Seismic Deformations in the Archaeological Sites of Varna, Bulgaria

• Published in: Izvestiya. Physics of the solid earth Vol. 58; no. 1; pp. 127 - 147
• Main Authors: Dimitrov, O. V., Vacheva, K. Z., Korzhenkov, A. M., Kouzov, Chr, Larkov, A. S., Ovsuchenko, A. N., Ranguelov, B. K., Rogozhin, E. A., Strelnikov, A. A.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01-02-2022; Springer Nature B.V
• Subjects: Archaeological sites; Archaeology; Building damage; Cathedrals; Construction; Cracking (corrosion); Cracking (fracturing); Deformation; Destruction; Direction; Earth and Environmental Science; Earth Sciences; Earthquake damage; Earthquakes; Entrances; Fortresses; Geological hazards; Geophysics/Geodesy; Hazard assessment; Historic buildings & sites; Historic sites; Historical buildings; Houses; Seismic activity; Seismic hazard; Soil; Stone; Subsidence; Bulgaria; Roman thermae; fortress wall; Odessos; Varna; archaeoseismology; kinematic indicators; gate; medieval period; Bulgaria; seismic deformations
• ISSN: 1069-3513; 1555-6506
• DOI: 10.1134/S1069351322010013

—The ruins of Odessos are located on the territory of Varna, Bulgaria. Archaeoseismological studies carried out at the archaeological sites of ancient Odessos revealed some significant damage and destruction in the ancient building constructions. These are numerous breaks, tilts, protrusions and turns of wall parts, longitudinal asymmetric deformation of the above-entrance arch, lateral in plan bending of the extended wall, outward knock-out of the stone blocks and intensive cracking. We attributed traces of subsequent shoddy repairs, which sometimes involved using single expensive stone blocks taken out of the ruins, as circumstantial evidence of strong earthquakes that had occurred here. Interestingly, medieval builders already knew about the possibility of strong earthquakes in the region and put wooden longitudinal beams in the walls to absorb vertical seismic vibrations. The degree of destruction and damage in medieval buildings built on loose fill soils increased due to subsidence. Archaeological research established 6 periods of construction in ancient Odessos: Using this periodization, we calculated the age of past seismic events: (1) around the 1st century A.D.—the destruction of the Late Hellenistic fortress wall; (2) the early 5th century—the early 6th century (a cluster of at least 3 strong earthquakes, judging also by deformations in the Bishop’s Basilica of Odessos (Minchev et al., 2020)): dismantling of the fortress wall, discontinued use of both thermae. The gate of Odessos remains in operation after repairs. There is low quality construction of houses. Accumulation of tile debris; (3) the destruction of the Orthodox church in the 1444 earthquake. We were able to determine the average direction of the maximum aggregate seismic movements that destroyed the large and small thermae, as well as the excavated segment of the fortress wall and the Odessos gate complex. They propagated along the WNW-ESE axis, most likely from the Shabla-Kaliakra seismogenic zone. The deformations of the 13th-century Orthodox church were caused by seismic vibrations that propagated in the submeridional direction from one of the seismically active faults south or north of Varna, although they may have been affected by ground subsidence. Thus, as a result of the studies, we were able to detect traces of yet unknown strong earthquakes, to determine their age and direction to the ancient epicentral zones. The intensity of seismic vibrations during all these earthquakes was
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= VIII—IX MSK-64. The data obtained can be used to improve the seismic hazard assessment of Varna and NE Bulgaria.