunction with other commonly observed occurrences in the Vrancea Seismic Zone, including magnetic and electrical anomalies that are regarded as precursors to earthquakes with magnitudes of M ≥ 3.9 (Enescu, 1999), (ENESCU, 2005), (Stanica, 2011); (ANDREI, 2025).

Conclusions

The paper shows that the seismic cycle in the Vrancea area can be coherently interpreted within a model of double percolation and vertical stress reset. This approach allows the explanation of the recurrence of major earthquakes without assuming the progressive degradation of the lithospheric material and provides a useful conceptual framework for the integration of existing seismological and geodynamic observations.

Abbreviations

No specific abbreviations are used in this study.

Acknowledgements

The author acknowledges the contributions of the national and international seismological community whose published datasets and analyses made this conceptual synthesis possible. The use of AI-based editorial assistance is also acknowledged for improving the clarity and structure of the manuscript.

Author’s contribution

Dumitru N ANDREI conceived the original scientific ideas, developed the conceptual framework, interpreted the observational evidence, and wrote the manuscript. He validated all scientific arguments and approved the definitive version of the paper. The AI assistant (ChatGPT) supported the author in structuring the manuscript, improving academic language and coherence, and assisting with formatting and citation consistency, under the author’s supervision.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Credit authorship statement

Dumitru N ANDREI: conceptualization; investigation; methodology; formal analysis; interpretation of data; writing original draft; writing review & editing; visualization. AI-assisted for editorial and linguistic support only. No scientific responsibility is attributed to the AI system.

Competing interests

The author declares that there are no competing interests.

References

Andreea Craiu, M. C. (2023). Vrancea intermediate-depth focal mechanism catalog: a useful instrument for local and regional stress field estimation. Acta Geophysica, 71, 29-52. Retrieved from https://doi.org/10.1007/s11600-022-00906-4

Andreea Craiu, T. P. (2022). A switch from horizontal compression to vertical extension in the Vrancea slab explained by the volume reduction of serpentine dehydration. Scientific Reports(12). Retrieved from https://doi.org/10.1038/s41598-022-26260-5

ANDREI, D. N. (2025). A Different Approach to the Dynamo Mechanism. Cunoasterea Stiintifica, 4(3), 57-64. doi:10.58679/CS88989

Enescu, B. D. (1999). Seismic-electromagnetic precursors of Romania’s Vrancea earthquakes. Romanian Journal of Physics, 44(7-8), 833-850.

ENESCU, D. (2005). Geomagnetic Anomalies – Possible Earthquake Precursors – Linked with 2004 Significant Seismic Activity in Vrancea (Romania). Romanian Journal of Physics, 50(9-10), 1213-1221.

Goetz Bokelmann, F.-A. R. (2014). Nature of the Vrancea seismic zone (Eastern Carpathians): New constarints from dispertion of first-arriving P-waves. Earth Planet. Sci. Lett., 390, 59-68. Retrieved from https://doi.org/10.1016/j.epsl.2013.12.034

Ivan Koulakov, B. Z. (2010). Delamination or slab detachment beneth Vrancea? New arguments from local earthquake tomography. Geochemistry, Geophysics, Geosystems, 11(3). Retrieved from https://doi.org/10.1029/2009GC002811

Laura Petrescu, B. E. (2025). Seismicity of a relic slab: space-time cluster analysis in the Vrancea Seismic Zone. Springer Natue Link, 77, 41. Retrieved from https://doi.org/10.1186/s40623-025-02136-6

Manea, T. P. (2021). Dehydration-induced earthquakes identified in a subducted ocdeanic slab beneath Vrancea, Romania. Scientific Reports, 11, 28. Retrieved from https://doi.org/10.1038/s41598-021-89601-w

Maria Rogozea, M. P. (2009). DETAILED SEISMICITY ANALYSIS IN THE VRANCEA AREA AND IMPLICATIONS FOR THE SEISMIC CYCLE EVOLUTION. Romanian Reports in Physics, 61(3), 740-747.

Stanica, D. S. (2011). Anomalous Pre-Seismic Behavior of the Electromagnetic Normalized Functions Related to the Intermediate Depth Earthquakes Occured in Vrancea Zone, Romania. Natural Hazards and Earth System Sciences, 11, 3151-3156. Retrieved from http://dx.doi.org/105194/nhess-3151-2011