Incorporating faults as independent seismic sources in Probabilistic Seismic Hazard Assessment (PSHA) significantly influences the ground motion values when compared to classical zoning methods (CZM). This practice holds particular relevance for populations situated top or near active faults. Some hybrid methods (HM) and fault-based methods, implemented in Spain, show that Peak Ground Acceleration (PGA) values increase in the vicinity of the fault traces. In some cases the hazard levels may double, consistent with the PGA observed in recent earthquakes (Rivas-Medina, A., 2018; Gómez-Novell O., 2020). Despite the existence of methods that combine zones and faults in seismic sources characterization, there is a lack of allocation of seismic potential between these two types of sources. Additionally, there is an increasing use of geological data since seismic catalogs alone are insufficient to fully characterize the seismic potential of faults. This limitation becomes particularly evident in slow deformation zones, such as southeastern Spain, where the recurrence period of faults exceeds the temporal coverage of the seismic catalog.

The present investigation addresses two key aspects: how to quantify the geological information of the faults and transfer it to recurrence models, and how to distribute the seismic potential of the region between faults and zone. This research contemplates two steps. In the first step, four methods were applied: 1) Moment rate-based method, 2) Slip rate-based method, both proposed by Bungum (2007); 3) the hybrid method developed by Rivas-Medina et al. (2018), which considers both zone-type and fault-type sources; and 4) SHERIFS, a fault system-based assessment proposed by Cartier et al. (2019). In the second step, Faults and Area Moment Sharing (FAMS) (Ornelas-Agrela et al., 2022*) is applied. This new method enables characterizing the faults based on their associated seismicity, improving the distribution of seismic potential between faults and zones. The five methods were applied to multiple seismogenic zones within southeastern Spain, recognized as one of the most seismically active areas in the country. An analysis was conducted, highlighting the sensitivity of the results of PSHA implementation. The preliminary results of the FAMS method application are presented.

_{* first presented by Ornelas-Agrela, A. et al. at the Iberfault2022 congress in Teruel, Spain.}

How to cite: Ornelas-Agrela, A. F., Gamboa-Canté, C., Benito, M. B., Rivas-Medina, A., and Quirós-Hernandez, L.: Review of methods that implement active faults characterization for PSHA in Southern Spain. Preliminary results of the application of the FAMS method., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18523, https://doi.org/10.5194/egusphere-egu24-18523, 2024.

The Indian plate is currently underthrusting beneath the Tibetan plateau along the Main Himalayan Thrust (MHT). The strain is getting accumulated during the inter-seismic period on MHT due to this ongoing convergence. The accumulated strain energy is released in a stick-slip fashion on MHT causing destructive earthquakes. Several strong earthquakes within the Himalayan zone have been reported in previous studies such as 1897 at Shillong (Mw 8.2), 1905 at Kangra (Mw 7.8), 1934 at Bihar-Nepal (Mw 8.1), 1950 at Assam (Mw 8.4), 2005 at Muzaffarabad (Mw 7.6), 2015 at Gorkha (Mw 7.8), and 2015 in Afghanistan (Mw 7.6). Based on the distribution of recent and historic earthquake activity in the Himalayas, three seismic gaps can be distinguished from west to east: the Kashmir Seismic Gap (west of the 1905 Kangra earthquake), the Central Seismic Gap (between the 1905 and earthquakes), and the Assam Seismic Gap (between the 1897 and 1950 earthquakes). The major objective of this study is to investigate the hinterland region in the NW Himalaya, which is relatively less explored in terms of its Active Tectonics & Paleoseismology as compared to the deformation front along the Main Himalayan Thrust (MFT). The Nahan salient and Kangra re-entrant are characterised by a complicated pattern of fault distribution which has been proposed in previous studies to get activated, either individually or alongside the Main Frontal thrust, during past large to great-magnitude earthquakes. A detailed tectono-geomorphic map has been prepared using the Cartosat-1 dataset, and geomorphic markers suggestive of recent tectonic activity have been identified in the study region. An exhaustive fieldwork has been conducted, during which a total of 13 samples were collected for Optically Stimulated Luminescence (OSL) dating and Carbon dating to identify signatures of past earthquakes and assess the possibility and degree to which the 1905 earthquake has affected the region. Based on the detailed tectono-geomorphological and paleoseismological investigations, we will be able to assess regional seismic hazards.

(*Author RH aka Rukhshar)

How to cite: Husain, R. and Malik, J. N.: Active Fault and Paleoseismological studies in the hinterland region of NW Himalaya, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1056, https://doi.org/10.5194/egusphere-egu24-1056, 2024.

In this study, we present scaling relationships between fault lengths, fault slip-rates and historical seismicity for an active normal fault system, seismically accommodating crustal extension within the upper plate of the Ionian subduction zone (southern Italy). This crustal extension is confirmed by historical seismicity and instrumental geodesy, with GNSS-derived values of horizonal deformation within a range of 2-3 mm/yr throughout Calabria and the Messina Strait region. We collated data for fault slip-rates, fault lengths and historical earthquakes for a given fault to explore whether fault slip-rates are correlated with fault size and their geometric moment.

We present new results showing a robust correlation between fault lengths and fault slip-rates, which supports the idea of a relationship for a given fault between fault slip-rates and the geometric moment.

We discuss our results in terms of how these correlations should be used if regional deformation is accommodated by localised strain on faults mostly arranged along strike rather than distributed strain on multiple faults across-strike. For instance, we compare our empirical correlation between fault lengths and fault throw-rates over the Middle-Late Pleistocene in Calabria and the Messina Strait with those from Central and Southern Apennines over the Holocene, characterized by strain distributed on multiple faults across-strike and strain localised on faults mostly arranged along-strike, respectively.

Tectonic and seismic hazard implications are discussed for future investigations based on fault slip-rates, fault size and historical seismicity.

How to cite: Meschis, M., Roberts, G., Sgambato, C., Michetti, A. M., Livio, F., Mildon, Z., Faure Walker, J., Iezzi, F., Robertson, J., Gattuso, A., Barberio, M. D., Randazzo, P., and Caracausi, A.: Empirical scaling correlations between fault lengths and fault slip-rates in seismically-active extensional regions: The Calabria and Messina Strait region (southern Italy) as case study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5155, https://doi.org/10.5194/egusphere-egu24-5155, 2024.

The Alboran Sea is one of the most seismically regions of Western Europe, accommodating an important part of the current NW-SE convergence between the African and Eurasian plates (4-5 mm/yr). Despite the Alboran Sea is considered as a region of relatively low tectonic deformation and diffuse seismicity, the major faults within have been responsible of large earthquakes (I_EMS>IX) since historical times (e.g., 1522 Almería, 1790 Oran, 1804 Alboran, 1910 Adra or 2016 Al-Idrissi earthquakes). One of the main issues for the characterization of the seismic hazard in the Alboran Sea, common to many low-deformation regions, lies in accurately constraining the seismic parameters that define fault activity and their behavior (i.e., slip rates, earthquake recurrence and multi-fault rupture capability, among others). This issue is further aggravated by the fact that these faults are mostly located offshore, making their investigation more challenging. As a result, most faults in the Alboran Sea have poor slip and activity rate estimates, while their capability to interact and rupture in complex rupture patterns has not been explored yet. In this study, we compute several models of earthquake rupture rates for the Alboran Sea with the SHERIFS code and using a systematic parameter exploration tree to determine the parameters of each model. We base the exploration tree on the slip rate and multi-fault rupture scenarios, allowing us to investigate the epistemic uncertainty linked to these parameters. To check the feasibility of the computed earthquake rates of each model, we compare them with the observed seismicity rates in the region. As a result, this enables us to identify which parameter combinations best match the recorded seismicity, prioritizing those that perform better for the hazard assessment. In addition, these optimal values might be used as indicators for further studies focused on better constraining the fault parameters in some faults, as well as for preliminary fault-based seismic hazard assessments. By extension, the limitations of the modelling in terms of slip rate budget distribution and fault rupture scenarios can also be used to determine which areas should be prioritized for further research. We expect that the results of our work enhance discussion among researchers working in the area and motivate further investigations into fault dynamics.

How to cite: Perea, H., Gómez-Novell, O., Jiménez, M. J., García, M., Lozano, L., García-Mayordomo, J., Sánchez-Roldán, J. L., Canari Bordoy, A., and Martínez-Loriente, S.: Exploring the uncertainties of the fault source parameters in the Alboran Sea for seismic hazard using earthquake rate modelling tools, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7698, https://doi.org/10.5194/egusphere-egu24-7698, 2024.

This research examines the influence of static Coulomb Stress Transfer (CST) in shaping the seismic cycles associated with the Central Apennine Fault System (CAFS), an active tectonic region that witnessed numerous destructive seismic events over the last millennium.

We selected 15 seismic events in the CAFS, dating from 1279 CE to the present, all exceeding Mw 6.0. Out of these, 9 events were specifically chosen for CST analysis, based on their spatial-temporal proximity to later activated faults. The study not only investigates static stress transfer for each event, but also considers the cumulative CST from recent earthquakes, providing a holistic view of the current stress environment. Following a novel approach, we utilized a three-dimensional fault modeling technique, with ellipses representing the 2D geometry of faults at depth. This approach accounts for strike variations and employs a variable strike three-dimensional elliptical model for enhanced precision in CST calculations.

Our case studies within the CAFS suggest that CST might have been a critical factor in either triggering or inhibiting fault activities. Instances of fault reactivation following high stress transfers and scenarios showing the dampening effects of stress shadows were observed. This intricate understanding of CST has practical implications, offering insights into potential future earthquake patterns and aiding in devising targeted risk mitigation strategies.

The complexity of CAFS reveals intricate stress patterns emerging from the interplay of different seismic episodes. These patterns of stress lobes interact in complex ways, influencing adjacent faults by amplifying, neutralizing, or diversifying their CST impacts. Through detailed analyses and cutting-edge modeling techniques, our study provides valuable insights for future research directions and practical approaches to seismic risk reduction. It underscores the strong impact of CST on shaping a region's seismic history and highlights the necessity of ongoing research in this vital area of geosciences.

How to cite: Valentini, G., Volatili, T., Galli, P., and Tondi, E.: A Millennial Perspective on Coulomb Stress Transfer Impact in the seismicity of the Central Apennine Fault System, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7956, https://doi.org/10.5194/egusphere-egu24-7956, 2024.

Earthquakes on normal faults cause negative Coulomb stress transfer (CST) onto receiver faults located across-strike, which, in theory, delays or prevents rupture. Nevertheless, earthquakes on such across-strike faults are frequently observed. This study explores why, and how, large earthquakes can be triggered on faults repeatedly receiving negative coseismic CST. Two triggering mechanisms are hypothesised: (1) Positively stressed patches or segments of faults, resulting from heterogeneous stress transfer, act as triggers to rupture earthquakes on faults that are on average negatively stressed. (2) Negative coseismic CST is compensated by interseismic loading and other processes building up positive stress on the source faults. To test both hypotheses, a 400-year earthquake sequence is modelled, located in the fault network of the Western Anatolian Extensional Province (SW Türkiye). The fault network features multiple active faults located along-strike and across-strike of another, as well as faults in a variety of orientations, suitable to explore stress-triggering mechanisms in a structurally complex setting. Detailed information on fault location, geometry, and mechanism is compiled from field investigations, literature review, and geodetic data. Based on instrumental and historical earthquake catalogues, and the suitability of the source fault network, a sequence of earthquakes is determined to investigate the two hypotheses by comparing the effects of coseismic CST and interseismic loading.

Results show that, out of 28 modelled large (MW ≥6) earthquakes, 6 were triggered on faults receiving significant negative coseismic CST. For five of these, negative coseismic CST is compensated by processes increasing CST. Only for one studied example, highly stressed positive fault segments on an otherwise negatively stressed fault could have been the driving mechanism leading to rupture of a large earthquake. Given all model uncertainties, stress-heterogeneities cannot be validated as a probable triggering mechanism for faults in the stress shadow of neighbouring faults. In contrast, earthquakes on normal faults located across-strike of another can only delay, but in most cases not prevent failure, as interseismic loading usually exceeds negative coseismic CST.

To reinforce these results, the impact of the modelled fault geometry and slip rates, used to calculate interseismic loading, is evaluated. Models of strike- and dip-variable faults, following the actual surface fault traces, are compared with simplified, planar fault models. Simplified models feature exaggerated areas of positive and negative CST on source faults prior to earthquakes, essentially distorting the stress field and causing stress-heterogeneities that are less pronounced in more realistic models. This observation highlights the necessity of modelling fault geometry as realistic as possible, especially when models are used in fault-based SHA. The impact of slip rates on model results is less drastic, so long as slip rates are used that are determined on similar time scales as the model duration. For the studied fault network short-term (geodetic) and long-term (‘geologic’) slip rates vary from the ‘Holocene’ slip rates by an order of magnitude. If used for modelling interseismic loading, the stress state and recurrence intervals of faults would be drastically under- or overestimated.

How to cite: Diercks, M., Mildon, Z., Boulton, S., Hussain, E., Alçiçek, C., Aykut, T., and Yıldırım, C.: Fault interactions and Coulomb stress-triggering in complex fault networks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10216, https://doi.org/10.5194/egusphere-egu24-10216, 2024.

The Northwestern Himalayas have been host to many earthquakes, with the recent 1905 Kangra Mw 7.8. Studies suggest the seismic gap in the Northwestern Himalayas to be more than Mw 7.8 in general, and of about Mw 8.4 in the Nahan region of the Northwestern Himalayas. There are studies suggesting the rupture of the Himalayan Frontal Thrust (HFT) and hinterland subsidiary faults by the Earthquakes in the region. In this study, we focused the Khetpurali Taksal Fault (KTF), which is one of the corroboration. It is an out-of- sequence ~ 250 km long dextral strike-slip fault with an NNW-SSE trend. KTF, which is bounded to the west by the Nahan Salient, and in the east by the Dehradun re-entrant; marks the boundary between the Central Himalayas (convergence rate ~ 18 ± 1 mm/y and obliquity ~0°) and the Northwestern Himalayas (convergence rate ~13.6–14 ± 1 mm/yr and obliquity ~ 15°-30°), which runs through the ~ 100km locked width of the Main Himalayan Trust (MHT). The dextral strike-slip motion has caused the displacement of some quaternary deposits along the KTF. It plays a key role in the slip partitioning between the active thrust and the oblique faults with the HFT displaying the thrusting and oblique component in the Pinjore Garden fault, Jhajra fault, and Barsar fault of the same region in the Northwestern Himalayas. This study is focused on the active fault along the KTF. We prepared the geomorphic map and delineated the extent of the KTF using the high-resolution Cartosat-1 data. Our studies show the presence of displaced terraces, lateral offset of streams, sag ponds, and pressure ridges along the KTF. A displacement of 250m to 1350m has been observed. Samples from the displaced terraces are analyzed by OSL dating technique to find out the slip along the KTF. Understanding the slip along KTF will enhance the understanding of slip partitioning taking place in the Nahan Region as a whole, which will help understand the geodynamics of the region and thus in seismic hazard assessment.

How to cite: Narayana, P. and Mailk, J. N.: Tectono-Geomorphic Studies along the Khetpurali Taksal Fault, Northwestern Himalayas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1045, https://doi.org/10.5194/egusphere-egu24-1045, 2024.

The study of normal fault-generated landforms, such as fault scarps, is commonly performed to investigate fault evolution and the recurrence and magnitude of earthquakes. The Amorgos region (Cyclades, Greece), located in the central part of the Aegean Sea, is structured by ~70km large NE-SW normal faults accommodating the back-arc extension of the Hellenic arc and the Anatolian extrusion. These faults are able to generate large earthquakes such as the Amorgos event (Ms=7.8) on July 09, 1956, followed by a second shock (Ms=7.2) 12 minutes later. This destructive event was the largest Mediterranean earthquake of the 20th century and caused severe damage, especially on Santorini Island. It also triggered a tsunami with reported run-ups reaching locally 30m along the southern coast of Amorgos Island. The submarine Amorgos fault, structuring the island’s southern coast and cumulating a ~2 km high vertical offset, is suggested to be the source of the 1956 main shock and tsunami. However, the accurate position of the 1956 rupture and the magnitude of the slip at surface are unknown, as the fault outcrops at 700m below sea level, as well as the pace at which this fault breaks. Considering that normal faults frequently accommodate the deformation on multiple splays, and within their damage zone, we searched whether the onland faults found within the cumulative scarp of the Amorgos fault ruptured during the 1956 event. We first performed a morphological study of the Chozoviotissa fault segment with satellite imagery, Structure-from-motion modelling, and field observations. We found evidence of recent deformation along this fault, in particular a ~70 cm high fresh ribbon at the base of the fault scarp. To provide chronological constraints, we sampled along-dip the carbonate-rich fault scarp for TCNs (Terrestrial Cosmogenic Nuclides) dating using the chlorine-36 element. This paleoseismic approach provides new insights on the recent slip history of this secondary fault, which is important to better evaluate the activity of the Amorgos fault system and improve the hazard assessment of the archipelago.

How to cite: Palagonia, S., Leclerc, F., Larroque, C., Benedetti, L., Feuillet, N., Nomikou, P., Henriquet, M., Guillou, V., and Manta, F.: Deciphering the recent activity of normal faults in the Hellenic Volcanic Arc from combined morpho-tectonics analysis and TCNs method dating (36Cl), Amorgos Island (Greece) , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15619, https://doi.org/10.5194/egusphere-egu24-15619, 2024.

Understanding the source processes and wave propagation in heterogeneous rock media is one of the most challenging frontiers to improve the seismic risk assessments in densely populated areas. In this framework, the sector of the Voltri Massif (NW Italian Alps) forming the hinterland of the city of Genoa is a natural laboratory to investigate (i) the interaction between rock faulting and fluid circulation during (potential) paleo-seismic activity and (ii) the detection, location, and source characterization of micro-earthquakes along tectonic lineaments developed inland and offshore the city area (i.e., in the Ligurian Sea). Our multi-scale and multidisciplinary study is part of the PNRR research program RETURN (“Multi-risk science for resilient communities under a changing climate”): it will include the structural and petrographic characterization of fault rocks (i.e., serpentinite breccias), the quantification of serpentinite carbonatization and its impact on the fault strength, and the analysis of the network of inland-offshore tectonic lineaments. This work, coupled with the analysis of historical seismic clusters, is crucial to identify suitable areas for the deployment of high-resolution seismometers and for tracing the spatial-temporal evolution of micro-earthquakes and their static and dynamic source parameters.

The detailed structural mapping of selected fault zones has revealed a complex, multi-stage deformation history, with older ductile structures (paragenesis: antigorite + ilmenite ± chlorite ± pyrite ± chalcopyrite, likely ascribed to the alpine-subduction and collision stages) cut by steeply dipping fault planes NNE-SSW striking. These latter are subparallel with the (low magnitude) seismic clusters detected in the area and develop multiple, anastomosed fault cores consisting of serpentinite-rich ultracataclasites, locally bound by chrysotile-rich shear bands. The faults damage zones textures (e.g., breccias and microbreccias), the paragenesis of newly formed shear bands and associated veins (chrysotile + chlorite) and the orientation of these faults (NNE-SSW striking, subparallel to the Miocene-age lineaments detected in the Gulf of Genoa) suggest recent tectonic reactivation at the regional scale.

Future developments of the research project will include more detailed, high-magnification microscopy of selected samples (e.g., raman, field emission SEM, EBSD and microprobe), regional scale morphotectonic characterization by satellite image analysis, and integration of field and seismic data. This will clarify the link between inland-offshore tectonic lineaments and the (low magnitude) seismicity of the area.

How to cite: Locatelli, M., Crispini, L., Scambelluri, M., Federico, L., Spallarossa, D., Morelli, D., and Cianfarra, P.: Analysis of active and fossil seismic structures: a multidisciplinary study for the seismic risk assessment in low-seismicity regions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16169, https://doi.org/10.5194/egusphere-egu24-16169, 2024.

The January 10, 2022, MS 6.9 Menyuan, China, earthquake occurred caused by strike-slip faulting in the tectonically complex region of the northeastern Tibetan Plateau, and ten people were injured in Menyuan City. The September 8, 2023, MS 6.9 Morocco, earthquake occurred in the African Plate at shallow depth with oblique-reverse faulting. At least 2,900 people were killed and more than 5,000 injured in Morocco till to September 13, 2023. International media reports of such kind of disasters by the Morocco earthquake only resulted from poor building structure design and low-solidity housing, such as in Marrakech, southwest of the epicenter. The surface wave magnitude (MS) of the two earthquakes is the same, and the moment magnitude (MW) and energy magnitude (Me) of the Menyuan mainshock are slightly lower than those of the Morocco event. Although the scalar moment and radiated seismic energy from Morocco dynamic rupture are only 2~3 times of the Menyuan earthquake, the density of urban residents nearby and around the epicenter of the Morocco mainshock is at least more than a hundred times higher than that around the epicenter of the Menyuan even. For the Morocco sequence, the USGS reported the number of aftershocks higher than MW4.0 is only seven and the largest is 4.9. In contrast, there are two aftershocks higher than MS5.0 in the Menyuan sequence recorded by the China Earthquake Networks Center, 5.1 and 5.2, respectively. Normally, a similar magnitude does not reflect the equivalent seismic moment, release of radiated energy, and the occurrence of strong aftershocks. Meanwhile, devastating loss of life and injuries are not only due to the design of the building and the quality of the house.

Acknowledgment: This research is supported by the Spark Program of Earthquake Sciences (XH22012YC)

How to cite: Meng, L., Zang, Y., and Xie, M.: The Differential of Casualties, Energy Radiation, and Characteristics of Sequences from the Same MS: The Menyuan MS 6.9 2022 and Morocco MS 6.9 2023 Earthquakes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3159, https://doi.org/10.5194/egusphere-egu24-3159, 2024.

The efficiency of an earthquake to cross a barrier can be evaluated based on geometric and frictional properties of faults, and specific seismic parameters such as the stress drop during an earthquake. Numerical modelling of seismic cycles allows to generate thousands of seismic events and to explore the effect of the physical properties with respect to a barrier effectiveness criteria. The probability of an event passing a barrier can thus be evaluated on the basis of this barrier effectiveness criteria. Such approach has been used for frictional barriers and fault bends. In this study we focuses on earthquake fault jumps which has been observed on multiple occasions such as the latest 2024 M7.5 earthquake in Japan. We use the quasi-dynamic algorithm VEGA, which numerically simulates seismic cycles of 2D fault networks and is based on rate and state friction. The problem is simplified to two planar faults separated by a gap. Among other parameters, we explore the effect of the overlap, distance and angle between the two faults. The loading of a fault network can be done in multiple ways. We thus explore the impact on the dynamics of sequences of earthquakes either from a far-field stress loading or from imposing a back slip rate loading on each fault. We also look at the effect of adding creeping - velocity strengthening - sections at the borders of the faults. We finally compare our results with the statistics of jump probabilities from published observed seismic events. Our study allows for a rapid assessment of thousands of earthquake scenarios and is a promising approach to facilitate the integration of earthquake physics into seismic hazard.

How to cite: Michel, S., Scotti, O., Hok, S., Bhat, H., khairdast, N., Almakari, M., and Cheng, J.: Evaluating probabilities of earthquake fault jumps from 2D numerical simulation of seismic cycles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18480, https://doi.org/10.5194/egusphere-egu24-18480, 2024.