Deformations in the Episcopal Basilica in Varna: Evidences of Strong Historical Earthquakes in Northwestern Bulgaria

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.