Ring-Shaped Seismicity Structures in the Northern Balkan Region: Possible Preparation of Large Earthquakes

Abstract

Certain seismicity characteristics in the northern Balkan region are studied. A method is used based on identification of ring-shaped seismicity structures that often form prior to large earthquakes in continental regions. Relatively large, shallow (h = 0–33 km), ring-shaped structures are identified within two areas where no relatively large earthquakes (М ≥ 6.5) have occurred since 1900. The meridional structure with a threshold magnitude M_th = 4.2 and major axis length L ~ 150 km formed within the western area bounded by coordinates of 42.0–44.5° N and 15.5–17.5° Е. A larger structure (М_th = 4.1, L ~ 180 km) has been revealed within the eastern area (41.0–43.5° N and 21.5–23.5° Е). Apart from shallow rings, deep seismicity bands (h = 34–70 km) formed within both areas. Magnitudes of possible large events are estimated based on previously obtained correlation dependences of the seismicity ring parameters on the energy of large earthquakes with different focal mechanisms: M_w = 7.2 ± 0.2 and M_w = 6.7 ± 0.1 for the western and eastern areas, respectively. By analogy with some continental regions (primarily, Greece), we suggest that the epicenters of possible large earthquakes can be located near the intersections of shallow rings and deep seismicity bands. The conclusion is made that a large event is the most probable within the eastern area in the near years.