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We conducted archaeoseismological studies at the Djanavara Monastery Complex in 2019. It consists of early medieval buildings of different ages, the main of which was the church. The church and the atrium were first badly damaged and then destroyed. There were attempts to repair them, as evidenced by the laying of a doorway in the north wall of the atrium. The age of this seismic event may coincide with the destruction of the “second” Episcopal Basilica of Varna, also built in the second half of the 5th century CE. This earthquake took place several decades later. Judging by the kinematic indicators in the building structures, the source of seismic movements was apparently located north-northeast of the Episcopal Basilica of Varna and, accordingly, of the Djanavara Monastery Complex. In this direction, the largest seismotectonic node is located at the intersection of sublatitudinal faults with the Shabla-Kaliakra seismogenic zone. Subsequent renovation of the Djanavara Church and construction of poor-quality buildings around it took place after this seismic event. However, even in these, later, walls, we see traces of younger seismic deformations, evidenced by numerous buttress walls attached to the walls that survived the second earthquake. Judging by the buttress walls attached to the original meridional walls from the east and west, the seismic movements of the second earthquake propagated along the east–west axis. The third seismic event put an end to the activity of the monastery complex. It can be seen that the repaired low-quality walls were again deformed: the later masonry that covered the doorway moved outward, and there was also a joint deformation of the meridional wall and its buttress. The westward movement of the latest masonry testifies to the source of seismic oscillations of this time, located west of the Djanavara Complex. However, the meridional walls show clockwise rotation of their parts, while a perpendicular wall was rotated counterclockwise. Such a systematic deformation presupposes the location of the seismic source not strictly to the west of the monastery complex, but to the southwest, which coincides with the direction to the source, which we determined for the deformations in the episcopal basilica of Varna. Some researchers believe that the Djanavara Monastery Complex was destroyed and finally abandoned in 614–615 CE during the Avar–Slavic invasion. However, there are materials indicating that residents left ancient Odessos even before the attack, most likely due to a strong (third in our case) earthquake, during which the entire city was engulfed in fires and many buildings were destroyed. We were unable to accurately estimate the local seismic intensity for each of the three described ancient earthquakes, however, such significant deformations as systematic inclination and shifting of walls, as well as rotations of their parts around the vertical axis, indicate that the intensity of seismic oscillations during the studied earthquakes reached IL ≤ 9 on the MSK-64 scale. The intensity of seismic oscillations may have been enhanced by unfavorable soil conditions at the site of the construction of the Djanavara Monastery Complex.

Archeoseismological studies of the ruins of the Djanavara monastery complex (Varna, Bulgaria) have been carried out. The complicated character of the investigated destruction required different methods of archeoseismological investigation. Their description constitutes the first part of the paper. Different types of seismic deformations in building structures caused by surface ruptures and displacement of building elements along them are considered, as well as secondary deformations, i.e., when destruction and damage in building constructions occurred due to strong seismic motions produced by a distant source. We discuss the possible interpretations of the peculiar destruction types as kinematic indicators in modern buildings and archeological objects. We provide examples of different types of destruction and collapses of constructions, the tilting and rotation of building elements, as well as displacements without rotation. The possibility of locating the earthquake epicenter based on the data on orientations of collapses and methods of dating and seismic event parameterization are also discussed. The results of applying these methods in order to study the ruins of the Djanavara Cloister complex will be given in the second part of the paper.

Our archeoseismological investigations in the Episcopal basilica in Varna have revealed signs of seismic deformations in its walls: numerous shifts, tilts, and rotations of the wall parts around the vertical axis, as well as subvertical through-going joints cutting several stone blocks. A cluster of (at least) three seismic events occurred for a hundred years (from the early 5th to the early 6th centuries AD). Judging by kinematic indicators in the basilica structures built of high-quality stone blocks, the source of seismic motions was located north-northeast of the basilica. Strong seismic oscillations propagated apparently from the southwest. The seismic sources were along seismically active faults localized north and south of Varna. The local seismic intensity of seismic oscillations could reach Il ≤ 9 on MSK-64 scale. The intensity of the seismic oscillations increased due to unfavorable ground conditions at the location of the basilica. These data should be taken into account when assessing the seismic hazard assessment of Varna to build a new map of seismic zoning of northeastern Bulgaria.

We have conducted paleo- and archaeoseismological surveys in the Durankulak Eneolithic archaeological settlement in northeastern Bulgaria, identifying many earthquake-related deformations of different ages. The ubiquitous presence of counterfort walls added to damaged original (nearly north–south) walls on the eastern side provides evidence of the first significant earthquake in Durankulak that seems to have occurred during Middle to Late Eneolithic time (4650–4100 B.C.). This seismic event made the north–south walls tilt westward, so that the ancient residents had to build counterfort walls to prevent the original walls from collapsing. However, the upper excavations in the archaeological settlement show that the next, later, seismic deformation induced a tilting and collapse of remains of the original walls eastward, producing a gap of a few tens of centimeters between the original walls and the counterfort walls. Afterwards the settlement was abandoned. It may be surmised that the later earthquake was stronger than the earlier one, which left room for reconstruction work. The local seismic intensity of the earlier earthquake seems to have been at least Il ≥ VIII, while the later event had Il ≥ IX on the МSК-64 scale. Considering that the masonry of both walls, the original and the counterfort ones, is about the same age, the time interval between the two seismic events must have been short, one or two hundred years. Nevertheless, the epicenters of both of these earthquakes were to different directions from Durankulak. The seismic motion (compressive waves and shocks) went from the west during the first earthquake and from the east during the second. The rupture zone of the first seismic event was obviously related (1) to an earthquake-generating zone in continental Bulgaria situated west of the archaeological settlement in question, or (2) to the Intramoesian active fault that was responsible for the large earthquake of 1444. The rupture zone of the second seismic event was nearby, in the coastal part of the Black Sea; the responsible feature seems to have been the Shabla–Kaliakra seismogenic zone. Considerable seismic deformations were also identified in structures on the southern slope of the island in a dwelling dating back to the First Bulgarian Kingdom (the 9th century A.D.). This seismic event produced distortions in the walls (in map view), sigmoidal patterns, as well as fractures and rotations in the walls. The local seismic intensity due to this earthquake is supposed to be at least Il ≥ VIII. The rupture zone of a third seismic event was likely to be nearby, in the coastal part of the Black Sea, and the responsible feature must have been the Shabla–Kaliakra seismogenic zone.

The ancient city of Gorgippia is surrounded by well-known (North Black Sea, Kerch, and South Azov) seismogenic zones, all of which are submarine, and only some of their branching faults reach the surface in the coastal zone. On the surface, in the vicinity of the ancient city, there are also seismically active faults and flexure rupture zones, which can generate strong earthquakes. Our investigations in the ancient city of Gorgippia have revealed a number of seismogenic deformations and destructions in its walls: systematic tilts and shifts of the upper parts of the walls, as well as their arc-shaped bends in plan view; breaks of the walls; rotations of walls and other building elements around the vertical axes; and squeezing of the mouths and shafts of water wells. Some of the mentioned deformations could be formed during the strong historical earthquakes in the 3rd and 1st centuries BC, as well as those in the 1st and 3rd centuries AD, and all these seismic events were accompanied by local destructions and fires. Summarizing all materials on historical earthquakes of the Taman Peninsula, the regional seismic potential can be determined by the possible occurrence of a crustal earthquake with M ≥ 7.0 and average recurrence interval of once in few hundred years. Seismic shaking in the source zones of such natural events can be I0 = IX or even more.

Correlation studies have been made between the gravitational field (Bouguer anomalies - G) and the magnetic field (common vector (T) and the vertical component dZ) for the territory of Bulgaria. The map data is in digital format, which makes it easier to calculate. The correlation itself is estimated from -1 to 0 and from 0 to +1, which means that a positive correlation is possible in the case of increased values of both parameters and vice versa - decreasing values are associated with reduced values. This forms a positive correlation. A negative correlation is observed when both parameters have minimum values. Zero correlation means - NO correlation (both parameters are min- max or max-min). The calculations from the maps give a clear picture of the positive and negative correlations and the ability to calculate areas with significant positive and / or negative correlations The results might be used for the interpretation of the studied potential fields in case of structural, regional and / or local changes in the gravity and magnetic fields, and their respective sources.

The EWS (Early Warning system) is a new technology applied in the advanced and prone to the earthquake risk countries. According of the new project SIMORA new methodology is performed to the seismogenic structure and a seismotectonic model of the region is developed. As the activated fault was not known (so called “blind faults”) deep geophysical complex investigations have been applied. It includes detailed gravity and magnetic fields measurements, new interpretations, comparison with the known and unknown seismic active structures and detailed study of the space-time seismicity patterns. Due to the applied research together with more detailed additional field measurements by electric resistive methods, radioactivity measurements and magnetic susceptibility profiling is expected to make a clear view of the local and regional geophysical fields. After the model is established the new approach for the selection of the strong motion devices locations is executed. Field measurements about the seismic noise (anthropogenic or natural) are performed. The spectral characteristics and amplitude levels are clarified according to the international requirements about reliability. The devices are intended to be put around the seismic source. In parallel such devices can serve as an instrument of the early warning to the local authorities and population.

Several models with illustrative pictures are presented covering different seismic possible explicit hazardous situations in Bulgaria: -Vrancea seismic source and distances model -Vrancea seismic source and differences in travel times P and S waves model -Pernik city source kinematics model - distances and travel times of the seismic waves The reliability of the models and obtained results strongly depends on several factors: -The exact positions of the seismogenic faults -The locations of the ground strong motion devices -The kinematics of the travel times of P and S waves -The triggering levels of the device sensitivities -The velocity of data transfer and the reliability of the communication links, etc. The legislation issues and the levels of the warnings need further development due to the sensitivity of the public to the “true and false” alarms. All described issues are under investigations in this paper.

The tsunamigenic sources of nonseismic origin are investigated. The relevant sources for the Black Sea (except earthquakes) might be: - underwater slides - meteorological impact (sashes) - meteorites The relevant sources about tsunamis in the Baltic Sea might be: - meteorological impact (storm surges) - meteorites - fast ferries (anthropogenic impact) Many examples and modeling results are presented illustrating the above mentioned nonseismic tsunami sources with quantitative characteristics of the modeled waves.

The problems of the geodynamic regime in the Aegean active zone are under investigation many years now. A number of authors ( Papazachos, 1966 ; Caputo, 1970 ; Artemjev, 1984 ; Ranguelov, 1987 , etc.) have suggested different hypotheses on the development of this complicated - from a geodynamic point of view – regional zone. Our hypothesis is based on the generated two models, for the North part (so called NASDM) and for the South subducted part (called SAASZM) and their interaction newly compiled and presented by us. The combined actions of the forces of the South subducted zone (SSZ) and the main element of the North part – North Anatolian Transform Fault (NAF) and their interactions, generate, in our opinion, the main geodynamic and geomorphologic peculiarities of the whole zone. The extensional areas are followed by areas with the compression regime, which suggests a very complicated recent geodynamic regime. All assumptions are based on the observed phenomena reflecting the recent development of the investigated zone (such as seismicity – expressed by the strong and week earthquakes, GPS measured plate movements, geomorphologic expressions observed on the relief, etc.). The main conclusion is that such a model can explain most of the observed structures and geodynamic facts in the region: strong earthquakes occurrence and their complicated positions and mechanisms, the alternation of the compressional and extensional areas from south to north, aseismic zone north of Crete, the recent volcanism expressed in the volcanic arc, the big curvature of the subducted part of the Aegean zone between the African and the Eurasian plates, the locations of many geomorphologic features, etc.

The paper examines the possibility that tsunami event recorded in the western Black Sea on 7 May 2007 was triggered by a traveling atmospheric disturbance. As meteotsunamis are favored by specific synoptic conditions, we inspected available ground and sounding observations, European Centre for Medium‐Range Weather Forecasts reanalysis fields, and satellite‐based products and compared them to the documented Mediterranean meteotsunamis. We found an atmospheric disturbance traveling toward 30° (NNE) with amplitude of 2–3 hPa and propagation speed of about 16 m/s, passing through few tens of kilometers wide pathway over the region affected by the tsunami. This disturbance occurred in the lower troposphere, but it was capped by instable convective cell that preserved gravity disturbance's coherence over a region at least 150 km long. An ocean modeling study showed that such a disturbance is capable of generating large tsunami waves and strong currents over the shallow regions, following the observations over the region where maximum sea level oscillations have been documented. Therefore, this event has a potential to be classified as a meteotsunami, the first of such kind in the Black Sea.

[1] A tsunami-like sea disturbance of nonseismic origin was observed on 7 May 2007 on the Bulgarian Black Sea coast. The maximum wave amplitude was three meters, and the sea oscillations lasted for several hours. The anomalous behaviour was seen in many places of the coast of Bulgaria and was stronger in the north. Several accounts of eyewitnesses as well as reports of local port authorities and three tide-gauge recordings are available and have been collected and analysed. The tsunami could have been produced by a submarine landslide or by atmospheric pressure pulses. Here, we explore the former hypothesis, i.e., the possibility that the tsunami was induced by a landslide that may have occurred on the shelf margin off Bulgaria. By making use of numerical modelling we find that submarine mass movements taking place within a certain delimited source area off Bulgaria may have generated tsunamis compatible with the observations.