The faults and the fault zones are essential to determine the seismotectonic features and the regime of a region and to produce seismic hazard and risk maps. Many geophysical methods, such as seismic, magnetic and gravity are used to determine the location and direction of faults. This study aims to detect potential fault edges located in the south-southwest of Nisyros Island and west of Tilos Island in the southeastern Aegean Sea and to interpret the seismic activity and faulting characterization. To this end, the potential fault edges were initially detected using the total horizontal derivative method on the gravity dataset collected from the WGM2008 database. The obtained spatial locations were combined with the seismicity and the focal mechanisms of the earthquakes. The detected fault has a northeast-southwest orientation with normal faulting. It was proved that this obtained fault is not included in the active faults of the Eurasia database (AFEAD), European Fault - Source Model 2020 (EFSM20) database and the Mineral Research and Exploration General Directorate (MTA) active fault map. Moreover, this observed fault was detected as longer than the fault shown on the National Observatory of Athens (NOAFAULTs) in terms of length. Consequently, it is highly recommended that this potential fault, which produces earthquakes with a moment magnitude greater than 4.0 (on instrumental period) must be added to such fault databases to increase and spread the knowledge of seismological and seismotectonic research.

How to cite: Tamtaş, B. D. and Toktay, H. D.: Seismotectonic interpretation and edge detection of the potential fault on the SE Aegean Sea using the total horizontal derivative method, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-785, https://doi.org/10.5194/egusphere-egu24-785, 2024.

Collision and subsequent convergence of Arabia and Eurasian plates during the late Alpine time caused extensive intracontinental deformation in the Caucasus region (e.g., Cavazza et al., 2023). Inversion of back-arc basins, exhumation, and crustal thickening took place in the far-field zone, forming two orogens, and leading to a convergence between the Lesser Caucasus (LC) and Greater Caucasus (GC). Continuous convergence between the LC and GC caused incremental deformation of the Kura foreland basin (Alania et al., 2023). Our study area is located within the LC-GC convergence zone in the western Kura foreland basin and is represented by the lenticular-shaped compressional basin formed by northward and southward-directed thrusting. 2D seismic reflection profiles revealed the presence of triangle zones at the frontal part of the LC retro-wedge and GC pro-wedge. According to analog, seismic, and sequential kinematic modeling results, the frontal part of the GC is represented by a double wedge-dominated triangle zone and is characterized by the presence of passive, active wedges and passive-forethrust. In the frontal part of the LC orogen, the syn-kinematic (Middle-late Miocene) strata have been deformed and uplifted by passive-back thrusting at the triangle zone. Seismic reflection profiles show north-vergent duplex and structural wedge at the triangle zone beneath the thrust front monocline and is represented by Cretaceous-Paleogene strata. Western part of the triangle zone of the Kavtiskhevi-Akhalkalaki area is introduced by south-vergent imbricated fan (passive-back thrusts). The imbricated fan is characterized by fault-propagation folding. The kinematic evolution of south-vergent fault-propagation folds is related to northward propagating duplex. Recent and historical earthquakes and paleoseismic data indicate that the frontal part of LC and GC is tectonically active (e.g., Stahl et al., 2022; Tsereteli et al., 2016).

Reference

Alania, V., et al. (2023). Interpretation and analysis of seismic and analog modeling data of triangle zone: a case study from the frontal part of western Kura foreland fold-and-thrust belt, Georgia. Frontiers in Earth Sciences 11, 1195767.

Cavazza, W., et al. (2023). Two-step exhumation of Caucasian intraplate rifts: a proxy of sequential plate-margin collisional orogenies, Geoscience Frontiers 15, 101737.

Stahl, T. A., et al. (2022). Recent Surface Rupturing Earthquakes along the South Flank of the Greater Caucasus near Tbilisi, Georgia. Bull. Seism. Soc. Am. XX, 1–19.

Tsereteli, N., et al. (2016). Active tectonics of central-western Caucasus, Georgia. Tectonophysics 691, 328-344.

How to cite: Shikhashvili, T., Mariam Bekurashvili, M., Giorgadze, A., Razmadze, A., Enukidze, O., and Alania, V.: Active triangle zones within the two orogens convergence zone, central Caucasus, Georgia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1278, https://doi.org/10.5194/egusphere-egu24-1278, 2024.

Our study area, Tbilisi urban area is located in the eastern Achara-Trialeti fold-and-thrust belt. The Achara-Trialeti fold-and-thrust belt, which is one of the good examples of collision-driven far-field deformations, is located within the northernmost part of the Lesser Caucasus orogen and is associated with Arabia-Eurasia convergence. Our interpretation has integrated seismic reflection profiles, several oil wells, and surface geology data to reveal the deformation structural style of the eastern Achara-Trialeti fold-and-thrust belt. Fault-related folding theories were used for seismic interpretation. Seismic reflection data reveal the presence of south-vergent and north-vergent fault-propagation folds, duplexes, and structural wedge. Interpreted seismic reflection profiles, structural cross-section, and recent earthquakes reveal the presence of an active blind thrust fault and structural wedge(s) beneath Tbilisi and the surrounding area. The structural model shows that the 2002 Mw 4.5 Tbilisi earthquake was related to a north-vergent blind thrust and the Kumisi and Teleti earthquakes are related to a north-vergent blind wedge thrust system.

How to cite: Bekurashvili, M., Shikhashvili, T., and Alania, V.: Structural architecture of the active eastern Achara-Trialeti fold-and-thrust belt (Tbilisi urban area), Georgia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1557, https://doi.org/10.5194/egusphere-egu24-1557, 2024.

In the Valsugana area (Southern Italian Alps) a NE-SW trending pre-permian Southalpine phyllitic basement is intruded by an Early to Middle Permian granitoids (Cima d’Asta,190 km2) and includes volcanic calderas of the Athesian Volcanic Group (125 km2) of the same age, and is locally covered by Upper Permian to Miocene sedimentary sequences. A complex system of faults juxtaposes these different geological domains. In particular, the Permian tectonic structures have been repeatedly reactivated during Mesozoic and Tertiary. The phyllitic basement, which suffered Variscan metamorphism and deformation, and the granitoids were dismembered by NNW-SSW and N-S tectono-magmatic faults associated with the opening of permian calderas. The main tectonic system of this area is the ENE-WSW oriented Valsugana fault system of Middle-Late Miocene age (Heberer et al., 2017 ). The master fault separates the metamorphic basement from the sedimentary sequences (e.g., M. Armentera, M. Civerone and M. Lefre). At the footwall of the master fault, other faults deformed in a compressive to transpressive regime the sedimentary sequences. Some of these are extensional faults were reactivated many times from Triassic to Lower Jurassic. The Valsugana fault system ends against the Permian to Mesozoic Calisio fault to the SW, while it continues to the NE towards Brocon and Cereda Passes (Gianolla et al. 2022). The Valsugana fault system is cut across by the Val di Sella fault of Late Miocene-Pliocene age, oriented c.a. E-W which deformed the northern walls of the Asiago Plateau (Barbieri & Grandesso, 2007), transporting slices of metamorphic basement and Permo-Mesozoic sequences to the north, over the Middle Miocene sandstones and marls of Valsugana. The most recent tectonic system consists of a set of NNW-SSE to N-S faults which cut across the ENE-WSW Valsugana and E-W Val di Sella fault systems. The N-S Grigno-Tolvà fault cuts across the Cima d’Asta magmatic complex from Val Vanoi to the north to Asiago Plateau to the south over XX km, and dislocates the Belluno and Val di Sella thrust faults at the footwall of the Valsugana fault system. All these faults are still seismically active.

Barbieri G. & Grandesso P. (2007) - Note Illustrative della Carta Geologica d'Italia alla scala 1:50.000, F. 082, Asiago.

Heberer B., Reverman R.L., Fellin M.G., Neubauer F., Dunkl I., Zattin M., Seward D., Genser J. & Brack P. (2017) - Postcollisional cooling history of the Eastern and Southern Alps and its linkage to Adria indentation. International Journal of Earth Sciences (Geologische Rundschau), 106:1557–1580.

Gianolla G., Caggiati M. & Riva A. (2022) - Note Illustrative della Carta Geologica d'Italia alla scala 1:50.000, F. 046, Longarone.

How to cite: Martin, S., Montresor, L., Zambotti, P., Monegato, G., Roghi, G., Piccin, G., Modesti, A., and Gosio, F.: The Valsugana area (TN, Italy): a structural knot, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1713, https://doi.org/10.5194/egusphere-egu24-1713, 2024.

The Kefalonia Transform Fault Zone (KTFZ) in central Ionian Islands (Kefalonia and Lefkada Islands), Greece, a dextral strike slip transform zone, exhibits the fastest rates of relative plate motion in the Mediterranean. Strong (M_w>6.0) earthquakes associated with the fault segments comprised in KTFZ are frequent as historical information and instrumental record reveals. Four main shocks of this order magnitude, among other earthquakes of M_w>5.0 occurred since 2003 on almost along strike adjacent fault segments, with high aftershock productivity extended far beyond the fault tips forming aftershocks zones remarkably wider than expected of strike slip faulting. This enhanced aftershock seismicity is associated with the shear deformation zone surrounding the mainshock rupture plane. It has been suggested that it results from the reactivation of secondary faults, its width decreases as a power law with cumulative fault displacement being narrower around mature faults than around immature faults. As the structural maturity may have a strong impact on earthquake behavior, such as magnitude, stress drop, distribution of slip, rupture velocity, ground motion amplitude, and number of ruptured segments, as prior studies have suggested, we aim at investigating the shear deformation zone along KTFZ. We used the relocated aftershock seismicity of the four recent (2003 – 2015) strong (M_w>6.0) main shocks that occurred on almost along strike positioned strike slip faults for defining the fault width in conjunction with the fault length, as we have already done in the case of the 2015 Lefkada main shock, and the activation of secondary faults, most probably triggered by stress changes and stress redistribution due to the main shock coseismic slip. Detailed slip models, when available, are engaged for this scope for investigating possible triggering of the secondary fault segments taking part in each seismic excitation, due to the stress transfer, taking also into account the stress sensitivity on the geometry and faulting properties of the minor faults, along with the accurately relocated aftershock seismicity. After the main fault width definition, we investigated the decrease of aftershock frequency and spatial density as a function of distance from the main fault.

Acknowledgments: Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or European Commission – Euratom. Neither the European Union nor the granting authority can be held responsible for them.

How to cite: Karakostas, V. and Papadimitriou, E.: The shear deformation zone along the KTFZ: implications for faulting properties, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1802, https://doi.org/10.5194/egusphere-egu24-1802, 2024.

The convergence between the Arabian and the Eurasian plates resulted in the development of the Greater Caucasus (GC) and the Lesser Caucasus fold-and-thrust belts, separated across most of their length by the Transcaucasian depression. The whole sub-horizontal shortening of the Caucasus was quantified at hundreds of kilometers and, according to several studies, reached its maximum rate in the Miocene-Pliocene. At present, convergence between the Eurasian and African-Arabian plates is still active, producing widespread deformation within the mountain belt and in surrounding regions, as testified to by seismological, paleoseismological and GPS data.

Understanding the active tectonics of the Caucasus is of paramount importance for a better assessment of geohazards, especially seismic hazard. Moreover, there is a major concentration of residents in Tbilisi, the capital of Georgia, hosting 1.2 million citizens, and in Baku, the capital of the Azerbaijan Republic, with over 2.3 million citizens; both cities are located in active tectonic basins at the southern foothills of the GC. Hundreds of rural villages are scattered in the mountain regions, and all were built without taking into consideration antiseismic criteria. All the above shows that the Caucasus and Transcaucasus regions are subject to an extreme seismic hazard and risk.

Here, we describe the active kinematics of the Greater Caucasus (territories of Georgia, Azerbaijan and Russia) through an integrated analysis of seismological, structural-geological and GPS data. Alignments of crustal earthquake epicentres indicate that most seismic areas are located along the southern margin of the mountain belt and in its north-eastern sector, in correspondence of major, activeWNW-ESE faults, parallel to the mountain range. Focal Mechanism Solutions (FMS) delineate dominant reverse fault kinematics in most sectors of the mountain belt, although swarms of strike-slip FMS indicate the presence of active transcurrent faulting, especially along the southeastern border of the Greater Caucasus. The mountain belt is characterized by dominant NNE-SSW-oriented P-axes. In the central-southern sector, in correspondence of the local collision between the Lesser and Greater Caucasus, P-axes are mainly NNW-SSE oriented. GPS data show dominant motions to the NNW, with rates increasing in eastward direction. All observations are consistent with a component of eastward escape of the central-eastern part of the Greater Caucasus.

How to cite: Pasquaré Mariotto, F., Tibaldi, A., Bonali, F. L., Corti, N., Pedicini, M., Gulam, B., and Nino, T.: Active Kinematics of the Greater Caucasus from Seismological and GPS Data: A Review, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2168, https://doi.org/10.5194/egusphere-egu24-2168, 2024.

 Discontinuities affect the Earth’s dynamics, yet the Earth is often represented in geodynamical models as a continuous material. The challenge of representing discontinuities in numerical models has been addressed in several ways in literature. The split node method, originally introduced by Jungels (1973) and Jungels and Frazier (1973) for elastic rheology and then modified by Melosh and Raefsky (1981) to simplify its implementation, allows the introduction of discontinuity into a finite element model by imposing an a-priori slip at a designated node, where the displacement depends on the element which the node is referred to. Originally, this method requires that the discontinuity’s geometry and slip are pre-established.

More recently, Marotta et al. (2020) modify this approach by introducing a coupling factor that indicates the percentage difference between the velocities of the element to which the slip node belongs, while the velocity consistently derives from the dynamic evolution of the system. However, this method still requires the pre-establishment of the discontinuity’s geometry.

We here present a new technique that enables the dynamic identification of the discontinuity’s during the thermomechanical evolution of the system, based on physical parameters and without predefining the slip or the geometry.

We have implemented a new algorithm that identifies one or more discontinuities in a finite-element scheme operating through two phases: nucleation and propagation. Nucleation involves selecting a yield physical property and identifying the potential slip nodes, i.e., nodes on which the chosen physical property exceeds a yield value. The nucleus is then identified as the potential slip node where the chosen property most exceeds the yield. Propagation can be performed by choosing between three approaches of propagation: single simple fault, multiple simple fault and single double fault; and three schemes for the identification of neighboring nodes: grid-bounded, pseudo-free and free. The resulting discontinuity is the line connecting the nucleus and the propagation nodes.  Once the discontinuity has been identified, a coupling factor is introduced and the algorithm continues to operate following the Marotta et al., (2020)’s scheme.

The results of several benchmark tests, performed through both simple and complex finite-elements models, confirm the success of the algorithm in recognizing yield conditions and introducing a discontinuity into a finite-element model and demonstrate the correctness of the propagation’s geometry.

References

Jungels P.H.; 1973: Models of tectonic processes associated with earthquakes. PhD thesis.

Jungels P.H., Frazier G.A. Frazier; 1973: Finite element analysis of the residual displacements for an earthquake rupture: source parameters for the San Fernando earthquake. Journal of Geophysical Research.

Marotta A.M., Restelli F., Bollino A., Regorda A., Sabadini R.; 2020: The static and time-dependent signature of ocean-continent and ocean-ocean subduction: The case studies of Sumatra and Mariana complexes. Geophysical Journal International.

Melosh H.J., Raefsky A.; 1981: A simple and efficient method for introducing faults into finite element computations. Bulletin of the Seismological Society of America.

How to cite: Fedeli, V. and Marotta, A. M.: A dynamic identification of continuous discontinuities in geodynamic numerical models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5338, https://doi.org/10.5194/egusphere-egu24-5338, 2024.

The Greater Caucasus orogen is a typical active double wedge orogen that accommodates the crustal shortening due to far-field effects of the collision between the Arabian and Eurasian plates. Our study area is a central part of the Greater Caucasus pro-wedge and represented by the Dzirula-Imereti Uplift zone and epicentral area of Racha 1991 (M_W=7) and 2009 (M_W=6) earthquakes - Southern Slope of Greater Caucasus. Here we present a new structural model based on interpreted seismic profiles and regional balanced cross-sections. Seismic reflection data reveals the presence of a basement structural wedge, fault-related folds, triangle zones, and active- and passive-roof duplexes. The most important observation from our study is that the Dzirula massif is a basement wedge and forms a triangle zone. The Dzirula structure is defined by mid-crustal detachment and a roof thrust, constituting a tectonic wedge with a passive back thrust. The regional balanced cross-sections show that the Racha earthquakes are related to crustal-scale duplexes.

Acknowledgments. This work was funded by Shota Rustaveli National Science Foundation (SRNSF) (grant# FR-21-26377).

How to cite: Alania, V., Enukidze, O., Kvavadze, N., Giorgadze, A., Merkviladze, D., Razmadze, A., and Xevcuriani, T.: Active crustal-scale duplexes and structural wedge in the central part of Greater Caucasus orogen pro-wedge, Georgia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6771, https://doi.org/10.5194/egusphere-egu24-6771, 2024.

The 2021 M_S 6.4 Yangbi earthquake occurred near the southwestern boundary of the Chuandian block in the SE Tibetan Plateau. Previous studies found that the mainshock occurred to the west of the main boundary fault – the Weixi-Qiaohou-Weishan fault, but the detailed seismogenic structure of the Yangbi earthquake sequence remains unclear. Accurate spatiotemporal characteristics of aftershocks and precise fault-zone structure are crucial for a more thorough understanding of the seismogenic structure and nucleation mechanisms of the Yangbi earthquake. Five days after the mainshock, we deployed a dense array with 200 short-period nodal seismometers spaced approximately 2 to 3 km in the vicinity of the Yangbi earthquake source region. In this study, we use the three-month-long continuous recordings of the dense array to study aftershock source parameters and fault-zone structures. We first obtained an earthquake catalog containing 88934 high-resolution event locations and 625 high-quality focal mechanisms. The entire aftershock sequence has a complete magnitude of -0.2, and a b-value of 0.97, with earthquake depths concentrated between 1 and 7 km below sea level. Combining precise aftershock locations and focal mechanisms, we constructed a detailed three-dimensional fault-zone structure model for the Yangbi earthquake sequence. The results reveal significant geometric variations in both strike and dip directions of the main fault. Along the dip direction, a conspicuous flower-like structure is present in the shallow part, transitioning to a listric structure inclined to the southwest at deeper levels. Along the strike direction, a noticeable bend exists in the central part. Several conjugate or intersecting faults are also present around the main fault. One of these faults, intersecting the main fault at a depth of about 4 km and dipping to the northeast, is likely the seismogenic fault of the largest foreshock. Additionally, below the main fault, there is another listric fault which appears to connect to the known Weixi-Qiaohou-Weishan fault at the surface. Our study provides new insight into the mechanism of the 2021 Yangbi earthquake sequence. The 3D fault-zone geometry can potentially be used for dynamic source modeling to better understand the initiation and rupture process of the mainshock.

How to cite: Zeng, X., Yu, C., and Zhang, G.: Complex fault-zone structure of the 2021 Yangbi Earthquake sequence revealed by dense array observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7731, https://doi.org/10.5194/egusphere-egu24-7731, 2024.

We present the main features of the frontal structure, known as Kur Fault, of the Plio-Quaternary Kura fold-and-thrust belt in the Greater Caucasus (Azerbaijan). The Kur Fault has been analysed thanks to geological-structural and geomorphological surveys of its whole length, integrated by a relocation of instrumental seismicity, data on historical seismicity, new focal mechanism solutions, and ambient vibration measurements across the fault trace. The in-depth study of the frontal structure can: i) provide insights into the shallow propagation of a regional reverse fault, ii) contribute to a better understanding of the earlier stage of development of a young continent-continent collision, and iii) have implications for seismic hazard assessment because the area is seismically active and hosts the most important infrastructure for energy production in the country. The results show that the fault deforms the surface for a total length of 123 km. The shallow expression is given by four main scarp segments, with a right-stepping arrangement, which have different structural significance; they are represented by an alternation of fault-propagation folds, folds with offset frontal limbs, and shallow faulting. Analyses of the age of deformed deposits and landforms suggest activity from Mid-Late Miocene times to the Holocene. The fault attitude and its reverse kinematics are coherent with the Holocene and present-day state of stress, characterised by a N-S to NNE-SSW horizontal s₁, suggesting the capability for seismic reactivation. Earthquake focal mechanism solutions indicate from pure reverse motions to transpressional kinematics in the area. Calculation of potential Mw indicates values in the range 7.5-7.9 if we consider its entire fault length, 6.1-7.2 if we consider the single segments.     

How to cite: Tibaldi, A., Bonali, F., Pasquaré Mariotto, F., Oppizzi, P., Tsereteli, N., Havenith, H., Babayev, G., and Pánek, T.: Structure, morphology and seismicity of the frontal part of a propagating fold-and thrust belt: The Holocene 123-km-long Kur Fault, Greater Caucasus, Azerbaijan , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10101, https://doi.org/10.5194/egusphere-egu24-10101, 2024.

The large amount of long-term fault slip rate measurements allows for determining the long-term tectonic moment rate for the central Apennines (Italy). Converting the long-term moment rate in seismicity rates requires a correct estimation of the long-term seismic coupling due to aseismic creep, possibly responsible for some of the observed dislocations. Ignoring this issue could induce a gross overestimation of the regional seismic hazard. Consequently, we must quantify the fraction of tectonic deformation released as earthquakes to correctly use the fault-based approach for forecasting long-term seismicity and assessing the seismic hazard of a region. To this aim, we introduce the seismic coupling in the calculation and treat it as a statistical variable. The probabilistic approach also considers the uncertainties in each parameter of active faults (depth of seismicity cutoff, length, and slip rate). The parameters of a regional Tapered Gutenberg-Richter distribution are determined to reproduce the observed earthquake size distribution. We found that about three-quarters of the long-term tectonic moment rate goes into earthquake activity, suggesting a non-marginal role of aseismic deformation for active faults in the central Apennines.

How to cite: Di Lorenzo, C., Carafa, M., Di Naccio, D., and Kastelic, V.: Fault offset rate and historic earthquake catalogue-based seismic coupling estimate for the central Apennines (Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10669, https://doi.org/10.5194/egusphere-egu24-10669, 2024.

Our study extensively explores the analysis of successive swarms and seismic sequences that followed the 2009 L'Aquila mainshock (Mw 6.3) in the southern-central Apennines of Italy—a region historically recognized for seismic events, some reaching ~M7.

The study area is characterized by active fault alignments well-documented in the geological literature. Bounded eastward by the Fucino-Marsicano-Barrea SW-dipping faults and westward by the southern termination of the Ernici NE NNE-dipping fault systems, it is also crosscut by the Villavallelonga-Pescasseroli–San Donato-Val Comino and the innermost right-stepping en-echelon Balsorano-Posta Fibreno sets, as well as the Sora fault.

Despite its active fault systems, the study area exhibits a low level of seismicity, prompting extensive investigation in previous experiments conducted by temporary seismic networks (Bagh et al., 2007; Romano et al., 2013) and more recent contributions by Frepoli et al. (2017). Notwithstanding, the paucity of seismicity, coupled with the deep geometry of faults and their association with historical earthquakes, remains a topic of ongoing debate in the literature.

Through the application of template matching techniques (Vuan et al., 2018) to enhance the available seismic catalog, we unveil previously undetected low-level seismicity with a completeness magnitude ML ~ 0.0. The space-time evolution of the intense seismicity, along with the characterization of each seismic episode, migration velocity, and Vp/Vs ratios, combined with the 3D distribution of seismicity and geological data, revealed both a clear tectonic influence and the role of fluids in seismic processes. This exploration illuminated previously unknown geometric aspects and provided the first evidence of the WNW-ESE deformation zone in the southernmost segment of the Villavallelonga fault at depths ranging from 11 to 15 km. Our results indicate that deeper seismicity (>16 km) suggests an ascending trend of possible mantle-derived CO2.

These findings significantly contribute: (1) to the understanding of tectonic seismic swarms in extensional domains, (2) to provide insights into fluid involvement in seismic processes, and (3) to the discussion of the seismotectonic setting in high seismic-risk areas. The acknowledgment of the value of microseismicity for regional seismotectonic studies serves as a compelling conclusion, underscoring the broader significance of these findings.

Bagh, S., Chiaraluce, L. et al. 2007. Background seismicity in the Central Apennines of Italy: The Abruzzo region case study. Tectonophysics, 444,80–92.

Frepoli, A, Cimini, G.B et al., 2017. Seismic sequences and swarms in the Latium-Abruzzo-Molise Apennines (central Italy): New observations and analysis from a dense monitoring of the recent activity, Tectonophysics, 312-329.

Vuan, A., Sugan, M., et al., 2018. Improving the Detection of Low‐Magnitude Seismicity Preceding the Mw 6.3 L’Aquila Earthquake: Development of a Scalable Code Based on the Cross Correlation of Template Earthquakes. BSSA 108, 471–480.

Latorre, D., Di Stefano, R., et al., 2023. An updated view of the Italian seismicity from probabilistic location in 3D velocity models: The 1981–2018 Italian catalog of absolute earthquake locations (CLASS): Tectonophysics, 846, 229664.

Romano, M. A., de Nardis, et al., 2013. Temporary seismic monitoring of the Sulmona area (Abruzzo, Italy): A quality study of microearthquake locations. NHESS, 13, 2727–2744.

How to cite: de Nardis, R., Vuan, A., Carbone, L., Talone, D., Romano, M. A., and Lavecchia, G.: Dual Role of Fluids in Activating Extensional Seismicity at Middle and Lower Crust Depth: the Deformation Corridor Throughout the Southern-Central Apennines (Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11665, https://doi.org/10.5194/egusphere-egu24-11665, 2024.

The present-day tectonics of the northernmost Africa region is dominated by the oblique convergence
between the Nubia and Eurasia plates initiated 35 Ma ago. GNSS-derived velocities indicate that the
relative plate motion of only ~5 mm/yr is accommodated within a wide region involving inland
and offshore tectonic structures, from Morocco to Tunisia and from the Mediterranean Sea to the Sahara
platform. Due to limited and sparse geodetic measurements, the main zones of strain accommodation as
well as the strain partitioning between thrust and strike-slip faults remain unresolved. However, despite
the low strain budget, significant seismicity and destroying earthquakes have been recorded in the region
with five M>6 events nucleated on both inland and offshore faults during the last decade.

To better identify the active inland faults and constrain their interseismic behavior for a better seismic
hazard assessment in northernmost Africa, we used multi-temporal InSAR analysis to produce the first
regional-scale interseismic velocity map of the region. The primary data consists of up to ~8 years of
SAR imagery from the Sentinel-1 satellite constellation for 6 tracks in both the ascending and descending
orbits. The data processing and the generation of InSAR-based time series describing the spatio-temporal
evolution of surface deformation were performed using the New Small Baselines Subset processing chain
(NSBAS, Doin et al. 2011). We followed a three-blocks processing strategy leading to (1) interferograms
generation, (2) phase unwrapping, and (3) time series estimation. Within the first block, interferometric
networks combining image pairs with short and long temporal baselines were defined to mitigate potential
bias introduced by changes in soil properties (e.g., snow, vegetation growth, dunes). Before unwrapping,
interferograms flattening, multi-looking, and atmospheric phase screen (APS) corrections based on the
ECMWF ERA-5 atmospheric model were implemented. The resulting time series of surface displacement
along the line-of-sight direction (LOS) for the 2014-2022 period were decomposed into the near-vertical
and horizontal (E-W) components and expressed into a Eurasia-fixed reference frame using the GNSS
velocities in Billi et al. (2023).

Our estimated deformation maps reveal multi-scale present-day motions, with large- and small-scale
signals suggesting tectonic origin and ground response to anthropogenic activity or landslides, respectively.
The oblique plate convergence involves the interseismic loading of a series of E-W oriented right-lateral
strike-slip inland faults. Between the longitudes ~3°W and ~9°E, this inland deformation is localized
within a 25-75 km wide zone consistent with a unique linear strike-slip fault without any clear uplift related
to thrusting on already mapped transfer structures. The rates and directions of surface displacements
suggest that the shortening component of the Nubia-Eurasia relative plate motion is almost entirely
accommodated by offshore tectonic structures which has an important impact on the assessment of
seismic and tsunamigenic hazards.

How to cite: Viltres, R., Doubre, C., Doin, M.-P., and Masson, F.: Active deformation within the slow-straining northernmost Africa region constrained by InSAR time-series, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12862, https://doi.org/10.5194/egusphere-egu24-12862, 2024.

Our goal is to comprehend fluid circulation within fault zones and its impact by monitoring hydrochemical and hydrophysical aspects across multiple dimensions. We also monitor nearby micro-seismicity and surface deformation. MAGIC (Multidimensional Active fault of Geo-Inclusive observation Center) is located at the Chihshang creeping fault, situated at the boundary of tectonic plates in eastern Taiwan. This fault exhibits aseismic creep at a rate of 2 cm per year, alongside high seismic activity.

The earthquake sequence of 2022 began with a magnitude 6.4 event on September 17 (local time), followed by a magnitude 6.8 earthquake the next day. The epicenters were within the Central Range fault system, positioned to the west of our monitoring network. This presents a significant opportunity to observe co-seismic changes in pore pressure within the footwall and fault zone.

The findings revealed that after the initial earthquake in the adjacent fault system, the pore pressure within the footwall swiftly decreased by nearly 100 cm within 15 hours. Simultaneously, the pore pressure within the fault zone increased by approximately 25 cm during the same period. Subsequently, the footwall's pore pressure continued to decrease with the second earthquake and then slowly recovered. These alterations in pore pressure suggest that fractures underwent both opening and closure during stress migration.

How to cite: Mu, C.-H., Fu, C.-C., Kuochen, H., Chen, K.-H., and Chen, K.-W.: Monitoring earthquake-induced pore pressure changes in the creeping fault system triggered by the 2022 Chihshang earthquake sequence, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13981, https://doi.org/10.5194/egusphere-egu24-13981, 2024.

The 2017 Pohang earthquake (M_L 5.4) ranked as the second-largest instrumental earthquake in Korea and the most destructive seismic event. Before this event, the absence of documented instrumental seismic activity and no mapped Quaternary faults near the epicenter raised questions about the paleoseismic history of the region. This study aims to gather and trace evidences of the paleoseismic ruptures along the surface projection of seismogenic fault, reported by previous study, and interpret their implications. To achieve it, we conducted comprehensive paleoseismological investigations, including geomorphic analysis, fieldwork, drilling survey, trench excavation and numerical age dating. Through geomorphic analysis and drilling survey, we identified two lineaments: NNE–SSW-striking Fault-1 and NE–SW to NNE–SSW-striking Fault-2. In the excavation site of fault-1, stratigraphic features and numerical ages indicate that the PE event occurred between 11±1 ka and 2.6±0.1 ka, and then the MRE event activated after 0.17±0.01 ka.  On the other hand, the combined results of two outcrops of Fault-2 show that the MRE and PE of Fault-2 could be constrained to have occurred between 148±7 ka and ca. 40 ka and around 200 ka, respectively. Our findings present that even before the 2017 Pohang earthquake, seismic events causing surface ruptures of moderate to large magnitude have occurred at least three times in this area during the late Quaternary.

This work was supported by a grant (2022-MOIS62-001) of National Disaster Risk Analysis and Management Technology in Earthquake funded by Ministry of Interior and Safety (MOIS, South Korea).

How to cite: Lee, S., Cheon, Y., Han, J., Ha, S., Choi, J.-H., Seong, Y. B., Kang, H.-C., and Son, M.: Paleoseismic records around epicenter of the 2017 Pohang earthquake, SE Korea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14319, https://doi.org/10.5194/egusphere-egu24-14319, 2024.

In the Central Apennines (Italy), the project RETRACE-3D provided a reliable 3D model of the crust in the area affected by the 2016-17 Amatrice-Visso-Norcia seismic sequence, highlighting that the coseismic rupture at the surface can involve old inherited normal faults while the seismogenic sources lay at depth, possibly reactivating and inverting previous thrust faults, as in the case of the Mw 6.5 Norcia earthquake (30 october 2016). Here we present a 2D gravity model across the Central Apennines, spanning from the Tyrrhenian coast to the Adriatic Sea, aimed at completing and verifying the crustal geometries resulting from the 3D model itself. The cross-section was built integrating different types of data, such as surface geology, hydrocarbon wells, seismic lines, and results from receiver function analysis. It was then checked against gravity anomalies and the velocity distribution from Local Earthquake Tomography (LET), adding further details, and, finally, against seismicity recorded during the 2016-2017 sequence. The results substantiate the reliability of the geometries proposed in the RETRACE 3D model, as they fit well, except for some local misfits, with the other independent data, such as the Bouguer anomalies and the velocity distribution from LET. Furthermore, the integration of different types of data allowed us to describe in detail the structural setting of the Apennine chain also in the surroundings of the RETRACE study area, where the cross-section length exceeds the 3D model, and to add some new elements at seismogenic depths, that exceed those typical of hydrocarbon exploration. In particular, we were able to investigate the nature of the basement top and its relationship with seismotectonics.

How to cite: Tiberti, M. M., Maesano, F. E., Buttinelli, M., De Gori, P., Ferri, F., Minelli, L., Di Nezza, M., and D'Ambrogi, C.: Multidisciplinary geophysical approach to investigate the deeper portion of the Central Apennines (Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15305, https://doi.org/10.5194/egusphere-egu24-15305, 2024.

The Caucasus region, which is one of the good examples of collision-driven far-field deformations, is located in the northernmost part of the Arabia/Eurasia collision zone and is classified with moderate seismic activity. Racha region, a part of Greater Caucasus orogen, in particular is characterized with the highest activity. In recent year several moderate earthquakes were observed in the region, notably 2009 September 7 (Mw = 6.0), 2011 August 18 (Mw = 4.8), and also 2024 January, 2 (Mb = 4.4), with depths varying from 10 to 15 km. The strongest event during the instrumental period recorded in Racha was observed in 1991 April 29, with magnitude Mw=7.0 and Depth = 17.2 km. Which was the largest earthquake recorded in the region, causing casualties. This event caused a number of fore- and aftershocks. Following the Racha 1991 earthquake temporary, dense seismic network was installed around the area to study aftershock activity, around 2000 aftershocks of different magnitudes were recorded by this network in total. This data was analyzed in several papers (for example Triep, et al., 1995, Fuenzalida, et al., 1997), including hypocenters and focal mechanisms. Depths of the aftershocks were observed ranging from 0 to 15 km. The mechanism of Racha 1991 main shock was shown to be reverse faulting on a gently sloping (35°) plane. Also, strong earthquake mechanisms show reverse faulting with strike-slip components. This work was oriented to analyze existing earthquake data and see its correlation to a 2-3D structural model created for the area.

Acknowledgments. This work was funded by Shota Rustaveli National Science Foundation (SRNSF) (grant# FR-21-26377).

How to cite: Kvavadze, N., Alania, V., and Enukidze, O.: Earthquake data analysis in the Racha region, Georgia: Implication for 3D structural model active faults, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16075, https://doi.org/10.5194/egusphere-egu24-16075, 2024.

We present the findings of a multimethodological study made on the Budoia-Aviano Thrust System conducted as part of the NASA4SHA PRIN Project “Fault segmentation and seismotectonics of active thrust systems: the Northern Apennines and Southern Alps laboratories for new Seismic Hazard Assessments in northern Italy”.

The NW dipping, SW-NE striking Budoia-Aviano Thrust System represents the southeast-verging external front of the Plio-Quaternary Eastern Southalpine Chain, occurring at the front of the western Carnic Pre-Alps, between Polcenigo and Montereale (PN).

The recent activity of the Budoia-Aviano thrust is evidenced by numerous geological and morphostructural features, including the exposure of Mio-Pliocene reliefs emerging from the Last Glacial Maximum alluvial plain, the upwarping of the LGM fan of the Artugna stream, and geomorphic anomalies of the surface hydrographic network (Poli et al., 2014).

Moreover, to identify sites to perform excavation aimed at exploring the Holocene movements of the thrust system, we performed multiscale geophysical investigations, such as Deep Electric Resistivity Tomography, Electric Resistivity Tomography and Ground Penetrating Radar across the entire thrust system. They revealed the presence of a series of possible north-verging shear planes, whose trace seemed to correspond to the more pronounced up-ward convex profile of the Artugna alluvial fan.

Based on geophysical results, two NNW-SSE paleoseismological trenches were excavated along selected GPR profiles. The trenches exposed late LGM-to-Holocene alluvial fan deposition of the Artugna stream. Notably, a paleosoil embedded in alluvial sequence yielded a ¹⁴C age interval between 16 Ky and 5 ky BCE.

In both trenches, the entire excavated late Quaternary succession was affected by a set of north-verging reverse planes, coinciding with the discontinuities identified in GPR profiles. In each trench, the total vertical displacement measured across all thrust planes is of about 4.5 meters, resulting from at least three displacement events occurred in the last 7,000 years. Moreover, the involvement of the bottom of the ploughed soil supports the hypothesis of the backthrust being activated during the M_W 6.3 Alpago-Cansiglio earthquake (Rovida et al., 2022) as suggested by Galadini et al. (2005).

The integration of paleoseismology, photogrammetry, and high-resolution geophysics enabled the construction of a detailed 3D model of the Budoia-Aviano Thrust System, revealing a 20-meter-wide deformation zone on the hanging wall of the main south-verging Budoia-Aviano Thrust. In the perspective of seismic hazard assessment and regional planning, it is worth to consider the proximity of this active and capable fault to industrial complexes, urban centres, and vulnerable structures characterizing the Budoia and Aviano area.

REFERENCES

• Galadini, M.E. Poli, Zanferrari, A. (2005). Seismogenic sources potentially responsible for earthquakes with M≥ 6 in the eastern Southern Alps (Thiene-Udine sector, NE Italy). Geophysical Journal International, 161(3).
• M.E. Poli, G. Monegato, A. Zanferrari, E. Falcucci, A. Marchesini, S. Grimaz, P. Malisan, E. Del Pin (2014). Seismotectonic characterization of the western Carnic pre-alpine area between Caneva and Meduno (Ne Italy, Friuli). DPC-INGV-S1 Project.
• Rovida, M. Locati, R. Camassi, B. Lolli, P. Gasperini, A. Antonucci (2022). Catalogo Parametrico dei Terremoti Italiani (CPTI15), versione 4.0. Istituto Nazionale di Geofisica e Vulcanologia (INGV). https://doi.org/10.13127/CPTI/CPTI15.4

How to cite: Patricelli, G., Poli, M. E., Falcucci, E., Gori, S., Paiero, G., Rizzo, E., Marchesini, A., and Caputo, R.: First evidence of Holocene activity and surface diplacement of the Budoia-Aviano Thrust System in north-eastern Italy, unravelled through the integration of geological, geophysic and paleoseismological analyses, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16527, https://doi.org/10.5194/egusphere-egu24-16527, 2024.

There is a growing interest among seismologists on the nonlinearity of seismic processes, corresponding to the synergetic approach to the development of the Earth as a nonlinear, open, self-organizing system. In the analysis of very complex multi-component geological-geophysical systems, it is important to assess the degree of their nonlinearity based on a set of control parameters. For the analysis of the seismic process, we propose to use seismic-tectonic stress in the Earth's crust and lithosphere as a control parameter. Also, in the analysis of processes releasing tectonic stresses in the form of earthquakes, the synergetic of choosing the initial state becomes important.

To choose the initial state of the Earth's crust and lithosphere, we consider them as an oscillatory block-hierarchical system, interconnected and developing over time but differing in various layers of the Earth's crust – sedimentary, consolidated crust, lower crust, and mantle beneath the Moho boundary, as each layer has its lithological properties and is in different physical conditions, at least due to the change in pressure. Block sizes are determined based on the thickness of the layer, and vertical planes of the block are determined based on the morphology of the layer, as well as the presence of a fault inside the block. Changes in the seismic-tectonic deformation vector indicate a change in the initial state of seismic-tectonic deformation of each block. Thus, for the analysis of the nonlinear seismic process, we obtain a continuous series of changes in seismic-tectonic stresses in different layers and for each block of the layer. Thanks to the known time of deformation changes, as well as the known size of each block, we have an approximation of the nonlinear oscillatory process in the block-hierarchical system of the Earth's crust and lithosphere, manifested as seismicity. Within the designated time interval, the seismic-tectonic deformation process in each block follows a linear law. We have tested our methodological approaches to the analysis of the nonlinear seismic process in the example of the Crimean-Black Sea region. We have processed earthquakes contained in the new catalog of seismic events for the period 1970-2012, recalculated according to the Seismological Bulletins of the USSR and Ukraine. The method of Aki was used to determine the seismic-tectonic deformation regime by constructing averages foci by the signs of the first arrivals of P-waves taken from the bulletins. Time series of changes in seismic-tectonic deformation regimes in different layers of the Earth's crust and lithosphere show that each layer has its peculiarities of deformation but fits into the general context of tectonic processes in the region. One of the interesting features of the tectonic process in the region turned out to be the unexpected variability of regimes and their duration from 2 to 10 years in different areas of the Crimean-Black Sea region. An important consequence of our study is the definition of clear spatial-temporal frameworks for the analysis of the amounts of seismic energy release in each of the blocks, which will improve the quality of seismic risk assessment for individual regions.

How to cite: Shumlianska, L. and Vilarrasa, V.: Approximation of nonlinear seismic processes based on space-time series of changes in seismotectonic deformation regimes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-641, https://doi.org/10.5194/egusphere-egu24-641, 2024.

Characterizing the structures of buried faults is inherently challenging due to a lack of data. However, the seismic survey has demonstrated the potential to reveal geometrical features of faults in the subsurface. In this study, we utilized three-dimensional seismic data and its associated attributes of variance, edge detection, and azimuth for investigating the distribution and structural characteristics of the buried faults in the Bohai Basin, eastern China. The results indicate: (1) there are two 7-km NNE strike-slip carbonate faults (F1, F2, in the Figure) dominated in this region, with fault sub-systems of horst and graben, stepped combinations, and "Y"-shaped; (2) F1 fault is approximately 6.5km with a fault damage zone of about 0.75km in width, and F2 fault is about 7.5km with a fault damage zone of 1.0km in width; (3) The width of fault damage zones increases from south to north, with increasing fault displacement. The maximum fault displacements of F1 and F2 are estimated at 420m and 700m, respectively. We argue that fault displacement leads to the growth of fault damage zones, which potentially controls the evolution of fault architecture. The geometrical information from the subsurface may provide crucial insights for understanding the fault mechanisms and associated earthquakes.

Figure 1. (A) Seismic attribute map of edge detection and time structure showing the two carbonate faults (F1, F2); (B) Measured fault displacement and damage zone width along the fault striking direction of F1 and (C) F2.

How to cite: Yang, N. and Liao, Z.: Geometrical characteristics of buried fault damage zones in the Bohai Basin, eastern China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9839, https://doi.org/10.5194/egusphere-egu24-9839, 2024.