Towards a seismotectonic map of Bulgaria

Several large earthquakes reaching up to M>7 have affected Bulgaria, and testify a significant active tectonics whose associated seismicity threatens large portions of the territory. A detailed knowledge of the main active faults and the understanding of the current tectonic activity and seismicity is therefore fundamental to get a comprehensive picture of the seismotectonics of the region. We have undertaken a study based on field survey, seismicity and paleostress analysis, remote sensing and digital topography interpretation, and integrated this information in a comprehensive map of active faults. This exhaustive dataset provides fundamental information on the state of stress and the seismic proneness of different areas, and represents a first valuable step toward the elaboration of a complete and actualized seismotectonic map of Bulgaria. This has obvious implication for the study of seismic hazard, urban planning and civil protection and gives basic information to the knowledge of the active deformation and geodynamics of the region. Few examples highlight the information that can be derived from such a seismotectonic map. Southern Bulgaria, south of the Balkan mountains, is mainly affected by normal active faults, which accommodate a roughly N-S to NNE-SSW extension, which is likely related to the northern extent of the Aegean extensional system. Activity of these extension-related structures is responsible for some of the destructive earthquakes which affected Bulgaria in historical times, notably the 1904 Krupnik earthquake (7.2<M<7.8) due to slip on a major E-W-trending normal fault system. In the Balkan mountains area the situation is more complex, with different kinematics affecting the eastern, central and western sectors of the chain. In the eastern sector, focal mechanisms range from purely compressional to strike slip, with P-axis oriented about ESE-WNW. Nonetheless, major normal faults characterize the southern front of the Balkan mountains. Faults are roughly E-W oriented and arranged in an en echelon pattern, to form a right lateral shear zone, compatible with the P-axis indicated by focal mechanisms. In the western sector, the southern front of the Balkan mountains results as well composed of a series of almost E-W directed normal faults arranged in an en echelon pattern, whose architecture reveals a component of left lateral shear zone, coherent with a roughly E-W directed horizontal compression. In the central sector, the southern border of the Balkan mountain is instead marked by a single frontal thrust. This apparent inconsistency of the tectonic style along the front may be explained only by a partial lateral extrusion of this sector, implying bending of the chain in response to a stress parallel to its axis. Our study shows therefore that the complex coexistence of different tectonic styles and stress regimes in the active deformation of the region may be explained in a simple coherent geodynamic framework.