Plamena Raykova

Bulgaria, situated in the eastern part of the Balkan Peninsula, is located within the Alpine–Himalayan seismic belt, characterized by high seismic activity. Earthquake activity in Bulgaria and the adjacent region reflects the ongoing geodynamic processes affecting the Balkan Peninsula, where seismicity exhibits significant spatial and temporal variability. In this study, a macroseismic dataset for shallow earthquakes that occurred in and near Bulgaria since the beginning of the 20th century was analyzed. The dataset includes events with moment magnitudes (MW) ranging from 5.2 to 7.6 and observed intensities from VII to X according to the European Macroseismic Scale (EMS-98). The spatial attenuation of macroseismic intensity was modelled using an elliptical approach, taking into account the regional characteristics of earthquake sources. The results demonstrate a strong correlation between observed and modelled intensities, with correlation coefficients of r > 0.9 and R2 > 0.8, confirming the reliability of the proposed attenuation model. The proposed model provides a consistent representation of macroseismic effects and their spatial distribution for strong earthquakes in the region. The proposed approach is applicable to seismic hazard and risk assessment in major urban areas of Bulgaria, particularly Sofia and Plovdiv. The results provide valuable input for seismic risk mitigation, urban planning, infrastructure assessment, and engineering decision-making.

The uplift of the Himalayan Mountains is a result from the continental collision between the Indian and Eurasian plates. The region near the India-Eurasia plate boundary has a history of frequent and significant seismic activity. Over the past century, numerous earthquakes of magnitude 6 and above have occurred in the study area. On January 7, 2025, a magnitude 7.1 earthquake was generated the southern Tibetan Plateau, north of the boundary between the Eurasian and Indian plates. The earthquake and the fault are located along the plate boundary, precisely where the eastward rotation of a microplate occurs. To analyze the displacements caused by the January 7, 2025, earthquake, we processed Sentinel-1 TOPS SAR images from two different viewing geometries. A total of eight radar images were used, acquired from three overlapping ascending and descending orbits (ascending: orbit 12; descending: orbits 48 and 121) of the Sentinel-1A satellite, operated by the European Space Agency (ESA). The detected Line-of-Sight (LoS) displacements were decomposed into vertical displacement and the horizontal (west-east component) of deformation. We tested several focal mechanism solutions from different data centers to estimate the vertical coseismic displacement using the Okada model. To better match the InSAR measurements and accurately describe the displacement field seen in the DInSAR maps, we modeled the fault as a five-segment structure. This approach shows good agreement with the DInSAR results, and the estimated eastward and vertical displacements closely match those derived from the Okada model.

Strong earthquake with moment magnitude MW7.5 hit the city of İzmit, Turkey with devastating effect in August 1999. The earthquake occurred in the North Anatolian Fault (NAF),and was followed by an intensive aftershock activity. It is assumed that the studied earthquake is a part of a seismic sequence that started in 1939 along the North Anatolian Fault. Over a period of 60 years or more, a series of large earthquakes moved progressively from east to west by the fault. The analysis of the space-temporal pattern of seismic clusters (foreshocks and aftershocks) distribution of the 1999 Turkey earthquake, is presented in the research. The properties of aftershock sequence (distinct cluster in space and time) allow for a time-dependent prediction of aftershock probabilities. Aftershock activity can sometimes cause more destruction than the main earthquake, due to factors such as radiation pattern, location and the cumulative nature of building damage. If the distribution of aftershocks over time is considered as a non-stationary Poisson process, the maximum likelihood method is used to estimate the parameters of the modified formula of Omori - power-low decay. A transformation from the time scale t to a frequency-linearized time scale τ is applied to test the correspondence between different statistical models and the occurrence of an aftershock sequence. As a final step of the study, a criterion - Akaike’s information criterion is used to determine the best model for the temporal distribution of aftershocks. The results of the study show that the precursor aseismic gap before the August 1999 MW7.5, Turkey earthquake is not observed. The aftershock sequence shows a complex temporal pattern - one secondary aftershock sequence with a different value of p – parameter.

Background: Earthquakes are the deadliest natural disasters affecting the human environment. Global seismic risk and vulnerability to earthquakes are increasing steadily as urbanization occupy more areas that a prone to effects of strong earthquakes. Purpose: Successful preventive measures and planning of post-earthquake activities are based on scenarios of expected damage, destruction and casualties from predicted strong seismic impacts. Methods: The study presents the comprehensive and advanced earthquake hazard assessment for the city of Blagoevgrad. The earthquake hazard for the city is estimated using both probabilistic and deterministic methods. The ground-shaking representations for the city of Blagoevgrad are produced through a GIS, by combining: active faults capable of generating strong earthquakes; location and geometry of the scenario earthquake source with assigned magnitude; an appropriate GMPE’s, consistent with the seismotectonic features of the region; and account soil amplification effects using geological zonation of the considered urban area. Results: The deterministic and probabilistic estimates of maximum acceleration vary across the city in the range of 0.26 g - 0.45 g for MW7.2 earthquake and from 0.23g to 30.40 g for the 475- year return period respectively. The highest acceleration and velocity values are in the central and south western parts of the Blagoevgrad city. Conclusion: The results of the study show that most of the Blagoevgrad territory is classified at the highest seismic hazard level - PGA larger than 0.3 g and PGV more than 24 cm/s. The results from the study will assist decision-makers involved in local and state government to manage potential damage and loss of life from strong earthquakes occurred in and near the city of Blagoevgrad.

We apply statistical analysis to study the temporal pattern of earthquake occurrence in the aftershock sequence of the 2023 Romania MW 5.4 earthquake. The temporal clustering of aftershocks is a dominant non-random element of the seismicity, so that, when the clusters are removed, the remaining activity can be modelled as a Poisson process (as first approximation). The maximum likelihood method was used for estimating the parameters (K, c and p) in the modified Omori formula for the aftershock sequence. The goodness of fit between the aftershock occurrence and different statistical models has then been tested using a transformation from the time scale t to a frequency-linearized time scale τ. It is assumed that on this time axis the aftershocks are distributed in time as a standard stationary Poisson process if the choice of the parameters K, c and p is correct. We further used the Akaike Information Criterion as a measure for selecting the best among competing models. The aftershock sequence of the February 14th, 2023, MW 5.4 Romania earthquake is best modelled by the combination of one ordinary and one secondary sequence.

Earthquakes are some of the deadliest natural disasters affecting human environment. The kinematic and dynamic parameters of earthquakes are the basis of almost all research in the Earth sciences. The results of theoretical and applied seismology are used to explain the main social and economic problems associated with earthquakes and their consequences. Knowledge of the earthquake source parameters enables us to understand better the recent tectonic setting and earthquake generation processes. One usable method for estimating source parameters of earthquake is the spectral analysis of seismic waveforms. The earthquake source parameter – seismic moment, M0, has the largest practical and scientific application. From M0 seismic moment magnitude MW (that is a measure of the energy released by an earthquake) can be computed. In the present study, we aim to improve the scaling between MP (local magnitude applied in Bulgarian seismological routine practice) and MW. We extend the MW estimates to lower magnitudes via a spectral fitting method. Applying P-wave displacement spectra we estimate MW for 127 small to moderate earthquakes with magnitude ranging between 2.0–5.3 which occurred in Bulgaria and surroundings from 2008 to 2024. The considered earthquakes are recorded at stations of Bulgarian Seismological Network – NOTSSI. As a result, we propose a new empirical scaling relation between local magnitude MP and seismic moment magnitude MW. Additionally, 148 MW estimates from the seismological centres – ISC, EMSC and Harvard are used. The well-defined relation between MW and MP can be accurately used for compiling homogeneous, with respect to magnitude, earthquake catalogue for Bulgaria.

An earthquake is the most destructive natural phenomenon, with its sudden onset and rapid spread impacting large areas. Among the various geohazards, seismic ones dominate in terms of their social and economic effects on human life and the urban environment. In the present study, a deterministic earthquake ground motion scenario for the city of Sofia (the capital of Bulgaria) is presented. The earthquake risk posed to Sofia is quantified by considering the city’s seismic context, which contributes to its hazard and risk. The assessment of seismic hazards and the generation of earthquake scenarios make up the first step of seismic risk evaluations, as risk reduction strategies can only be developed with a better understanding of these threats. Risk assessment and its associated management comprise the most effective approach to estimating the impact of seismic hazards on the city of Sofia, which exhibits high seismic activity. Our findings provide a basis for local governments to review their susceptibility and preparedness. The consideration of earthquake scenarios in the creation of policies for seismic risk reduction will allow us to focus on the prevention of earthquake effects rather than on the activities following these disasters.

The devastating earthquake with moment magnitude MW7.8, (local magnitude ML=7.5) which occurred in the East Anatolian Fault Zone, westwards of Gaziantep, Turkey on February 6, 2023 was followed by an intensive aftershock activity. Studying time distribution of aftershocks is important for understanding physics of the earthquake generation process. In the present study temporal pattern of aftershock sequence of the 2023 MW7.8 Turkey earthquake is analyzed. Because of the factors such as location and radiation pattern and the cumulative nature of building damage, aftershocks can cause more damage than the main shock. The temporal clustering of aftershocks is a dominant non-random element of the seismicity, so when the clusters are removed, the remaining activity can be modelled (as first approximation) as a Poisson process. On the assumption that aftershocks are distributed in time as a non-stationary Poisson process, we use the maximum likelihood method for estimating the parameters (K, c and p) of the modified Omori formula. For testing the goodness of fit between the aftershock occurrence and different statistical models, a transformation from the time scale t to a frequency-linearized time scale τ is applied. Akaike's information criteria is used to select the best model for the temporal distribution of aftershocks. It has been found that temporal distribution of the 2023 Turkey earthquake aftershocks is dominated by the classic power law decay in time and suggests the existence of secondary aftershock activity.

Among natural disasters, earthquakes dominate with regard to their social and economic effects on human life and the urban environment. Appropriate risk reduction strategies can be developed only with a better understanding of seismic hazards as well as the population and building growth rates in the corresponding region. In the present study, deterministic and probabilistic earthquake (seismic) ground-motion scenarios are developed and presented for one of the most earthquake-prone cities in Bulgaria, the city of Plovdiv. We apply a method for integrating the spatial distribution of seismic hazards with human exposure to assess hazard impacts. Earthquake scenarios and social vulnerability metrics are combined in a geographic information system (GIS) to identify the spatial vulnerability of the exposed population to seismic risk and the locations for targeting mitigation actions in areas with high exposure and vulnerability levels. The results of our study show that most of the territory of the city is classified at the highest seismic hazard level: PGA 0.2. High and major levels were estimated for the population exposure index (PEI) for nearly 60% of the residential buildings in the city (inhabited by approximately 96% of the Plovdiv population). The spatial distribution of the results is available in GIS format and can be used in scientific and practical applications to reduce the risk and consequences of possible strong earthquakes in future.