Traces of Strong Eneolithic and Medieval Earthquakes Hitting the Durankulak Archaeological Settlement in Northeastern Bulgaria

Abstract

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.